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4Astronomy& Astrophysicsmanuscript no. draft c© ESO 2014October 2, 2014

Imprints of galaxy evolution on H ii regionsMemory of the past uncovered by the CALIFA survey

S. F. Sánchez1, E. Pérez2, F. F. Rosales-Ortega3, D. Miralles-Caballero4, A. R. López-Sánchez5,6, J.Iglesias-Páramo2,7, R. A. Marino8, L. Sánchez-Menguiano2,9, R. García-Benito2, D. Mast10, M.A. Mendoza2, P.

Papaderos11, S. Ellis12,5, L. Galbany13,14, C. Kehrig2, A. Monreal-Ibero15, R. González Delgado2, M. Mollá16, B.Ziegler17, A. de Lorenzo-Cáceres18, J. Mendez-Abreu18, J. Bland-Hawthorn12, S. Bekeraite19, M. M. Roth19, A.

Pasquali20, A. Díaz4, D. Bomans21, G. van de Ven22, L. Wisotzki19, and The CALIFA collaboration

1 Instituto de Astronomía,Universidad Nacional Autonóma deMexico, A.P. 70-264, 04510, México,D.F.2 Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, Aptdo. 3004, E18080-Granada, Spain

e-mail:sanchez@iaa.es.3 Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis E. Erro 1, 72840 Tonantzintla, Puebla, Mexico.4 Departamento de Física Teórica, Universidad Autónoma de Madrid, 28049 Madrid, Spain.5 Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia6 Department of Physics and Astronomy, Macquarie University, NSW 2109, Australia7 Centro Astronómico Hispano Alemán, Calar Al to, (CSIC-MPG), C/Jesús Durbán Remón 2-2, E-04004 Almería, Spain8 CEI Campus Moncloa, UCM-UPM, Departamento de Astrofísica yCC. de la Atmósfera, Facultad de CC. Físicas, Universidad

Complutense de Madrid, Avda. Complutense s/n, 28040 Madrid, Spain.9 Dpto. de Física Teórica y del Cosmos, University of Granada,Facultad de Ciencias (Edificio Mecenas), 18071 Granada, Spain

10 Instituto de Cosmologia, Relatividade e Astrofísica - ICRA, Centro Brasileiro de Pesquisas Físicas, Rua Dr.Xavier Sigaud 150,CEP 22290-180, Rio de Janeiro, RJ, Brazil

11 Centro de Astrofísica and Faculdade de Ciencias, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal.12 Sydney Institute for Astronomy, School of Physics A28, University of Sydney, NSW 2006, Australia.13 Millennium Institute of Astrophysics, Universidad de Chile, Casilla 36-D, Santiago, Chile14 Departamento de Astronomía, Universidad de Chile, Casilla36-D, Santiago, Chile15 GEPI, Observatoire de Paris, CNRS UMR8111, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France16 Departamento de Investigación Básica, CIEMAT, Avda. Complutense 40 E-28040 Madrid, Spain.17 University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria18 School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, U.K. (SUPA)19 Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany.20 Universitaet Heidelberg, Zentrum fuer Astronomie, Astronomisches Rechen Institut, Moenchhofstr. 12 - 14, 69120 Heidelberg,

Germany.21 Astronomical Institute of the Ruhr-University Bochum Universitaetsstr. 150, 44801 Bochum, Germany.22 Max-Planck-Institut für Astronomie, Heidelberg, Germany.

Received —– ; accepted —-

ABSTRACT

Context. H ii regions in galaxies are the sites of star formation and thus particular places to understand the build-up of stellar massinthe universe. The line ratios of this ionized gas are frequently used to characterize the ionization conditions. In particular the oxygenabundances are assumed to trace the chemical enrichment of galaxies.Aims. We explore here the connections between the ionization conditions and the properties of the overall underlying stellarpopula-tion (ionizing or not-ionizing) in Hii regions, in order to uncover the real physical connection between them.Methods. We use the Hii regions catalogue from the CALIFA survey, the largest in existence with more than 5000 Hii regions, toexplore their distribution across the classical [Oiii] λ5007/Hβ vs. [N ii] λ6583/Hα diagnostic diagram, and how it depends on theoxygen abundance, ionization parameter, electron density, and dust attenuation. The location of Hii regions within this diagram iscompared with predictions from photoionization models. Finally, we explore the dependence of the location within the diagnosticdiagram on the properties of the host galaxies, the galactocentric distances and the properties of the underlying stellar population.Results. We found that the Hii regions with weaker ionization strengths and more metal rich are located in the bottom-right areaof the diagram. On the contrary, those regions with strongerionization strengths and more metal poor are located in the upper-leftend of the diagram. Photoionization models per se do not predict these correlations between the parameters and the line ratios. TheH ii regions located in earlier-type galaxies, closer to the center, and formed in older and more metal rich regions of the galaxies arelocated in the bottom-right area of the diagram. On the otherhand, those regions located in late-type galaxies, in the outer regions ofthe disks, and formed on younger and more metal poor regions lie in the top-left area of the diagram. The two explored line ratiosshow strong correlations with the age and metallicity of theunderlying stellar population.Conclusions. These results indicate that although Hii regions are short lived events, they are affected by the total underlying stellarpopulation. One may say that Hii regions keep a memory of the stellar evolution and chemical enrichment that have left an imprinton the both the ionizing stellar population and the ionized gas.

Key words. Galaxies: abundances — Galaxies: fundamental parameters —Galaxies: ISM — Galaxies: stellar content — Techniques:imaging spectroscopy — techniques: spectroscopic – stars:formation – galaxies: ISM – galaxies: stellar content

1

1. Introduction

Classical Hii regions are large, low-density clouds of partiallyionized gas in which star formation has recently taken place(<15Myr). The short-lived blue stars forged in these regions emitlarge amounts of ultraviolet light that ionizes the surroundinggas. They show very different physical scales, from few parsecs,like Orion nebula (D∼8 pc), or even smaller (Anderson 2014), tohundreds of parsecs, such as 30 Doradus (D∼200 pc), NGC 604(D∼460 pc) or NGC 5471 (D∼1 kpc) as reported by Oey et al.(2003) and García-Benito et al. (2011). These latter ones arethe prototypes of the extragalactic giant Hii regions found fre-quently in the disks of spiral galaxies (e.g. Hodge & Kennicutt1983; Dottori 1987; Dottori & Copetti 1989; Knapen 1998), orstarburst and blue compact galaxies (e.g. Kehrig et al. 2008;López-Sánchez & Esteban 2009; Cairós et al. 2012).

Baldwin et al. (1981) first proposed the [Oiii] λ5007/Hβversus [Nii] λ6583/Hα diagnostic diagram (now known asthe BPT diagram) to separate emission-line objects accord-ing to the main excitation mechanism: normal Hii regions,planetary nebulae, and objects photoionized by a harder radi-ation field. A hard radiation field can be produced by either apower-law continuum from an AGN, shock excitation, plane-tary nebulae or even post-AGB stars (e.g. Binette et al. 1994,2009; Morisset & Georgiev 2009; Flores-Fajardo et al. 2011;Kehrig et al. 2012; Singh et al. 2013; Papaderos et al. 2013).Veilleux & Osterbrock (1987) and Osterbrock (1989) extendedand refined this classification scheme, incorporating new diag-nostic diagrams. Osterbrock (1989) used theoretical photoion-ization models to infer the shape of the classification line be-tween star-forming (SF) and AGN galaxies, and they addedtwo new diagnostics diagrams that exploit the [Oi]/Hβ ver-sus [Sii]/Hα line ratios. Dopita et al. (2000) and Kewley et al.(2001) combined stellar population synthesis and photoioniza-tion models to build the first purely theoretical classificationscheme for separating pure AGN from galaxies containing starformation, and Kauffmann et al. (2003) used SDSS data to ob-servationaly constrains these classifications.

In essence, these models assume that the main factors thatcontrol the emission line spectrum are the chemical abundancesof the heavy elements in the gas phase of an Hii region (oxygenbeing the most important), the shape of the ionizing radiationspectrum, and the geometrical distribution of gas with respect tothe ionizing sources. Generally speaking, all the geometrical fac-tors are subsumed into a single factor, the ionization parameterq(with dimensionscm s−1) or the (dimensionless) ionization pa-rameteru = q/c. They also assumea priori that these parametersare independent, and thus these models are presented as grids ofoxygen abundance, ionization parameter, and electron densitiesfor a given ionizing population.

Different demarcation lines have been proposed for this di-agram. The most popular ones are the Kauffmann et al. (2003)and Kewley et al. (2001) curves. They are usually invoked to dis-tinguish between star-forming regions (below the Kauffmann etal. 2013 curve) and AGNs (above the Kewley et al. 2001 curve).The location between both curves is normally assigned to a mix-ture of different sources of ionization. Additional demarcationlines have been proposed for the region above the Kewley et al.(2001) curve to segregate between Seyfert and LINERs (e.g.,Kewley et al. 2006; Cid Fernandes et al. 2010). More recentlyit has been explored the use of 3D diagnostic diagrams to seg-regate the different physical ionizaion sources in a cleaner way(e.g. Vogt et al. 2014).

There are well known trends of the ionization conditionsacross the BPT diagram. For example, as the gas-phase chem-ical abundances are increased, the primary effect on the Hii re-gion is to decrease the electron temperature, since there ismorecooling per photoionization. This affects the temperature sen-sitive line ratios such as [Oiii]λ4959,λ5007/λ4363, but alsomany others (e.g. Marino et al. 2013), and in particular the twoline ratios used in the BPT diagram: [Oiii] λ5007/Hβ versus[N ii] λ6583/Hα. Indeed, early photographic spectra revealedthat, in disk galaxies, Hii regions located in the outer spiralarms displayed large [Oiii] λ5007/Hβ ratios, while Hii re-gions located in the inner spiral arms show relatively large[N ii] λ6583/Hα ratios (e.g. Sanduleak 1969; Searle 1971). Itwas soon realized that this is due to the existence of a globalabundance gradient across the galaxy, connecting for the firsttime the location of Hii regions within the BPT diagram withthe chemical evolution in disk galaxies.

Since these early studies, many others have unveiled someaspects of the complex processes at play between the chem-ical evolution of galaxies and the ionization conditions inH ii regions. These studies have been successful in deter-mining important relationships, scaling laws and systematicpatterns: (i) luminosity-metallicity, mass-metallicity, and sur-face brightness vs. metallicity relations (Lequeux et al. 1979;Skillman 1989; Vila-Costas & Edmunds 1992; Zaritsky et al.1994; Tremonti et al. 2004); (ii) effective yield vs. luminos-ity and circular velocity relations (Garnett 2002); (iii) abun-dance gradients and the effective radius of disks (Diaz 1989);(iv) systematic differences in the gas-phase abundance gradi-ents between normal and barred spirals (Zaritsky et al. 1994;Martin & Roy 1994); (v) characteristic vs. integrated abun-dances (Moustakas & Kennicutt 2006); (vi) galactocentric abun-dance gradients (e.g. Bresolin et al. 2009; Yoachim et al. 2010;Rosales-Ortega et al. 2011; Marino et al. 2012; Bresolin et al.2012), etc.

However, many of these studies, in particular the older ones,present evident selection effects. In most of the cases the spectro-scopic surveys of Hii regions rely on previous imaging surveysusing narrow-band images (e.g. Martin & Roy 1994). Those im-ages are then processed to provide with a catalogue of Hii re-gions suitable to be observed using long-slit spectroscopy. Inthis selection the signal-to-noise of the detected emission linein the narrow-band images plays an important role. Thereforemost of these surveys are biased towards high contrast (equiv-alent width of emission lines) Hii regions, located in the outerdisks of galaxies. They tend to be focused on late-type spirals(Sc and Sd), which offer the best constrast, due to their lowersurface-brightness in the continuum. This may has biased ourunderstanding of the properties of Hii regions, since in mostof the cases the studies were restricted to regions ionized byyounger and/or massive stellar clusters . Indeed, Kennicutt et al.(1989) recognized that Hii regions in the center of galaxies dis-tinguish themselves spectroscopically from those in the disk bytheir stronger low-ionization forbidden emission. The nature ofthis difference was not clear. It may be due to contaminationby an extra source of ionization, such as diffuse emission or thepresence of an AGN. However, other stellar processes, such asnitrogen enhancement due to a natural aging process of Hii re-gions and the surrounding ISM, can produce the same effect.These early results were confirmed by Ho et al. (1997), whodemonstrated that inner star-forming regions may populatetheright branch of the BPT diagram at a location above the demar-cation line defined later by Kauffmann et al. (2003).

S. F. Sánchez et al.: Imprints of galaxy evolution on Hii regions

Another bias that may be introduced by classical slit-spectroscopy surveys of Hii regions is the aperture effects. Asalready noticed by Dopita et al. (2014), Long-slit observationsdo not integrate over the whole of an Hii region, particularly inthe case of observations of nearby galaxies. By picking out thebrightest regions of such Hii regions, we bias the data towardthe high-excitation regions and the spectrum is not representa-tive of the whole Hii region. This is clearly illustrated whenthe integrated and spatial resolved properties of Hii regionsare compared (e.g. Sánchez et al. 2007c; Relaño et al. 2010;Monreal-Ibero et al. 2011).

The advent of Multi-Object Spectrometers and Integral FieldSpectroscopy (IFS) instruments with large fields of view offersus the opportunity to undertake a new generation of emission-line surveys, based on samples of hundreds of Hii regions andfull two-dimensional (2D) coverage of the disks of nearby spiralgalaxies (e.g. Rosales-Ortega et al. 2010; Sánchez et al. 2012a,2014). Large catalogs of Hii regions have been produced, withthousands of objects, over statistical samples of galaxiescover-ing a wide range of morphological types (Sánchez et al. 2012b,2013). These catalogs led to the discovery of a clear correlationbetween the oxygen abundance of individual Hii regions andthe stellar mass densities of the underlying stellar population(Rosales-Ortega et al. 2012; Sánchez et al. 2013), a radial gra-dient of different ionization conditions (Sánchez et al. 2012b),and the existence of a common abundance gradient in all disk-dominated galaxies (Sánchez et al. 2014). Interestingly, theseresults confirmed the previous ones presented by Kennicutt et al.(1989) and Ho et al. (1997), regarding the presence of Hii re-gions above the demarcation line defined by Kauffmann et al.(2003). Those Hii regions are not restricted to the central regionsand can be found at any galactocentric distance, even at morethan 2re, which excludes/minimizes a possible contaminationby a central source of ionization. A more plausible explanationis that we are observing more aged Hii regions as indicated be-fore.

All these relations point towards a connection between theionization conditions in Hii regions and the properties of the un-derlying stellar population, and therefore, the overall evolutionof the host galaxies. This connection is not taken into account byphoto-ionization models. Evolutionary processes of the ionizingcluster (e.g. Dopita et al. 2006a) and/or constrainsts impossed bythe evolution of the host galaxies (e.g. Dopita et al. 2006b)arerequired to exaplain these relations.

In the current study we explore the distribution of Hii re-gions across the BPT diagram to try to understand the na-ture of this connection. To do so, we use the largest catalogof H ii regions with spectroscopic information currently avail-able. This catalog was extracted from the CALIFA dataset(Sánchez et al. 2012a), and comprises more than 5000 Hii re-gions (Sánchez et al. 2014). The structure of this article isas fol-lows: Section 2 presents the sample of galaxies and Hii regionsused along this study. Section 3 summarizes the analysis of thedata. Section 3.1 comprises the comparison between the distri-bution across the BPT diagram with photo-ionization models. InSec. 3.2 we explore how the parameters that define the ionizationconditions actually change across the BPT diagram. Sections 3.3and 3.4 explore the change in the distribution of Hii regions asa function of the morphology of the host galaxy and their galac-tocentric distance. Finally, Sec. 3.5 explores the dependency ofthe location within the BPT diagram with the properties of theunderlying stellar population. Section 4 discusses the results ofthe analysis, and the conclusions are summarized in Sec. 5.

2. Sample of galaxies and dataset

The galaxies were selected from the CALIFA observed sample.Since CALIFA is an ongoing survey, whose observations arescheduled on a monthly basis (i.e., dark nights), the list ofob-jects increases regularly. The current results are based onthe 306galaxies observed before May 2013. Their main characteristicswere already described in Sánchez et al. (2014).

The details of the survey, sample, observational strategy,andreduction are explained in Sánchez et al. (2012a). All galaxieswere observed using PMAS (Roth et al. 2005) in the PPAK con-figuration (Kelz et al. 2006), covering a hexagonal field-of-view(FoV) of 74′′×64′′, sufficient to map the full optical extent ofthe galaxies up to two to three disk effective radii. This is possi-ble because of the diameter selection of the sample (Walcheretal., 2014). The observing strategy guarantees complete coverageof the FoV, with a final spatial resolution of FWHM∼3′′, cor-responding to∼1 kpc at the average redshift of the survey. Thesampled wavelength range and spectroscopic resolution (3745-7500 Å, λ/∆λ ∼850, for the low-resolution setup) are morethan sufficient to explore the most prominent ionized gas emis-sion lines, from [Oii]λ3727 to [Sii]λ6731 at the redshift of ourtargets, on one hand, and to deblend and subtract the underly-ing stellar population, on the other (e.g., Sánchez et al. 2012a;Kehrig et al. 2012; Cid Fernandes et al. 2013, 2014). The datasetwas reduced using version 1.3c of the CALIFA pipeline, whosemodifications with respect to the one presented in Sánchez etal.(2012a) are described in detail in Husemann et al. (2013). Insummary, the data fulfil the predicted quality-control require-ments with a spectrophotometric accuracy that is better than15% everywhere within the wavelength range, both absolute andrelative with a depth that allows us to detect emission linesin in-dividual Hii regions as faint as∼10−17erg s−1 cm−2, and with asignal-to-noise ratio of S/N∼3-5. For the emission lines consid-ered in the current study, the S/N is well above this limit, and themeasurement errors are negligible in most of the cases. In anycase, they have been propagated and included in the final errorbudget.

The final product of the data reduction is a regular-grid dat-acube, withx andy coordinates that indicate the right ascensionand declination of the target andza common step in wavelength.The CALIFA pipeline also provides the propagated error cube, aproper mask cube of bad pixels, and a prescription of how to han-dle the errors when performing spatial binning (due to covari-ance between adjacent pixels after image reconstruction).Thesedatacubes, together with the ancillary data described in Walcheret al. (2014), are the basic starting points of our analysis.

2.1. H ii detection and extraction

The segregation of Hii regions and the extraction of the corre-sponding spectra is performed using a semi-automatic procedurenamed HIIexplorer1. The procedure is based on some basic as-sumptions: (a) Hii regions are peaky and isolated structures witha strong ionized gas emission, which is significantly above thestellar continuum emission and the average ionized gas emis-sion across the galaxy. This is particularly true for Hα because(b) H ii regions have a typical physical size of about a hundredor a few hundred parsecs (e.g., González Delgado & Perez 1997;Lopez et al. 2011; Oey et al. 2003), which corresponds to a typ-ical projected size of a few arcsec at the distance of the galaxies.

1 http://www.caha.es/sanchez/HII_explorer/

3

S. F. Sánchez et al.: Imprints of galaxy evolution on Hii regions

These basic assumptions are based on the fact that most ofthe Hα luminosity observed in spiral and irregular galaxies is adirect tracer of the ionization of the interstellar medium (ISM)by the ultraviolet (UV) radiation produced by young high-massOB stars. Since only high-mass, short-lived stars contribute sig-nificantly to the integrated ionizing flux, this luminosity is a di-rect tracer of the current star-formation rate (SFR), independentof the previous star-formation history. Therefore, clumpystruc-tures detected in the Hα intensity maps are most probably as-sociated with classical Hii regions (i.e., those regions for whichthe oxygen abundances have been calibrated).

The details of HIIexplorer are given in Sánchez et al.(2012b) and Rosales-Ortega et al. (2012). In summary we cre-ate a narrow-band image centred on the wavelength of Hα at theredshift of the object. Then we run HIIexplorer to detect andextract the spectra of each individual Hii region, adopting theparameters presented in Sánchez et al. (2014). The algorithmstarts looking for the brightest pixel in the map. Then, the codeaggregates the adjacent pixels until all pixels with flux largerthan 10% of the peak flux of the region and within 500 pc or3.5 spaxels from the center have been accumulated. The dis-tance limit take into account the typical size of Hii regions of afew hundreds of parsecs (e.g. González Delgado & Perez 1997;Lopez et al. 2011). Then, the selected region is masked and thecode keeps iterating until no peak with flux ex- ceeding the me-dian Hα emission flux of the galaxy is left. Mast et al. (2014)studied the loss of resolution in IFS using nearby galaxies ob-served by PINGS (Rosales-Ortega et al. 2010). Some of thesegalaxies were simulated at higher redshifts to match the char-acteristics and resolution the galaxies observed by the CALIFAsurvey. Regarding the Hii region selection, the authors concludethat atz ∼0.02 the Hii clumps can contain on average from 1 to6 of the Hii regions obtained from the original data atz∼ 0.001.Another caveat is that this procedure tends to select regions withsimilar sizes although real Hii regions actually have differentsizes. However, the actual adopted size is of the order of theFWHM of the CALIFA data for the used version of the data re-duction (Husemann et al. 2013).

Then, for each individual extracted spectrum we modelledthe stellar continuum usingFIT3D2, a fitting package describedin Sánchez et al. (2006b) and Sánchez et al. (2011). A simpleSSP template grid with 12 individual populations was adopted.It comprises four stellar ages (0.09, 0.45, 1.00, and 17.78 Gyr),two young and two old ones, and three metallicities (0.0004,0.019, and 0.03, that is, subsolar, solar, or supersolar, respec-tively). The models were extracted from the SSP template li-brary provided by the MILES project (Vazdekis et al. 2010;Falcón-Barroso et al. 2011). The use of different stellar ages andmetallicities or a larger set of templates does not qualitativelyaffect the derived quantities that describe the stellar popula-tions. Indeed, we compared two of these parameters (luminosityweighted age and metallicity), derived using this simple templateand the more elaborate one described in Cid Fernandes et al.(2013, 2014) and we found an agreement within∼0.25 dex inboth parameters.

After subtracting the underlying stellar population, the fluxintensity of the strong emission lines was extracted for each gas-pure spectrum by fitting a single Gaussian model to each line,re-sulting in a catalog of the emission line properties (Sánchez et al.2012b). Finally, the Hii regions were selected from this final cat-alog based on the properties of the underlying stellar continuum.It was required that they present a young stellar population, com-

2 http://www.caha.es/sanchez/FIT3D/

patible with the observed equivalent width of Hα (Sánchez et al.2014).

The final catalog comprises the strongest emission line andemission line ratios from [Oii]λ3727 to [Sii]λ6731 for 5229H ii regions, together with their equivalent widths and the lumi-nosity weighted ages and metallicities of the underlying stellarpopulation. So far, this is the largest catalog of Hii regions andagregations with spectroscopic information. It is also oneof thefew catalogs derived for a statistically well-defined sample ofgalaxies representative of the entire population of galaxies in thelocal Universe (Walcher et al. 2014).

3. Analysis and Results

The main goal of this study is to understand how the ioniza-tion conditions in Hii regions are related to the properties of thegalaxies, and if there is a connection between the ionization con-ditions and the overall evolution and chemical enrichment of thestellar populations.

To do so we use the catalog of emission-line fluxes and ra-tios, and the corresponding properties of the stellar populationsof the Hii regions sample described before (Sánchez et al. 2014).

3.1. Distribution of H ii regions across the BPT diagram

Figure 1 shows the distribution of our sample of Hii regionsacross the BPT diagram, compared with different photoion-ization models. The Kauffmann et al. (2003) and Kewley et al.(2001) demarcation lines have been included as general ref-erences. The two tophand panels include the expected lo-cation based on the MAPPINGS-III photoionization models(Allen et al. 2008), adopting two different ionizing populations(Kewley et al. 2001). The top left hand panel shows the grid cor-responding to an instantaneous zero-age starbust model based onthe PEGASE spectral energy distribution (Kewley et al. 2001;Dopita et al. 2000). Each red solid-line corresponds to a differ-ent metallicity, which is indicated with the correspondinglabel(ranging from 0.2 to 2.0 solar abundances), and each blue solid-line corresponds to a different ionization parameter of the radia-tion field (ranging from log(u)=-3.7, at the bottom, to log(u)=-2,at the top). The top right hand panel shows a similar plot; theonly difference being that here we assumed a continous starburstmodel 4 Myr since its ignition. In all cases it is assumed an av-erage electron density ofne =350 cm−3.

As expected, most of the Hii regions are located below theenvelope defined by the instantaneous star-burst photoinizationmodels, which was proposed by Dopita et al. (2000) as a de-marcation line to define the regions that are clearly ionizedby star-formation. This line is very similar those defined byKauffmann et al. (2003), empirical, and Stasinska et al. (2006),theoretical. We adopted here the Kauffmann et al. (2003) one,but the use of any of the three will produce similar results.However, there is a non-negligible fraction of∼14% of Hii re-gions located between this envelope and the one defined by aa model with continuous star formation, as we already noticedin Sánchez et al. (2013) and Sánchez et al. (2014). Kewley et al.(2001) noticed that starburst galaxies are preferentiallylocatedin this region. They considered these galaxies to have experi-enced on-going star formation over at least several Myr, andthis prolonged star-formation activity is reflected in the BPT dia-grams. A similar interpretation can be applied to the Hii regionsabove the Dopita et al. (2000) and/or Kauffmann et al. (2003)demarcation lines.

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S. F. Sánchez et al.: Imprints of galaxy evolution on Hii regions

log([NII] λ6583/Hα)

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1

Fig. 1. Top-left panel:[O iii] λ5007/Hβ vs. [N ii] λ6583/Hα diagnostic diagram for the∼5000 Hii ionized regions included in oursample. The color solid-lines represent the expected line ratios derived from the MAPPINGS-III ionization models, based on aninstantaneous zero-age starbust model based on the PEGASE spectral energy distribution (Kewley et al. 2001; Dopita et al. 2000),and a fixed electron density of 350 cm−3. Each orange-to-red solid-line corresponds to a different metallicity, which is indicatedwith the corresponding label and color (ranging from 0.2 to 2.0 solar metallicities), and each blue-to-cyan solid-linecorrespondsto a different ionization parameter of the radiation field (ranging from log(u)=-3.7, for the dark blue, to log(u)=-2, at the cyanblue).Top-right panel:Similar plot, with the only difference that now the color solid-lines represent the expected line ratios for acontinuous starburst model 4 Myr since its ignition.Botton-left panel:Similar plot, with the only difference being in this case that thecolor solid-lines and the dotted-lines correspond to the expected line ratios for a photoionization model based on an instantaneouszero-age starburst, with a constant metallicity (indicated with a label and represented with a fixed color), but for two differentelectron densities (10 cm−3, for the dotted-line, and 350cm−3, solid one).Botton-right panel:Similar plot, with the only differencethat in this case the color solid-lines and the dotted-linescorrespond to the expected line ratios for a photoionization model basedon an instantaneous zero-age starburst, with a constant metallicity, and a fixed electron density shown in top-left panel, but fortwo different dust attenuations, (no dust, for the solid-line, andAV =3 mag, for the dotted one). In all the panels, the dot-dashed-and dashed-line represent, respectively, the Kauffmann et al. (2003) and Kewley et al. (2001) demarcation curves. They are usuallyinvoked to distinguish between classical star-forming objects (below the dashed-dotted line), and AGN powered sources (above thedashed-line). Regions between both lines are considered intermediate ones.

In both cases, a single burst or a continuous star formation,the distribution of Hii regions across the BPT diagram does notcover the entire parameter space forseen by the considered pho-toionization models. Thus,real H ii regions cover a smaller pa-rameter space than is predicted by the models. On the other hand,there is a degeneracy in the combination of ionization conditions

that predict a certain location in the diagram (abundance and ion-ization strength in this particular grid). Therefore, the location ofa particular Hii could be explained by different ionization con-ditions. For example, the curve described for a fixed solar abun-dance present values within the BPT diagram that range from(-1,0.5), in the upper-left corner, to (-0.2,-1), in the bottom-right

5

S. F. Sánchez et al.: Imprints of galaxy evolution on Hii regions

corner. Indeed, it describes almost an envelope of the distributionof H ii regions across the BPT diagram (Fig. 1). Only when fix-ing one of the two parameters involved in the grid it is possible toderive a trend with the other parameter across the BPT diagram.Both results have two direct consequences: (i) it is not possi-ble to derive which ioniziation conditions are associated with aparticular Hii region based only on their location in the BPT di-agram, and (ii) there are ionization conditions that are discardedby nature for reasons that cannot be predicted based only on pho-toionization models. This result is well known, and has limitedour ability to recover the ionization conditions of Hii regions fordecades (e.g. Pérez-Montero 2014).

We explore how the variation of other parameters affects theline ratios predicted by photoionization models. The bottom lefthand panel of Fig 1 illustrates the change in the location acrossthe BPT diagram when different electron densities are assumed.We show the same grid of photoinization models presented in thetop left hand panel, but assuming two different electron densi-ties (ne=10 and 350 cm−3). In general the photoionization mod-els predict that higher density regions of similar abundancesand ionization strengths should be located at higher valuesof[O iii]/Hβ for very similar values of [Nii]/Hα. Therefore, the re-gions located at the top right envelope of the distribution shouldbe the densest ones.

Finally, we explore the variation of the line ratios whendust attenuation is taken into account. The bottom right handpanel of Fig 1 shows the same grid of photoionization mod-els presented in top-lefthand panel, but assuming two differ-ent dust attenuations (AV=0 and 3.5 mag). The effects of thedust attenuation over the line ratios were simulated adopting theCardelli et al. (1989) extinction law, with a specific dust atten-uation of RV = 3.1. The BPT diagram is designed to be lessaffected by dust attenuation than other ones frequently used (e.g.Veilleux et al. 1995), since it involves emission lines at very sim-ilar wavelengths for each ratio. Thus, even in the case of verylarge dust attenuations the differences are very small. Therefore,no trend is expected with the dust attenuation in the distributionof H ii regions across the BPT diagram.

3.2. Empirical estimation of the ionization parameters

We have seen in the previous section that the location in theBPT diagram for a classical Hii region ionized by young starsof a certain age is well defined by three main parameters (a) theionization parameter or fraction of Lyman continuum photonswith respect to total amount of gas, (b) the metallicity or chemi-cal abundance of the ionized gas, and (c) the electron density ofthe gas. In addition, we have seen that the location should haveno dependency on the dust attenuation (for this particular dia-gram). However, the trends of the two first parameters acrosstheBPT diagram are not well defined based on photoinization mod-els. We investigate here how the oxygen abundance, ionizationstrength, electron density, and dust attenuation actuallychangeacross the BPT diagram, based on empirical estimations of theseparameters.

The oxygen abundances were derived using thecounterpart-method described by Pilyugin et al. (2012). This method usesthe dust corrected emission line ratios of a set of strong emis-sion lines including [Oii]/Hβ, [O iii]/Hβ, [N ii]/Hβ and [Sii]/Hβ,and compares the input values with a grid of line ratios ofH ii regions with known abundances derived using the directmethod (i.e., based on direct estimations of the electron tem-perature). This calibrator attempts to be an improvement ofthepreviously analytical method proposed in Pilyugin et al. (2010).

The values derived using this method were cross-checked withthe ones provided using the O3N2 calibrator (Alloin et al. 1979;Pettini & Pagel 2004; Stasinska et al. 2006),

O3N2 = log

[

I ([O iii] λ5007)/I (Hβ)I ([N ii] λ6584)/I (Hα)

]

(1)

using the recently updated calibration presented by Marinoet al.(2013), that improves the abundance estimation by includingmany more Hii regions than previous calibrations using this pa-rameter (e.g. Pettini & Pagel 2004), in particular in the rangeof higher metallicities. The advantage of this method is that itdepends on strong, well deblended emission lines, and in par-ticular does not depend on [Oii]λ3727, a line that may be af-fected by vignetting (Sánchez et al. 2012a), any inaccuracyinflux calibration in the blue-end of the covered spectral range(Husemann et al. 2013), and uncertainties related to the deriva-tion of the dust attenuation. The values for oxygen abundancesmatch one another within a standard deviation of∼0.07 dex, avalue of the order of the expected errors of the individual mea-surements (Sánchez et al. 2013) and the accuracy of the calibra-tors (Marino et al. 2013).

For the ionization parameter,u, we adopted two dif-ferent calibrators described by Díaz et al. (2000), based on[S ii]λ6717,31/Hβ, and [Oii]λ3727/Hβ, that require a previousknowledge of the oxygen abundance, and a third one describedby Dors et al. (2011), that depends on the [Oii]/[O iii] line ratio.This latter one is a recalibration of the classical one proposed byDíaz et al. (2000). We cross-checked the values obtained by thedifferent methods, and they seem to agree within the errors, al-though [Sii]/Hβ yields slightly larger values than the other two.We adopted the mean value of the three as our estimation ofthe ionization parameter. Recent results indicate that other lineratios, like [Oiii]/[S ii] can be used as good tracers of the ioniza-tion parameter (Dopita et al. 2013). However, there is no pub-lished calibrator corresponding to this ratio to our knowledge,and therefore we could not compare the results in an independentway. As a final sanity check we cross-checked that the derivedvalues follow the described relation between this parameter andthe equivalent width of Hβ (Diaz 1989).

The electron density,ne, was derived based on the line ra-tio of the [Sii] doublet (e.g., Osterbrock 1989), by solving theequation:

I ([S ii]λ6717)I ([S ii]λ6731)

= 1.491+ 3.77x1+ 12.8x

(2)

wherex is the density parameter, defined asx = 10−4net−1/2 andtis the electron temperature in units of 104 K (McCall et al. 1985).For this calculation we assumed a typical electron temperature ofT = 104K, an average value corresponding to the expected con-ditions in Hii regions Osterbrock (1989). This equation reflectsthe fact that the [Sii] doublet ratio is sensitive to changes in theelectron density only for a limited range of values. For highandlow values it becomes asymptotic and the actual derived valuehas to be treated with care, and should not be used for quantita-tive statements. However, the value will be still valid for the pur-poses of this study, allowing us to understand the global trendsacross the BPT diagram with the electron density.

Finally, the dust attenuation, AV , was derived based on theHα/Hβ Balmer line ratio. The extinction law by Cardelli et al.(1989) was assumed, with a specific dust attenuation ofRV =

3.1, and the theoretical value for the unobscured line ratio for

6

S. F. Sánchez et al.: Imprints of galaxy evolution on Hii regions

log([NII] λ6583/Hα)

log(

[OIII

]λ50

07/H

β)

-1.5 -1 -0.5 0 0.5

-1-0

.50

0.5

1

8.2

8.4

8.6

12+log(O/H)

log([NII] λ6583/Hα)

log(

[OIII

]λ50

07/H

β)

-1.5 -1 -0.5 0 0.5

-1-0

.50

0.5

1

-3.5

-3-2

.5log(u)

log([NII] λ6583/Hα)

log(

[OIII

]λ50

07/H

β)

-1.5 -1 -0.5 0 0.5

-1-0

.50

0.5

1

00.

51

1.5

2log(ne)

log([NII] λ6583/Hα)

log(

[OIII

]λ50

07/H

β)

-1.5 -1 -0.5 0 0.5

-1-0

.50

0.5

1

01

23

Av (mag)

Fig. 2. [O iii] λ5007/Hβ vs. [Nii] λ6583/Hα diagnostic diagram for the∼5000 Hii regions included in our sample. The contoursshow the density distribution of these regions within the diagram plane, with the outermost contour enclosing 95% of theregions,and each consecutive one enclosing 20% fewer regions. In each panel the color indicates the average value at the correspondinglocation in the diagram for one of the four parameters described in the text:top-left panel:the oxygen abundance;top-right panel:the ionization parameter;botton-left panel:the electron density; andbotton-right panel:the dust attenuation. In all panels, thedot-dashed and dashed lines represent, respectively, the demarcation curves described in Fig.1, and the grey solid-lines representthe expected line ratios derived from the photoionization models shown in Fig. 1, where each line corresponds to a different stellarmetallicity (orage-to-red solid lines in Fig. 1).

case B recombination of Hα/Hβ = 2.86, for Te=10,000K andne=100 cm−3 (Osterbrock 1989). For this study we have assumedthat the intrinsic Hα/Hβ line ratio does not vary significantly, al-though it is known that it presents a dependence with the electrondensity and the temperature (e.g. Sánchez et al. 2005).

3.2.1. Distribution across the BPT diagram

The top left hand panel of Fig. 2 shows the distribution of Hii re-gions across the BPT diagram, with a color code representingtheoxygen abundance in each region. The more metal poor Hii re-gions are located in the upper-left corner of the diagram, and themore metal rich ones are located towards the lower-right corner.None of these trends can be explained based only in the expected

values derived from photoionization models, as we have seeninthe previous section. The opposite trend is found for the ioniza-tion parameter. The top left hand panel of Figure 2, shows thesame distribution with a color code representing the value of theionization parameter in each region. There is a clear trend acrossthe diagram. The Hii regions with the largest ionization parame-ter are located in the upper-left corner of the diagram, and thosewith the lowest values of this parameter are located towardsthelower-right corner.

The bottom left hand panel of Fig. 2 shows the distribution ofH ii regions across the BPT diagram with a color code represent-ing the electron density. The denser regions are located towardsthe upper-right envelope of the distribution, as expected from thecomparison with photoionization models, presented in the pre-vious section. Regions with higher electron density are expected

7

S. F. Sánchez et al.: Imprints of galaxy evolution on Hii regions

in areas of the galaxy that have suffered an over-pressure, in par-ticular in the leading front of a density wave generated by spiralarms or towards the central regions of the galaxies. We will in-vestigate that possibility in the next sections.

Finally, we explore the relation between the distribution ofH ii regions across the BPT diagram and the amount of dust at-tenuation. The bottom right hand panel of Fig. 2 shows the samedistribution shown in previous panels with a color code repre-senting the dust attenuation derived from the Balmer decrement.There is a clear trend that is not taken in to account by photoion-ization models: by construction the location within the BPTdia-gram ought to be independent of dust attenuation. Regions withlower dust attenuation are located in the upper-left regionof thediagram, with AV increasing towards the lower-right, and thenincreasing again at the upper-right end of the diagram. Indeed,the regions with the largest dust attenuation are those located inthe so-called intermediate area, defined as the region betweenthe Kauffmann et al. (2003) and Kewley et al. (2001) curves.

We should note here that the derived parameters may presentcross-dependences due to the adopted formulae/calibration toderive them. We tried to minimize or quantify those possibledependences to exclude possible induced trends. For the oxy-gen abundance we adopted thecounterpart-method since it usesall the strong emission lines in the considered wavelength range,and not only those involved in the Hii diagram (like O3N2 or N2calibrators Marino et al. 2013). We repeated the analysis usingother abundance indicators, like R23 or N2S2, without signifi-cant differences in the described trends. The ionization parame-ter was derived using different calibrators, only one of them ispurely empirical (the one based on the [Oii]/[O iii] line ratio).The other two calibrators involve deprojections of photoioniza-tion models that try to remove the degenerancy in the derivationof the oxygen abundance and the ionization-parameter. We con-sider that this approach minimize the possible dependencies be-tween those two parameters induced by the adopted estimatorsare minimized.

The derivation of the electron density is based on a formuladerived from basic principles of the quantum physic (equation2). This equation provides with an electron density that dependson the electron temperature. We have fixed this second param-eter, and therefore we may have induced a possible secondarycorrelation with any other parameter that depends on the tem-perature, like the oxygen abundance or the ionization strength.However, the largest possible bias introduced by fixing the elec-tron temperature in the derivation ofne is a of the order of a fac-tor 2-3. This cannot explain the range of derived electron densi-ties, of two orders of magnitudes, or their pattern along theBPTdiagram.

Finally, in the derivation of the dust attenuation we as-sumed the a fixed value for the intrinsic Balmer line ratios:Hα/Hβ = 2.86. However, it is well known that this line ratio de-pends on the ionization conditions within the nebulae, and it canrange between 2.7 and 3.1 for usual Hii regions (e.g. Osterbrock1989). The possible effect of taking into account this detail inthe derivation of the dust attenuation has been explored by pre-vious studies and in general it has been found the differences inthe derived dust attenuations is very small (e.g. Sánchez etal.2007c). In general the possible trends of the derived AV with theelectron temperature and electron density are weaker than theobservational errors of the considered line ratio, or othereffects(like assuming an uncorrect dust attenuation law or a Milky-Wayspecific dust attenuation). In summary we consider that the de-scribed trends are genuine, and not induced by possible correla-tions implicit in the estimations used to derive each parameter.

-4 -3 -2

88.

28.

48.

68.

89

log(u)

12+

log(

O/H

)

Fig. 3. Distribution of the∼5000 Hii regions described in thetext across the oxygen abudance vs. ionization parameter plane.The contours and color show the density distribution, with theoutermost contour enclosing 95% of the regions, and each con-secutive one enclosing 20% fewer regions.

The described trends across the BPT diagram of the pa-rameters that define the ionization conditions imply that thereare correlations between those parameters that cannot be ex-plained based purely on photoionization models. Fig. 3 showsthe distribution of oxygen abundance along the ionization pa-rameter. As expected, there is a clear correlation. Those re-gions with higher oxygen abundance have the lowest ionizationstrengths, and those regions with the lowest oxygen abundanceshave the highest ionization strengths. This is a well known re-lation, described for the first time by Dopita & Evans (1986),using strong-line indicators of both parameters. More recently,Pérez-Montero (2014), explored this relation based on a col-lection of Hii regions with abundances derived using the di-rect method, ie., based on the electron tempeture, describedin Marino et al. (2013). The results of Pérez-Montero (2014)clearly show that this relation is not induced by the strong-linecalibrator used to derive the abundance.

As a sanity check, we have repeated the figure using differentabundance and ionization parameter estimators, and we foundthe same result for all of them. This trend cannot be explainedbased purely on photoionization models, which assume that bothparameters are totally independent.

We should note here that these representations of the dataare dominated by the average properties and they highlight thedominant trends. Secondary dependencies are mostly hiddenorblurred when adopting this scheme, and are appreciated onlywhen the dominant trend is removed.

3.3. Distribution by galaxy morphology

In the previous sections we have explored the distribution of theH ii regions across the BPT diagram, comparing it with the pre-dictions by photoionization models (Sec. 3.1) and the empiri-cal estimation of a set of parameters that defines the ionizationconditions (Sec. 3.2). Now we explore the dependence of thelocation of Hii regions across the BPT diagram on the mor-phology of the host galaxy. The morphological classification wasperformed by eye, based on independent analysis by five mem-bers of the CALIFA collaboration, and will be described else-

8

S. F. Sánchez et al.: Imprints of galaxy evolution on Hii regions

where in detail (Walcher et al. 2014). Different tests indicatethat our morphological classification is fully compatible withpre-existing ones, and the results agree with the expectationsbased on other photometric/morphological parameters, such asthe concentration index (Sánchez et al. 2013) or the Sersic index(Sersic 1968).

Figure 4 shows the distribution of Hii regions across the BPTdiagram segregated based on the morphology of their host galax-ies. Each panel shows the Hii regions corresponding to galaxiesfrom earlier to later morphological types, divided in four groupswith similar number of galaxies each one: (i) the top left handpanel comprises those regions found in E/S0 and Sa galaxies(we found Hii regions in only one galaxy classified as Elliptical,however, 2/3 of the classifiers selected them as S0 in Walcher etal. 2014); (ii) the top right hand panel comprises those regionsfound in Sb galaxies; (iii) the bottom left hand panel comprisesthose regions found in Sbc galaxies; and finally, (iv) the bottomright hand panel comprises those regions found in Sc/Sd and ir-regular galaxies.

There is a clear pattern in the distribution of Hii regionsacross the BPT diagram depending on the morphology of thehost galaxies. Early-type galaxies have Hii regions located inthe right-end of the distribution. For those galaxies the numberof regions within theintermediatearea is much larger (∼50%for E/S0 and Sa galaxies). Late-type galaxies have Hii re-gions located along the classical sequence of these regions(e.g.Osterbrock 1989), with a clear trend towards more regions intheupper-left end of the distribution the later the type of the galaxy.

A priori, we did not expect a connection between the locationin the BPT diagram that is related to the ionization conditions ofthe nebulae, and the morphological type of the host galaxies,since Hii regions are short-lived phenomena. This result indi-cates that it is not possible togenerateany kind of Hii region inany morphological type. I.e., there is a connection betweenthefeasibleionization conditions in a nebulae and the morphologi-cal type of the host galaxy.

3.4. Distribution by galactocentric distance

In the previous section we explored the distribution of the Hii re-gions across the BPT diagram based on the morphological typeof the host galaxies. We found that galaxies of different mor-phologies show different distributions of their star-forming re-gions across the BPT diagram. In this section we explore thedistribution of the Hii regions based on their galactocentric dis-tances.

To do so, we use the galactocentric distances normalized tothe effective radius presented in Sánchez et al. (2013). Those dis-tances have been derived from the catalog of Hii regions pro-vided by HIIexplorer (Sánchez et al. 2012b), corrected for theestimated inclination of the galaxy, and normalized to the diskeffective radius (see Appendix of Sánchez et al. 2013).

Figure 5 shows the distribution of Hii regions across theBPT diagram segregated based on their galactocentric distances.Each panel shows the Hii regions corresponding to galaxiesfrom smaller to larger galactocentric distances, divided in tofour groups with similar number of regions each one: (i) thetop left hand panel comprises those regions atR/Re <0.5; (ii)the top right hand panel comprises those regions found between0.5< R/Re <1 ; (iii) the bottom left hand panel comprises thoseregions found between 1< R/Re<2; and finally, (iv) the bottomright hand panel comprises those regions found at distance largerthanR/Re >2.

There is a trend similar to the one described for the morpho-logical type. The Hii regions in the central areas of the galax-ies are more frequently located in the right-end of the distribu-tion, with a larger fraction of them within the intermediatere-gion. On the other hand, the Hii regions in the outer regions fol-lows a distribution more consistent with the classical trend (e.g.Osterbrock 1989). This is somewhat expected, since classicalH ii regions were observed in the disk of late-type spiral galaxies(e.g. Diaz 1989). However, as indicated before, Kennicutt et al.(1989) and Ho et al. (1997) recognized that Hii regions in thecenter of galaxies distinguish themselves spectroscopically fromthose in the disk by their stronger low-ionization forbidden emis-sion lines. Initially it was thought that this may be due to con-tamination by an extra source of ionization, such as diffuse emis-sion or the presence of an AGN. However, Sánchez et al. (2014)found that these regions are located across the entire optical ex-tension of the galaxies, and most probably they are the conse-quence of a nitrogen enhancement due to a natural aging processof H ii regions and the surrounding ISM.

3.5. Distribution by the properties of the underlying stellarpopulation

In the two previous sections we explored the distribution ofthe Hii regions across the BPT diagram based on the morphol-ogy of the host galaxy and the galactocentric distance of theconsidered region. We found that regions located in the cen-tral areas of galaxies, and in earlier type galaxies are locatedin right/lower-right side of the distribution, while those regionslocated at larger galactocentric distances and in latter type galax-ies are more frequently located towards the upper-left sideof thedistribution. Since the stellar population of the center ofdiskgalaxies present similarities with that of early type galaxies (e.g.González Delgado et al. 2014b), the described trends may indi-cate a connection between the location with the BPT diagramand the properties of the underlying stellar population.

We derive the luminosity weighted ages and metallicities ofthe underlying stellar population for each Hii region, adoptingthe results from the analysis described in Sánchez et al. (2013)and Sánchez et al. (2014), summarized before.

The left hand panel of Figure 6 shows the distribution ofH ii regions across the BPT diagram with a color code repre-senting the age of the underlying stellar population. Thosere-gions with older underlying stellar population are locatedinthe right/lower-right end of the distribution, while those withyounger underlying stellar populations are located in the upper-left side one. We should note here that we are not talking aboutthe age of the ionizing population, the derivation of which wouldrequire a more detailed analysis (e.g. Miralles-Caballeroet al.2014), but the complete stellar population of each Hii region.

The right hand panel of Fig. 6 shows the distribution ofH ii regions across the BPT diagram with a color code repre-senting the metallicity of the underlying stellar population, inunits of [Z/H] (=log(Z/Z⊙), where Z⊙ =0.02). Those regionswith metal rich underlying stellar populations are locatedin theright/lower-right end of the distribution, while the metal poorones are in the upper-left side one. Despite the larger scatterthis trend is very similar to the one presented in Fig. 2 forthe oxygen abundance, indicating a correspondence betweentheoxygen abundance and the stellar metallicity of the underly-ing population. This correspondence has been recently describedby González Delgado et al. (2014a) and Sanchez-Blazquez et al.(2014), in two independent analyses of the same data.

9

S. F. Sánchez et al.: Imprints of galaxy evolution on Hii regions

log([NII] λ6583/Hα)

log(

[OIII

]λ50

07/H

β)

-1 -0.5 0

-1

-0.5

0

0.5

E/S0/Sa

log([NII] λ6583/Hα)

log(

[OIII

]λ50

07/H

β)

-1 -0.5 0

-1

-0.5

0

0.5

Sb

log([NII] λ6583/Hα)

log(

[OIII

]λ50

07/H

β)

-1 -0.5 0

-1

-0.5

0

0.5

Sbc

log([NII] λ6583/Hα)

log(

[OIII

]λ50

07/H

β)

-1 -0.5 0

-1

-0.5

0

0.5

Sc/Sd/Irr

Fig. 4. [O iii] λ5007/Hβ vs. [N ii] λ6583/Hα diagnostic diagram for the∼5000 Hii regions described in the text. The contoursshow the density distribution of these regions with the diagram plane, with the outermost contour enclosing 95% of the regions,and each consecutive one enclosing 20% fewer regions. In each panel, Hii regions corresponding to different galaxies with differentmorphological types are represented: (i) E/S0/Sa top left, (ii) Sb top right, (iii) Sbc bottom left, and (iv)Sc/Sd/Irr bottom right panel.In all panels, the dot-dashed and dashed lines represent, respectively, the demarcation curves described in Fig.1

We should remind again that these representations of the dataare dominated by the average properties and they highlight thedominant trends. For example, the left-to-right gradient of theage appreciated in the left hand panel of Figure 6 should not beinterpreted as a lack of dependence of [Oiii] with this parameter.The correct interpretation is that the dependence of [Nii] withthe age is much stronger, as we will see in the next section.

3.6. Correlations with the properties of the underlying stellarpopulation

The trends described in the previous section across the BPT di-agram with the properties of the underlying stellar populationpoint towards some kind of relationship between them and theline ratios shown in the diagram. We explored these relations inthe current section.

The left hand panel of Fig. 7 shows the distribution of the[N ii]/Hα line ratio along the luminosity weighted age of theunderlying stellar population for the Hii regions shown in theprevious figures, with the stellar metallicity representedwith acolor code. There is a clear correlation between both parame-ters, with a correlation coefficient of r = 0.58. This correlationdepends on the abundance, being stronger for the higher metal-licities (r = 0.70 for the super-solar metallicities). In general, theolder and more metal rich the underlying stellar population, thehigher the value of the [Nii]/Hα line ratio.

In order to illustrate more clearly the described correlationwe have split the data into bins of increasing metallicity, with awidth of 0.1 dex in [Z/H] for each one, and derived the meanvalue of the line ratio for different ages, separated by 0.25 dexin the logarithm of the age. The result is shown in Fig. 7, wherethe distribution of the mean line ratios (coloured circles)as afunction of the ages for each stellar metallicity are presented.

10

S. F. Sánchez et al.: Imprints of galaxy evolution on Hii regions

log([NII] λ6583/Hα)

log(

[OIII

]λ50

07/H

β)

-1 -0.5 0

-1

-0.5

0

0.5

0<R/Re<0.5

log([NII] λ6583/Hα)

log(

[OIII

]λ50

07/H

β)

-1 -0.5 0

-1

-0.5

0

0.5

0.5<R/Re<1.0

log([NII] λ6583/Hα)

log(

[OIII

]λ50

07/H

β)

-1 -0.5 0

-1

-0.5

0

0.5

1.0<R/Re<2.0

log([NII] λ6583/Hα)

log(

[OIII

]λ50

07/H

β)

-1 -0.5 0

-1

-0.5

0

0.5

2.0<R/Re<4.5

Fig. 5. [O iii] λ5007/Hβ vs. [N ii] λ6583/Hα diagnostic diagram for the∼5000 Hii regions described in the text. The contours showthe density distribution of these regions with the diagram plane, with the outermost contour enclosing 95% of the regions, andeach consecutive one enclosing 20% fewer regions. In each panel shows the Hii regions corresponding to different galactocentricdistances, normalized to the effective radius: (i)R/Re<0.5 top left, (ii) 0.5< R/Re<1.0 top right, (iii) 1.0< R/Re<2.0 bottom left,and (iv)R/Re>2.0 bottom right panel. In all panels, the dot-dashed and dashed-lines represent, respectively, the demarcation curvesdescribed in Fig.1

The described depedence of the line ratio on both parametersisclearly seen.

Thus, to first order the [Nii]/Hα line ratio could be describedas a linear combination of both the luminosity weighted agesandmetallicities of the underlying stellar population, for which thebest fitting linear regression is

log([N ii]Hα

) = 0.24 log(age/yr)+ 0.34 [Z/H] − 2.67 (3)

After correcting for this dependency, the standard deviationof the line ratio is reduced by∼70%. Indeed, the dispersionaround the best fitted surface of∼0.14 dex is similar to the meanerror of the considered line ratio∼0.13 dex, compared with theinitial dispersion (∼0.20 dex). This indicates that the underlyingstellar population dominates the definition of the [Nii]/Hα line

ratio, and any other physical conditions of the nebulae seemtohave little effect on it.

The right hand panel of Figure 7 shows the distribution ofthe [Oiii]/Hβ line ratio along the luminosity weighted age of theunderlying stellar population in our Hii region sample, with thestellar metallicity represented with a color code. The correla-tion between both parameters is weaker than the one found for[N ii]/Hα, with a correlation coefficient of r = 0.42. As in theprevious case, there is a dependency of this correlation with theabundance, being stronger for the higher metallicities (r = 0.60for the super-solar metallicities). The double dependenceis moreevident when it is applied using the binning procedure for metal-licity ranges and ages described before for [Nii]/Hα (solid cir-cles and lines in Fig. 7).

In general, the younger and more metal poor the underlyingstellar population, the higher the value of the [Oiii]/Hβ line ra-tio (contrary to the expected for the ionizing stellar population).

11

S. F. Sánchez et al.: Imprints of galaxy evolution on Hii regions

log([NII] λ6583/Hα)

log(

[OIII

]λ50

07/H

β)

-1.5 -1 -0.5 0 0.5

-1-0

.50

0.5

1

8.5

99.

5

log(age/yr)

log([NII] λ6583/Hα)

log(

[OIII

]λ50

07/H

β)

-1.5 -1 -0.5 0 0.5

-1-0

.50

0.5

1

-0.4

-0.2

0[Z/H]

Fig. 6. [O iii] λ5007/Hβ vs. [Nii] λ6583/Hα diagnostic diagram for the∼5000 Hii regions included in our sample. The contoursshow the density distribution of these regions within the diagram plane, with the outermost contour enclosing 95% of theregions,and each consecutive one enclosing 20% fewer regions. In each panel the color indicates the average value at the correspondinglocation in the diagram of the luminosity weighted age (left panel) and metallicity (right panel) of the underlying stellar population,respectively. The grey solid-lines represent the expectedline ratios derived from the photoionization models shown in Fig. 1, whereeach line corresponds to a different stellar metallicity (oragen-to-red solid lines in Fig. 1). In both panels, the dot-dashed and dashedline represent, respectively, the demarcation curves described in Fig.1

The best fitting linear regression between this line ratio and theproperties of the underlying stellar population is described bythe formula

log([O iii]

Hβ) = −0.25 log(age/yr)− 0.41 [Z/H] + 2.11 (4)

After correcting for this dependency the standard deviationof the distribution of [Oiii]/Hβ decreases only by∼55%. Forthis line ratio the dispersion around the best fitted surface(∼0.25dex) is considerably larger than the mean error of the consideredline ratio (∼0.13 dex). In contrast with [Nii]/Hα, it seems thatthe value of [Oiii]/Hβ does not depend only on the underlyingstellar population. Rather, it seems to be sensitive to additionalphysical conditions in the nebulae, such as the age of the ionizingpopulation, the electron density, the mass of the ionizing cluster,and the geometry of the ionized gas (e.g. Garcia-Vargas et al.1995a).

We repeated all the analysis using the luminosity weightedages and metallicities derived using STARLIGHT, as describedin González Delgado et al. (2014b) for the galaxies in common(∼60% of the sample). These values are derived from a more de-tailed analysis of the underlying stellar population, using a largerand more suitable SSP library. We derive very similar results.The trend with the age of the underlying stellar population isexactly the same, while the trend with metallicity seems to beslighly weaker. However, this later result seems to be the effectof the smaller statiscal number of points rather than a real differ-ence.

4. Discussion

So far explored the distribution of Hii regions across the BPT di-agram based on (i) the expected distributions derived from pho-

toionization models, (ii) an estimated set of parameters defin-ing the ionization conditions, (iii) the morphology of the hostgalaxies, (iv) the galactocentric distances of the Hii regions, and(v) the main properties of the underlying stellar population. Wefound that the location of an Hii region is not uniquely definedby a single set of parameters for any set of photoionization mod-els. I.e., their location in the BPT diagram could be reproducedby the models using different ionization conditions, with a cleardegenerancy between the ionization parameter, log(u) and theoxygen abundance. On the other hand, not all the possible lo-cations predicted by photoionization models are covered bytheobserved Hii regions (i.e., there are plausible combinations ofphysical parameters in Hii regions which can be empirically ex-cluded on the basis of the derived BPT diagrams). In summary,from all the feasible hyper-space of line ratios predicted by pho-toionization models, real Hii regions cover a reduced region inthis space, which is not predicted by the models.

Indeed, this result is not new. Other authors have previouslynoticed that from all the line ratios predicted by photoioniza-tion models, observed Hii regions cover only a particular setof them (e.g. Dopita et al. 2000; Kewley et al. 2001). However,our larger statistical sample ensures that we can cover a muchwider range of ionization conditions, and the results stillholds.Thus, there are ionization conditions described by photoioniza-tion models that are not found in real Hii regions. This basicallymeans that the observed Hii regions are restricted to a particularset of parameters among those ones explored by photoionizationmodels.

Furthermore, Hii regions do not only cover a particular re-gion in the hyper-space of possible ionization conditions.Wefound that there are clear trends across the BPT diagram depend-ing on the parameters defining the ionization. Hii regions in theupper-left side of the diagram are more metal poor, with higherionization strengths, and less dust attenuation that thoseregionsin the right/lower-right end of the diagram. Again, we are not the

12

S. F. Sánchez et al.: Imprints of galaxy evolution on Hii regions

first to notice this effect. Classical studies already found similartrends for more reduced number of regions and/or a fewer num-ber of galaxies Evans & Dopita (1985); Dopita & Evans (1986).More recently, (e.g. Bresolin et al. 2012) described the sametrend with the ionization parameter across the BPT diagram,for their sample of Hii regions. Indeed, these trends are be-hind some of the well-known empirical calibrators betweenthe oxygen abundance and the strong-line ratios, like O3N2(Alloin et al. 1979; Pettini & Pagel 2004; Stasinska et al. 2006;Marino et al. 2013) or N2 (Pettini & Pagel 2004; Marino et al.2013). Dopita & Evans (1986) already found that all the abun-dance calibrators based on strong line indicators break thedegenerancy between the ionization parameter and the abun-dance existing in photoionization models based on the existenceof an empirical correlation between both parameters. Indeed,Pérez-Montero (2014) have recently demonstrated that in theabsence of this relation it is not possible to derive a sensitiveabundance based on any combination of strong emission line ra-tios, even for the largest dataset of Hii regions with direct abun-dance estimations. The correlation describred by Pérez-Montero(2014) is very similar to the one presented in Fig. 3.

Several explanations have been proposed for the relationbetween the oxygen abundance and the ionization parameter:(i) a correlation between the IMF and the metallicity, and/orthe ionization parameter (Dopita et al. 2006b) (ii) the correla-tion is the result of the dust absorption, and (iii) there areen-vironmental effects which change the geometry of the Hii re-gions at higher metallicities. The first possibility was discussedby Evans & Dopita (1985), without deriving a conclusive re-sult. The latter two were discussed by Dopita & Evans (1986),who considered that the dust extinction required to producetheobserved correlation should be larger than the observed one.Finally, they considered that the third may be possible if thecorrelation is related to a radial gradient in the density oftheH ii regions, with inner regions being more dense. However, aswe have show in Fig. 2 and Fig. 5 the electron density does notseem to present an obvious radial distribution and it does notpresent a clear correlation with the other two parameters.

A possible alternative explanation to this relation could bethat for a given age, the ionizing stellar population becomesredder for higher metallicities. First, as the metallicityof thestars increases, so does the envelope opacity, causing a drop intheir effective temperatures and a shift of flux towards redderwavelengths (e.g. Pagel & Edmunds 1981). Second, the ultravi-olet emission is strongly influenced by metal line blanketing inthe stellar atmospheres (e.g. Lanz & Hubeny 2003). Its effect isnot as straight-forward to understand as the one of the opacity.Mokiem et al. (2004) found that the total number of Lyman con-tinuum photons emitted is almost independent of line blanketingeffects and metallicity for a given effective temperature. This isbecause the flux that is blocked by the forest of metal lines atwavelengths shorter than 600 Angstrom is redistributed mainlywithin the Lyman continuum. However, they also found thatblanketing produces a significant decrease in the effective tem-perature, as previously reported by Martins et al. (2002). Thusthe net effect is a decrease of the Lyman photons with the metal-licity (e.g. Lanz & Hubeny 2003).

Indeed, evolutionary models for giant Hii regions describea decrease of the ionization parameter with the metallicityfor agiven age and mass of the ionizing cluster qualitatively similarto the one described here (Garcia-Vargas et al. 1995a,b). The de-tailed dependency between both parameters is different for eachage and mass of the ionizing cluster, as can be derived fromthe trends between emission lines shown in Martín-Manjón etal.

(2010), and therefore a considerable scatter is expected inthe de-scribed relation (as observed in Fig. 3).

Finally, the correlation may be induced by a totally differentmechanism. (Dopita et al. 2006a) proposed a scenario in whichthe expansion and internal pressure of the ionized cloud is drivenby the net input of mechanical energy from the central cluster(due to winds of supernove events). Those models explain natu-rally the anticorrelation of ionization parameter with theoxygenabundance: (i) The stellar wind has a higher opacity for moremetal rich clouds, and therefore absorbs a greater fractionof theionizing photons, reducing log(u) in the surrounding Hii region;(ii) the atmosphere scatters the photons emitted from the pho-tosphere more efficiently when the atmospheric abundances arehigher, leading to a greater conversion efficiency from luminousenergy flux to mechanical energy flux in the stellar wind base re-gion. This also leads to a diminution of log(u) in the surroundingH ii region.

The distribution shown in Fig. 4 is connected to the oneshown in Fig. 2, upper left hand panel, via theM-Z relation(Tremonti et al. 2004; Sánchez et al. 2013). Early-type galaxiesare, in general, more massive, and therefore more metal richthanlate-type ones. Therefore their Hii regions are located towardsthe right/lower-right end of the distribution. On the other hand,the Hii regions of latter-type galaxies, more metal poor, are lo-cated towards the left/upper-left side. The distribution shown inFig. 5 can be explained based on the well known inverse galacto-centric abundance gradient, that seems to be present in all galaxytypes (e.g. Sánchez et al. 2012b, 2014, and references therein).Indeed, both distributions can be also explained based onΣ-Zrelation (Rosales-Ortega et al. 2012; Sánchez et al. 2013),sincethe central regions of galaxies present higher stellar massden-sities and therefore larger oxygen abundances. Thus, the loca-tion of H ii regions within the BPT diagram changes with themorphology and the galactocentric distance due to the describedtrend between this location and the oxygen abundance.

The trend found with the dust attenuation was not a con-sequence of the dependence of the line ratios with dust atten-uation, and it was not predicted by the adopted photoioniza-tion models. By construction the location within the BPT di-agram should be indepedent of this parameter (Fig. 2, bottomright hand panel). Based on the previous results this trend indi-cates that there should be an inverse gradient of the dust atten-uation with the galactocentric distance and a relation withthestellar mass density, similar to the ones described for the oxy-gen abundance. To our knowledge these trends have not beenreported. In Sánchez et al. (2012b) we found only a very weakinverse galactocentric gradient of the dust attenuation for a re-duced sample of face-on spiral galaxies. However, that samplewas dominated by Sc/Sd galaxies, which may have affected theresults. Similar results for individual objects were presented byLópez-Sánchez et al. (2004), López-Sánchez et al. (2006) andMonreal-Ibero et al. (2010). A possible explanation to thistrendcould arise from the known relation between the stellar dustat-tenuation and the Hi gas surface density (Bohlin et al. 1978).Since gas and stellar attenuations present a tight relation(e.g.Calzetti et al. 1994; Calzetti 2001), there should a correlation be-tween the dust attenuation in the gas and the gas mass density,as recently highlighted by Brinchmann et al. (2013a). Sincethegas surface density increases towards the center of spiral galax-ies (e.g. Young & Scoville 1991), it is expected that there will bea corresponding increase ofAV, gasin the inner regions. On theother hand, this trend could be a consequence of the relationbe-tween metals and the dust grains, and therefore, with dust atten-uation. Due to this relation the dust-to-gas ratio increases with

13

S. F. Sánchez et al.: Imprints of galaxy evolution on Hii regions

log(age/yr)

log(

[NII]

λ658

3/Hα

)

8 8.5 9 9.5 10-1.5

-1-0

.50

[Z/H]

-0.55-0.45-0.35-0.25-0.15-0.05+0.05+0.15

log(age/yr)

log(

[OIII

]λ50

07/H

β)

8 8.5 9 9.5 10

-1-0

.50

0.5

1 [Z/H]

-0.55-0.45-0.35-0.25-0.15-0.05+0.05+0.15

Fig. 7. Left panel:Distribution of the [Nii] λ6583/Hα line ratio along the luminosity weighted ages of the underlying stellarpopulation for the∼5000 Hii regions included in our sample (individual dots), with a color code representing the luminosityweighted metallicity of the underlying stellar population. The solid circles and individual lines illustrate how the considered ratiochange along the stellar population age for a certain metallicity range. Each circle represents the average line ratio within a bin of0.25 dex in log(age/yr) for a fixed range of metallicities (within a range of 0.1 dex). Consecutive circles for a particular metallicityare linked with a solid line, and both of them are representedwith a fixed color, different for each metallicity. The actual value of themetallicity corresponding to each color is indicated in theenhanced box.Right panel:Similar distribution for the [Oiii] λ5007/Hβline. Symbols, solid lines and color codes are the same as theone presented in the previous panel.

metallicity, as nicely summarized by Brinchmann et al. (2013b).Indeed, for the Milky Way is∼3 times larger than for the LargeMagellanic Cloud and∼1.5 times larger than for the SmallMagellanic Cloud, as direct consequence of the differences inmetallicity between these three galaxies (e.g Bohlin et al.1978;Fitzpatrick 1986; Rodrigues et al. 1997). In this case the galacto-centric decrease of the abundance may induce a similar gradientfor the dust attenuation.

Finally, the trend described for the electron density seemstobe explained naturally by photoionization models (Fig. 2, bot-tom left hand panel). Indeed, Kewley et al. (2013) has nicelyillustrated the effect recently (e.g., their Fig. 2). Electron den-sity is a proxy of the ionized gas density, which is susceptible tomodifications due to different dynamical mechanisms that mod-ified the pressure. In an isobaric density distribution, thedensityis defined in terms of the ratio of the mean ISM pressure,P,and the mean electron temperature,Te, throughne = P/Tek.For the typical electron temperature of ionized gas the den-sity is simply determined by the ISM pressure (Kewley et al.2013). Some authors have described a radial trend of the elec-tron density in spiral galaxies (e.g. Gutiérrez & Beckman 2010;Beckman et al. 2013), although it has not been tested in a statisti-cally significant large sample of galaxies. It is also expected thatthe electron density increases in the wave front of spiral arms(e.g. Gutiérrez & Beckman 2010), and due to external effects,like galaxy interaction (e.g. Krabbe et al. 2014). Highly activestar-forming galaxies, like ULIRGs, and high redshift ones, bothwith dense ionized gas are known to be located in the upper-endof the classical location for Hii regions (e.g., Brinchmann et al.2008; Lehnert et al. 2009; Yuan et al. 2010; Dopita et al. 2014),in agreement with our result. Finally, the electron densitycouldevolve with the age of the ionizing cluster, if we assume thatH ii region is evolving as a mass-loss bubble pressurized by thecombined ram pressure of the stellar wind and supernova explo-

sions (Dopita et al. 2014), as decribed before. In this scenariothe density and the radius of the Hii regions are closely coupled,and both of them would depend on the age of the ionizing clus-ter. Under this assumptions we expect a variation of the electrondensity across the optical extension of the galaxies.

All these results indicate that the location within the BPTdiagram is tightly related to properties of the underlying stel-lar population. Indeed, Fig. 7, shows that both line ratios in-volved in the BPT diagram present clear correlations with theage and metallicity of the stellar populations. In particular, the[N ii] λ6583/Hα value is totally defined by both parameterswithin the errors. This is somewhat expected since this lineratiois a proxy of the oxygen abundance (e.g. Marino et al. 2013).Since the gas is polluted by the last population of stars, andassuming that the radial mixing is not too strong− which canbe assumed based on the results by Sánchez et al. (2014) andSanchez-Blazquez et al. (2014)−, the oxygen abundance has tobe correlated with the metallicity of the younger population ofstars. Indeed, the gas abundance should be the envelope of thedistribution of metallicities within the underlying stellar popu-lation (as recently found by González-Delgado et al., 2014b).Therefore, a correlation between the abundance and the metal-licity is expected for two reasons: both the ionizing stars,theyoungest ones, should be more metal rich, and the ionized gasshould be polluted by the last generation of metal rich stars.

The correlation with the age is a consequence of the chemi-cal enrichment in galaxies. We know that disk-dominated galax-ies grow inside out, with the older and more metal rich stellarpopulations living in the central regions, where the stellar massdensity is larger (e.g. Pérez et al. 2013; González Delgado et al.2014b; Sanchez-Blazquez et al. 2014). Therefore, a correlationbetween the oxygen abundance of the Hii regions and the ageof the underlying stellar population is expected, and thereforebetween this latter parameter and [Nii] λ6583/Hα.

14

S. F. Sánchez et al.: Imprints of galaxy evolution on Hii regions

Finally, for [Oiii] λ5007/Hβ, it is also expected that the cor-relation is weaker. On one hand this line ratio is less sensitiveto the metallicity. The excitation of O++ requires a much higherenergy than that of N+, and therefore is more sensitive to vari-ations in the geometry of the nebulae, the gas density, and theproperties of the ionizing population. Although this also has animprint of the local chemical evolution of the galaxy, due toitsmetal content, its evolution affects this line ratio more than theformer one. Therefore, the location within the BPT diagram islargely explained by the chemical enrichment and star-formationhistory, but not totally.

Throughout this article we have explored the dependence ofthe location of Hii regions within the BPT diagram with theproperties of the underlying stellar population. We selected theBPT diagram as the archetype of the diagnostics diagrams, onone hand, and because the line ratios involved are less sensitiveto dust attenuation. However, the main results described inthisarticle would be similar if other diagnostic diagrams are used.In Appendix A, we illustrate how the distribution of Hii regionsacross other broadly used diagnostic diagrams depends on theproperties of the underlying stellar population. We prefernot topresent them in the main text for clarity.

5. Summary and Conclusions

In this study we have presented the distribution across the BPTdiagram for the largest catalog of Hii regions with spectroscopicinformation currently available (Sánchez et al. 2013, 2014). Wecompared their location in this diagram to that predicted byphoto-ionization models, finding that (i) Hii regions do not coverthe entire range of parameters predicted by these models, and (ii)the same location within the diagram can be reproduced by sev-eral combinations of ionization parameters and photo-ionizationmodels, in many cases.

We have shown that these parameters (ionization parameter,oxygen abundance, electron density and dust attenuation) fol-low well defined trends across the BPT diagram. Only the trenddescribed for the electron density can be explained directly byphoto-ionization models. The remaining trends must have a dif-ferent origin. Indeed, there is clear correlation between the ion-ization parameter and the oxygen abundance that cannot be ex-plained based on photoinization models. Different explanationsfor this correlation, extracted from the literature, have been dis-cussed. We propose an alternative one, in which the relationis aconsequence of the increase of the metal line blanketing andtheenvelope opacity of the ionizing stars with the metallicity.

We found that the location of Hii regions within the BPTdiagram depends to first order on the morphology of the hostgalaxy, the galactocentric distance, and ultimately, the proper-ties of the underlying stellar population (age and metallicity).The reason for these empirical trends are related to the chemi-cal evolution in galaxies. Oxygen abundance is a tracer of stellarmetallicity, and therefore a by-product of the star-formation his-tory and metal enrichment. Due to this, the metal sensitive lineratios are tighly correlated with the properties of the underlyingstellar population (i.e., the products of the SFH). Finally, due tothe correlation between the oxygen abundance and the ionizingparameter, those line ratios more sensitive to this latter physi-cal quantity depend on the properties of the underlying stellarpopulation too.

Despite the fact that Hii regions are short lived phenomena,their ionization conditions seem to be tightly related to the prop-erties of the underlying stellar population: i.e., they keep amem-

ory of the local star-formation history and metal enrichment atthe location where they are formed.

Acknowledgements.We thank Prf. Dr. M. Dopita for his comments and sugges-tions during the reviewing process that have help to improvesignificantly theinterpretation of our results and the ovarall manuscript.

SFS thanks the director of CEFCA, M. Moles, for his sincere support to thisproject.

This study makes uses of the data provided by the Calar Alto LegacyIntegral Field Area (CALIFA) survey (http://califa.caha.es/).

CALIFA is the first legacy survey being performed at Calar Alto. TheCALIFA collaboration would like to thank the IAA-CSIC and MPIA-MPG asmajor partners of the observatory, and CAHA itself, for the unique access totelescope time and support in manpower and infrastructures. The CALIFA col-laboration also thanks the CAHA staff for the dedication to this project.

Based on observations collected at the Centro Astronómico HispanoAlemán (CAHA) at Calar Alto, operated jointly by the Max-Planck-Institut fürAstronomie and the Instituto de Astrofísica de Andalucia (CSIC).

We thank theViabilidad , Diseño , Acceso y Mejorafunding program,ICTS-2009-10, for supporting the initial developement of this project.

S.F.S, M.A.M and L.S.M. thank thePlan Nacional de Investigación yDesarrollo funding programs, AYA2012-31935, of the SpanishMinisterio deEconomía y Competitividad, for the support given to this project.

S.F.S thanks the theRamón y Cajalproject RyC-2011-07590 of the spanishMinisterio de Economía y Competitividad, for the support giving to this project.

PP is supported by a FCT Investigator 2013 consolidation grant. He ac-knowledges support by the Fundação para a Ciência e a Tecnologia (FCT)under project FCOMP-01-0124-FEDER-029170 (Reference FCTPTDC/FIS-AST/3214/2012), funded by FCT-MEC (PIDDAC) and FEDER (COMPETE).

R.G.D , E.P., and R.G.B. thank thePlan Nacional de Investigación yDesarrollo funding program AYA2010-15081.

Support for LG is provided by the Ministry of Economy, Development,and Tourism’s Millennium Science Initiative through grantIC12009, awardedto The Millennium Institute of Astrophysics, MAS. LG acknowledges supportby CONICYT through FONDECYT grant 3140566.

R.A. Marino was also funded by the spanish programme of InternationalCampus of Excellence Moncloa (CEI).

A. M.-I. acknowledges support from Agence Nationale de la Recherchethrough the STILISM project (ANR-12-BS05-0016-02).

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Appendix A: Distribution of H ii regions acrossother diagnostic diagrams

This study has been focused on the study of the distribution ofH ii regions across the most widely used diagnostic diagram, theso-called BPT diagram (e.g., Fig. 1). In particular we have stud-ied the dependence of the location within this diagram and theproperties of the underlying stellar population (e.g., Fig. 6). TheBPT diagram has the advantage that it compares line ratios be-tween ions that are very near in their wavelengths, and thereforethey are equally affected by dust attenuation. Indeed, we haveillustrated that even for large amount of dust the location withinthe diagram is not significantly affected.

Throughout this article we have shown that indeed there isa tight correlation between the location within the BPT diagramand the properties of the underlying stellar population, and thisdependence is most probably a consequence of the chemical evo-lution of the galaxies.

In this Appendix we show that the imprint of the chem-ical evolution does not only affect the location within theBPT diagram. Indeed, in basically all the explored diagnos-tic diagrams we find clear trends related to the propertiesof the underlying stellar population. Figure A.1 shows fouradditional diagnostic diagrams, different than the BPT dia-gram, built based on the comparison of the following emission

16

S. F. Sánchez et al.: Imprints of galaxy evolution on Hii regions

line ratios: [Oiii] λ5007/Hβ, [N ii] λ6583/Hβ, [O ii] λ3727/Hβ,[S ii] λλ6717,31/Hβand [Oiii] λ5007/[O ii] λ3727. Each of theseline ratios are more sensitive to different properties of the ion-ized gas and the ionizing sources. It is beyond the scope ofthis study to describe in detail the particular dependencies ofeach one, which have been widely described in the literature(e.g. Veilleux & Osterbrock 1987; Dopita & Sutherland 1995;Cid Fernandes et al. 2010). However, regardless of which physi-cal properties are more sensible, Fig. A.1 shows that it is possibleto identify trends with the luminosity weighted age of the under-lying stellar population in the four diagrams. In some casesthetrend is stronger for one of the explored line ratios: e.g., in the[O iii] λ5007/Hβ vs. [Sii] λλ6717,31/Hβ diagram the stellar ageis more clearly associated with the first of the two parameters,while in the [Oiii] λ5007/[O ii] λ3727 vs. [Nii] λ6583/Hβ one,it is more clearly related to the second one. However, even inthecase of the diagnostic diagram comparing the two line ratioswiththe apparent weaker dependence with the age of the stellar pop-ulation ([Oiii] λ5007/[O ii] λ3727 vs. [Sii] λ6717,31/Hβ), thereare clear trends that depend on the two parameters at the sametime.

In summary, we have shown that the main conclusion of thecurrent study, i.e., that there is a tight relation between the prop-erties of the Hii regions and those of the underlying stellar pop-ulation, is independent of the diagnostic diagram selectedto ex-plore this depedence.

17

S. F. Sánchez et al.: Imprints of galaxy evolution on Hii regions

log([OII]λ3727/Hβ)

log(

[OIII

]λ50

07/H

β)

0 1 2

-1-0

.50

0.5

1

8.5

99.

5

log(age/yr)

log([SII]λ6717,31/Hα)

log(

[OIII

]λ50

07/H

β)

-1 -0.5 0 0.5

-1-0

.50

0.5

1

8.5

99.

5

log(age/yr)

log([NII] λ6583/Hα)

log(

[OIII

]λ50

07/[O

II]λ3

727)

-1.5 -1 -0.5 0 0.5

-10

8.5

99.

5

log(age/yr)

log([SII]λ6717,31/Hα)

log(

[OIII

]λ50

07/[O

II]λ3

727)

-1 -0.5 0 0.5

-10

8.5

99.

5

log(age/yr)

Fig. A.1. Distribution of the∼5000 Hii regions included in our sample across four classical diagnostic diagrams:top leftpanel, [O iii] λ5007/Hβ vs. [Oii] λ3727/Hβ; top right panel, [O iii] λ5007/Hβ vs. [Sii] λ6717,31/Hβ; bottom left panel,[O iii] λ5007/[O ii] λ3727 vs. [Nii] λ6583/Hβ andbottom right panel,[O iii] λ5007/[O ii] λ3727 vs. [Sii] λ6717,31/Hβ. In eachpanel, the contours show the density distribution of the Hii regions within considered diagram, with the outermost contour enclos-ing 95% of the regions, and each consecutive one enclosing 20% less regions. The color indicates the average luminosity weightedage of the underlying stellar population at the corresponding location in the diagram. The solid line represent the classical demar-cation line used to select Hii regions.

18