Star Formation on Galactic Scales - Princeton University
Transcript of Star Formation on Galactic Scales - Princeton University
![Page 1: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/1.jpg)
Star Formation on Galactic Scales
Petchara Pattarakijwanich
Princeton University
15 January 2013
![Page 2: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/2.jpg)
Outline
I Observation method overview.I How to measure star formation rate (SFR).I How to measure gas density in various phases.
I Observational results.I Disk-averaged.I Spatially-resolved.
I Theoretical efforts.
![Page 3: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/3.jpg)
Empirical Relations
I A few empirical relations are found to fit the data well.I ΣSFR = AΣN
gas (Kennicutt & Schmidt Law)
I ΣSFR = AΣgas
tdyn
I ΣSFR ∝ ηPtotal
I How do we measure these quantities?
![Page 4: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/4.jpg)
Measuring Gas Density
I Atomic Gas.I HI spin-flip hyperfine transition.I λ = 21cm, ν = 1.42GHzI MHI = 2.343× 105M�(1 + z)( DL
Mpc)2(R
Fνdv
Jy km s−1 )
I Molecular Gas.I H2 does not emit strongly.I Use other trace molecules, most commonly CO.I XCO = N(H2)R
TAdv= 1.58× 1020n
1/23 (e5.5K/Texc) cm−2
K km s−1
I Various issues:I CO line is optically thickI XCO varying.
![Page 5: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/5.jpg)
Why does H2 not emit strongly?
I Common interpretation:Lack of electric dipole.
I Actually: Small momentof inertia → Large energygap → Not populated.
I mass ∼ 15 times smaller,size ∼ 50% smaller.
I E = J(J+1)~2
2Iwhere
I = µr 2.
http://hyperphysics.phy-astr.gsu.ed
![Page 6: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/6.jpg)
Measuring Star Formation Rate
I Various ways, each have different issues and are sensitiveto different timescales.
I Star count.I Ultraviolet.I Emission lines.I Infrared.I X-ray.I Radio.
![Page 7: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/7.jpg)
Measuring Star Formation Rate: Star-count
I With complete data of individual stars in a group andgood stellar evolution model, one can model everything.
I 〈M∗〉 =Mu∑
M∗=Ml
N(M∗, t∗)M∗/t∗
I Problems:I Need very high quality data.I Resolution limit. Not possible outside local group.
![Page 8: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/8.jpg)
Measuring Star Formation Rate: UV & IR
I UVI Emitted by young stars.I Main database is GALEX.I Issue: Dust.
I IRI Emitted by dust cloud surrounding young starsI Various database (WISE, Spitzer, Herschel, Planck)I Issue: Lack of dust.
I Complementary to each other.
![Page 9: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/9.jpg)
Measuring Star Formation Rate: Emission Lines
I Emission lines are associated with HII regions.
I Commonly used lines:I Hα (Line of choice).I [OII]λ3727.I Lyα.I Paschen Series.I metal IR cooling lines.
I Problems:I Dust.I Calibration.
![Page 10: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/10.jpg)
Measuring Star Formation Rate: X-ray & Radio
I Both X-ray and radio emissions are associated with youngphase of stellar evolution.
I X-ray: X-ray binary, supernovae & remnants, youngstars.
I Radio: free-free from HII region, synchrotron fromsupernova remnants.
I Problem: AGNs also emit X-ray and radio wave.
![Page 11: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/11.jpg)
Measuring Star Formation Rate: Combination
Kennicutt & Evans (2012)
![Page 12: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/12.jpg)
Measuring Star Formation Rate: Calibration
Kennicutt & Evans (2012)
![Page 13: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/13.jpg)
Observables → Theoretical Quantities
Leroy et al (2008)
![Page 14: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/14.jpg)
Observational Results
I Apply these methods to measure real galaxies.
I Empirical correlations:I ΣSFR ∝ Σ1.4
gas (Kennicutt & Schmidt Law)
I ΣSFR ∝ Σgas
tdyn
I ΣSFR ∝ ηPtotal
I Constrains for theories and high-resolution simulations.
I Used as “subgrid” model for low-resolution simulations.
![Page 15: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/15.jpg)
Observational Results ReviewsI This starts with the prediction by Schmidt (1959) and
observation by Kennicutt (1998). The so-calledKennicutt-Schmidt Law.
Kennicutt (1998)
![Page 16: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/16.jpg)
More modern dataset
I This game can be playedwith large galaxy sample.
I Various types andselection methods.
I The correlation still holds.
Kennicutt & Evans (2012)
![Page 17: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/17.jpg)
Resolved KS law
I One can also do this withresolved data.
I Either point-by-pointbasis, or ring-averaged.
I Some complications onsmall scale.
Kennicutt & Evans (2012)
![Page 18: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/18.jpg)
Resolved KS law at high redshiftI Similar result for galaxies at redshift z ∼ 1 from recent
paper.
Freundlich et al (2013)
![Page 19: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/19.jpg)
Break in KS law at Σgas = 10M�pc−2
I Break from N ∼ 1.4 atΣgas ∼ 10M�pc−2
I Some papers argue thatthis is associated withgravitational instabilityscale. However this is notlikely the case consideringthe radial profile.
I This is more likely the signof second parameter.ΣSFR can take any valuefor Σgas ∼ 10M�pc−2
Leroy et al (2008)
![Page 20: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/20.jpg)
Break in KS law at Σgas = 10M�pc−2
I The correlation with Σmol
remains below 10M�pc−2.
I This indicates varyingfraction of molecular andatomic gas, as well as therelative importance of gasand stellar potential.
Leroy et al (2013)
![Page 21: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/21.jpg)
Theoretical aspects
I Challenges for Theorists:I Explain the form of
empirical relations.I Explain the generally
low SFR.I Naive expectation
would be ΣSFR ∼ Σgas
tffand this would lead to2 orders of magnitudehigher in SFR.
Kennicutt (1998)
![Page 22: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/22.jpg)
Stellar Feedback
I Sources of feedback:protostellar jets, stellarwinds, supernovae andradiation pressure fromyoung star.
I Momentum feedbackseems necessary, or theenergy is just radiatedaway easily.
I Hopkins et al (2011)implemented thismomentum feedback insimulation and showedthat it suppress SF. Hopkins, Quataert & Murray (2011)
![Page 23: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/23.jpg)
Stellar Feedback
Hopkins, Quataert & Murray (2011)
![Page 24: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/24.jpg)
Turbulence
I Krumholz & McKee (2005) proposed thatturbulence-regulated SF can work. This theory is derivedfrom first principle and is based on a few naturalassumptions.
I Clouds are virialized and supersonically turbulent.I Density distribution is log-normal.I SF happens in dense enough region where gravity is
stronger than turbulence.
I Predict the empirical relations found from observation.
I Does not specify the source of turbulence. Thisturbulence can be due to feedback.
![Page 25: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/25.jpg)
Turbulence
Krumholz & McKee (2005)
![Page 26: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/26.jpg)
Star Formation as Demand
I Traditional thinking: Star formation responds to supply.I Gas is the fuel, it collapses and forms stars.I Only a few percent of GMC mass is turned into stars.I Star formation is very inefficient.
I New way proposed by Ostriker, McKee & Leroy (2010):Star formation responds to demand.
I In order to keep ISM in equilibrium, energy andturbulence dissipated must be replenished. And pressuremust be provided to counter gravity.
I Star formation is responsible for these processes.I To sustain this with a few percent rate, star formation is
actually very efficient.
![Page 27: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/27.jpg)
Star Formation as Demand
I A few things have to bebalanced
I Heating vs. Energyloss via radiation.
I Momentum injectionvs. Turbulencedissipation.
I Pressure vs. Gravity.
Kim, Kim & Ostriker (2011)
![Page 28: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/28.jpg)
Conclusions
I One can learn a lot about galaxies with rightmeasurements.
I Star formation rate in galaxies follows certain empiricalrelationships.
I These observational results can be used as a test fortheories or input for simulations.
I Theories are being developed to explain theseobservations. None of them is complete yet. But withbetter dataset, simulations and theories the understandingof this process is advancing quickly.
![Page 29: Star Formation on Galactic Scales - Princeton University](https://reader033.fdocument.pub/reader033/viewer/2022042522/62621a67026af52afc0a688d/html5/thumbnails/29.jpg)
ReferenceI Bigiel et al (2008) “The Star Formation Law in Nearby Galaxies on Sub-Kpc
Scales”, AJ, 136, 2846
I Freundlich et al (2013) “Towards a resolved Kennicutt-Schmidt law at highredshift”, arXiv:1301.0628
I Hopkins, Quataert & Murray (2011) “Self-regulated star formation in galaxiesvia momentum input from massive stars”, MNRAS, 417, 950
I Kennicutt (1998) “The Global Schmidt Law in Star-forming Galaxies”, ApJ,498, 541
I Kennicutt & Evans (2012) “Star Formation in the Milky Way and NearbyGalaxies”, ARA&A, 50, 531
I Kim, Kim & Ostriker (2011) “Regulation of Star Formation Rates in MultiphaseGalactic Disks: Numerical Tests of the Thermal/Dynamical EquilibriumModel”, ApJ, 743, 25K
I Krumholz & McKee (2005) “A General Theory of Turbulence-regulated StarFormation, from Spirals to Ultraluminous Infrared Galaxies”, ApJ, 630, 250
I Leroy et al (2008) “The Star Formation Efficiency in Nearby Galaxies:Measuring Where Gas Forms Stars Effectively”, AJ, 136, 2782
I Leroy et al (2013) “Molecular Gas and Star Formation in Nearby DiskGalaxies”, arXiv:1301.2328
I Ostriker, McKee & Leroy (2010) “Regulation of Star Formation Rates inMultiphase Galactic Disks: A Thermal/Dynamical Equilibrium Model”, ApJ,721, 975O