The 21cm signature of the First Stars Xuelei Chen 陳學雷 National Astronomical Observatory of...
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Transcript of The 21cm signature of the First Stars Xuelei Chen 陳學雷 National Astronomical Observatory of...
The 21cm signature of the First Stars
The 21cm signature of the First Stars
Xuelei Chen陳學雷
National Astronomical Observatory of China
Xuelei Chen陳學雷
National Astronomical Observatory of China
KIAS workshop on cosmology and structure formation, Seoul, Korea, Sept 20, 2006
The Cosmic HistoryThe Cosmic History
z ~ 1000 z ~ 30 z ~ 6 z ~ 0
?
Hierachical structure formation, formation of first objects, and
reionization
Hierachical structure formation, formation of first objects, and
reionization
small perturbations
collapse to form dark halos
gravity
gas accreate and cool
form stars and galaxy
Barkana & Loeb 2001
21cm: Historical Review21cm: Historical Review21cm: hyperfine structure of neutral H atom
• van de Hulst (1945): theoretical prediction
• Ewen & Purcell; Muller & Oort (1951): detection in the Milky Way, discovery of spiral structure of Milky Way
• Field, ... (late 1950s): theoretical understanding of 21cm brightness determined by spin temperature and the role of Ly photons
• 1960, 1970s: observation of nearby galaxies, discovery of dark matter in galaxies, search for neutral InterGalactic Medium (null)
• 1980s: search for pancake clouds predicted by Zeldovich’s hot dark matter (neutrino) model (null)
• 1990: search for high redshift galaxy, loss of interest
• Madau, Meikesen, Rees (1997) probe reionization, revival of interest
The 21cm tomographic probeThe 21cm tomographic probe
Furlanetto, Sokasian, Hernquist 2003
Probe the reionization process with 21cm tomography (Madau, Meiksen & Rees
1997)
Ongoing 21cm projectsOngoing 21cm projects
Carilli 2005
MWA21CMA/PAST
21CMA/PAST
LOFAR prototypeMWA prototype
• interferometers• clustered dipoles• typical baseline of a few km, collecting area 105 m2
The physics of 21cm lineThe physics of 21cm line• spontanous transition
F=1
F=0
• Lyman series scattering (Wouthousian-Field mechanism) Ly
• collision induced transition
• CMB induced transition
CMB
n=0
n=1
21cm
Ly
n=2
Spin TemperatureSpin Temperature
Ly
collision
Thermal systems:
spin
atomic motion
CMBLy
photons
Lyman alpha photonsLyman alpha photons
• injected photons: photons emitted/scattered at Ly alpha frequency, produced by recombination
• Continuum photons: UV photon between Ly alpha and Ly beta, redshift to Ly alpha frequency
• Once enter Ly alpha frequency (Doppler core), resonant scattering & confined locally. At Ly alpha frequency, color temperature equals to kinetic temperature
• leaking by 2 photon process
• higher Lyman series
Modulation of 21cm signalModulation of 21cm signal
density (cosmic web, minihalo)
ionization fraction (galaxy, cosmic HII region)
spin temperature: temperature, density, Ly alpha flux
• dark age: density & peculiar velocity
• first light: density & spin temperature
• reionization: density & ionization
When Ts >> Tcmb: emission saturates
Models of 21cm Models of 21cm
• global edge: due to reionization or emission/absorption transition
• 21cm forest: HII region, cosmic web, minihalo
• tomography: bubble model of HII region
• tomography: density modulation due to cosmic web
• tomography: spin temperature due to collision (dark age)
• tomography: spin temperature due to Ly : large scale, individual quasar, galaxy, first star
Reionization ModelReionization Model
Expansion of ionized (HII) region
photon production rate: calculate the bound fraction of baryons instar forming halos (M>Mmin), then assume each baryon produce a number of photons
Evolution of ionization fraction
The bubble modelThe bubble model
For an isolated region, condition for ionization:
but
so ionized bubble if
Zahn 2006
Furlanetto et al 2004
distribution of bubble
Evolution of global spin temperature
Evolution of global spin temperature
CMB
gas
spin
star formation
z>150: Tk=Tcmb=Ts
50<z<150: Ts=Tk<Tcmb collisional coupling
25<z<50: Tk<Ts= Tcmb no coupling
15<z<25: Tk<Ts <Tcmb Ly alpha coupling
10<z<15: Tk>Ts >Tcmb Ly alpha coupling
The temperature of gasThe temperature of gas
spin temperature evolution
21cm brightness temperature
Chen & Miralda-Escude 2004
Heating of IGM:
• Shock
• ionizing radiation
• Lyman alpha? No (Madau, Meiksen, Rees 1997, Chen & Miralda-Escude 2004, Hirata 2006, chuzhoy & Shapiro 2006, Rybicki 2006, Meiksen 2006, Pritchard & Furlanetto 2006)
• X-ray
The absorption signaturesThe absorption signatures
• z>200, CMB and gas has about the same temperature, no 21cm signal
• 200>z>40, gas temperature < CMB temperature, absorption modulated by density
• z<40, before the presence of Lyman alpha background: absorption, density modulation in mini-halos
• Lyman alpha modulation, temperature modulation, density modulation, ionization modulation...
high redshift fluctuationhigh redshift fluctuation
Barkana & Loeb 2005
density fluctuation during dark age
large scale spin-temperature variation induced by first galaxies
Barkana & Loeb 2004
Formation of the first starsFormation of the first stars
• halo gravity exceeds gas pressure (Jeans mass)
• gas in the halo can cool
• molecule H cooling ~102-3 K
• atomic cooling ~ 104 K
Star could form in a dark halo only if
Simulations (Abel 2000, Bromm 2000) indicate first star may form in halos of 105-6 solar mass, one or a few per halo, with masses of a few hundred solar.
H
H2
Lyman alpha sphere around first stars
Lyman alpha sphere around first stars
first stars: 100 solar mass metal free star radiating at Eddington limit (Bromm et al 2001)
NOT TO SCALE
• life time of the star ~ 3 Myr, Hubble time ~ 108 yr
• light propagation time ~ size of Lya sphere (10 kpc) • halo virial radius ~ 0.1 kpc
Chen & Miralda-Escude 2006
Radiative TransferRadiative Transfer
For stellar mass of 25, 50, 100, 200, 400, 800 Msun.
continuum Lyman alpha photonscontinuum Lyman alpha photons
• Line photons confined to HII region (or where it is produced) by resonance scattering
• continuum photon: decrease as r-2
The secondary Lyman alpha photons
The secondary Lyman alpha photons
induction by X-ray photons: recombination, excitation, cascade
photon production rate ~ energy rate/E
frequency shift rate ~ H
flux ~ photon production rate / frequency shift rate:
Neglected order 1 correction factor and higher Lyman series
Ly sphere profileLy sphere profile
with injected Lyman photons very strong absorption
with only continuum Ly photons, weak absorption
Formation Rate of first stars
Formation Rate of first stars
• Minimal mass requirement:
Tvir > 2000 K
• One star formed per halo
• Star died after 3 Myr, so exist only in halos just formed
This breaks down at low redshift: (1) halo destruction (2) feed back
Biase & correlation function of halos (not stars)
Biase & correlation function of halos (not stars)
PS
ST
The effect of heatingThe effect of heating
no heating with heating
Cross Section MapCross Section Map
• Ly alpha background reduce contrast of Ly alpha sphere
• If gas heated above CMB, no absorption signal
• absorption signal much stronger than emission
ForegroundForeground
X. Wang et al astro-ph/0501081
ObservablityObservablity
measurement error
system temperature dominated by galactic foreground:
covering factor
signal to noise ratio
ObservablityObservablity
difficult to achieve high SNR: require almost-filled array
105 10 -51
Signal To Noise RatioSignal To Noise RatioAssume: z=20, M=400, t=1.5 Myr,1 year intergration covering factor=1
Some NumbersSome Numbers
baseline bandwidth beamwidth SNR
45 km 30 kHz 20 arcsec 5
65 km 30 kHz 14 arcsec 10
91 km 30 kHz 10 20
Very Challenging, far beyond the capability of current generation 21cm experiments.
But Not Impossible!
SummarySummary
• Redshifted 21cm observation provides powerful probe for dark age, first light, and reionization
• The 21cm signal is modulated by density, ionization fraction and spin temperature
• The spin temperature is determined by CMB temperature, kinetic temperature, density, and Ly background flux
• Models of 21cm fluctuation due to ionized region, density fluctuation, first star
• X-ray from first stars maybe important in heating and in creating “injected Lyman alpha photons”. This may be used to distinguish different models of first objects
ThanksThanks
Overlap of Lyman alpha spheres Overlap of Lyman alpha spheres
• cumulative number of halos within a certain distance
• caveat: may not form at the same time
• first star clusters: bigger Lyman alpha sphere
• the shape of Ly alpha sphere may be irregular due to nearby first star and density fluctuation