The 21cm signature of the First Stars

1
The 21cm signature of the First Stars

• Xuelei Chen
• ???
• National Astronomical Observatory of China

KIAS workshop on cosmology and structure
formation, Seoul, Korea, Sept 20, 2006
2
The Cosmic History
?
3
Hierachical structure formation, formation of
first objects, and reionization
Barkana Loeb 2001
4
21cm Historical Review
21cm 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
  Zeldovichs hot dark matter (neutrino) model
  (null)
• 1990 search for high redshift galaxy, loss of
  interest
• Madau, Meikesen, Rees (1997) probe reionization,
  revival of interest

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The 21cm tomographic probe
Probe the reionization process with 21cm
tomography (Madau, Meiksen Rees 1997)
Furlanetto, Sokasian, Hernquist 2003
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Ongoing 21cm projects
21CMA/PAST
MWA
Carilli 2005
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21CMA/PAST

• interferometers
• clustered dipoles
• typical baseline of a few km, collecting area
  105 m2

MWA prototype
LOFAR prototype
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The physics of 21cm line

• spontanous transition

n2

• collision induced transition

Ly ?

• CMB induced transition

n1

• Lyman series scattering
• (Wouthousian-Field mechanism)
  Ly ?

n0
F1
CMB
21cm
F0
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Spin Temperature
Thermal systems
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Lyman 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

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Modulation of 21cm signal

• density (cosmic web, minihalo)
• ionization fraction (galaxy, cosmic HII region)
• spin temperature
• temperature,
• density,
• Ly alpha flux

When Ts gtgt Tcmb emission saturates

• dark age density peculiar velocity
• first light density spin temperature
• reionization density ionization

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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

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Reionization Model
Expansion of ionized (HII) region
photon production rate calculate the bound
fraction of baryons in star forming halos
(MgtMmin), then assume each baryon produce a
number of photons
Evolution of ionization fraction
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The bubble model
Furlanetto et al 2004
For an isolated region, condition for ionization
but
so ionized bubble if
distribution of bubble
Zahn 2006
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Evolution of global spin temperature
zgt150 TkTcmbTs 50ltzlt150 TsTkltTcmb
collisional coupling 25ltzlt50 TkltTs Tcmb no
coupling 15ltzlt25 TkltTs ltTcmb Ly alpha
coupling 10ltzlt15 TkgtTs gtTcmb Ly alpha coupling
CMB
spin
gas
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The temperature of gas
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

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The absorption signatures

• zgt200, CMB and gas has about the same
  temperature, no 21cm signal
• 200gtzgt40, gas temperature lt CMB temperature,
  absorption modulated by density
• zlt40, before the presence of Lyman alpha
  background absorption, density modulation in
  mini-halos
• Lyman alpha modulation, temperature modulation,
  density modulation, ionization modulation...

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high redshift fluctuation
large scale spin-temperature variation induced by
first galaxies
density fluctuation during dark age
Barkana Loeb 2004
Barkana Loeb 2005
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Formation of the first stars
Star could form in a dark halo only if

• halo gravity exceeds gas pressure (Jeans mass)
• gas in the halo can cool
• molecule H cooling 102-3 K
• atomic cooling 104 K

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.
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Lyman alpha sphere around first stars
first stars 100 solar mass metal free star
radiating at Eddington limit (Bromm et al 2001)
Chen Miralda-Escude 2006

• 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

NOT TO SCALE
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Radiative Transfer
For stellar mass of 25, 50, 100, 200, 400, 800
Msun.
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continuum Lyman alpha photons

• Line photons confined to HII region (or where it
  is produced) by resonance scattering
• continuum photon decrease as r-2

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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
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Ly ? sphere profile
with only continuum Ly photons, weak absorption
with injected Lyman photons very strong absorption
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Formation Rate of first stars

• Minimal mass requirement
• Tvir gt 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
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Biase correlation function of halos (not stars)
PS
ST
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The effect of heating
no heating
with heating
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Cross 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

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Foreground
X. Wang et al astro-ph/0501081
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Observablity
measurement error
system temperature dominated by galactic
foreground
covering factor
signal to noise ratio
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Observablity
difficult to achieve high SNR require
almost-filled array
105
10 -5
1
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Signal To Noise Ratio
Assume z20, M400, t1.5 Myr, 1 year
intergration covering factor1
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Some Numbers
Very Challenging, far beyond the capability of
current generation 21cm experiments. But Not
Impossible!
baseline bandwidth beamwidth SNR
45 km 30 kHz 20 arcsec 5
65 km 30 kHz 14 arcsec 10
91 km 30 kHz 10 20
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Summary

• 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

35
Thanks
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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