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The Dark Age and Cosmology. Xuelei Chen ( 陈学雷 ) National Astronomical Observarories of China. The 2nd Sino-French Workshop on the Dark Universe, Aug 31st 2006, Beijing. z ~ 1000. z ~ 30. z ~ 6. z ~ 0. The Cosmic History. small perturbations. gravity. collapse to form dark halos. - PowerPoint PPT Presentation
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The Dark Age and Cosmology
Xuelei Chen (陈学雷 )
National Astronomical Observarories of China
The 2nd Sino-French Workshop on the Dark Universe, Aug 31st 2006, Beijing
The Cosmic Historyz ~ 1000 z ~ 30 z ~ 6 z ~ 0
First Stars and Reionization
small perturbations
collapse to form dark halos
gravity
gas accreate and cool
form stars and galaxy
Barkana & Loeb 2001
Observational Probes
• metal poor stars
• high redshift galaxies and quasars (IR, submm, radio)
• photometric drop-out technic
• high redshift GRB
• spectral lines: 21cm, Lyman alpha, H alpha, OI, OH...
• CMB
Hubble UDF
Lyman alpha Absorption
Fan et al 2006
Gunn-Peterson
neutral fraction
High Redshift Sources
Observation indicates that quasar density drops by a factor of 40 from z=2.5 to 6, they will not be sufficient to reionize Universe at z=6 if the shape of LF is similar to lower redshift. galaxies should be sufficient.
quasar galaxy
Reionization and CMB polarization
last scattering shell
reionization
To produce polarization, needs
• anisotropic radiation field• bulk velocity of electrons
Thompson scattering
WMAP Results
WMAP 1 year WMAP 3 year
WMAP results on Reionization
Simple Model of Reionization
Chen et al (2003)
Expansion of HII (ionized) region:
fstar predicted with structure formation theory
Decaying Dark Matter
Chen & Kamionkowski, 2004
If Dark Matter can decay, part of the energy is deposited in IGM and could ionize the gas
Annihilating Dark Matter
L. Zhang et al., astro-ph/0603425
If dark matter mass is 1-100 MeV instead of 100 GeV (neutralino), then the number density is much higher.Annihilation rate ~ n2
Boehm, Hooper & Silk 2003 Inconsistencies among e+e-, gamma-ray, CMB and Supernovae
The 21cm tomographic probe
Furlanetto, Sokasian, Hernquist 2003
Probe the reionization process with 21cm tomography (Madau, Meiksen & Rees
1997)
Foreground
X. Wang et al astro-ph/0501081
Ongoing 21cm projects
MWA21CMA/PAST
First generation dedicated projects: 21CMA, LOFAR, MWA• interferometers• clustered dipoles• typical baseline: a few km• collecting area: 104-5 m2
Using existing telescopes: VLA, GMRT
More to come: FAST, MWA5000, SKA
21CMA/PAST
FAST
The 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 Temperature
Ly
collision
Thermal systems:
spin
atomic motion
CMBLy
photons
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
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 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
Modulation 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
perturbation
Lyman alpha sphere around first starsfirst 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
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
The effect of heating
no heating with heating
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
The bubble model of reionization
For an isolated region, condition for ionization:
but
so
Zahn et al 2006
Bubble model
bubble size distribution
high redshift fluctuation
Barkana & Loeb 2005
density fluctuation during dark age
large scale spin-temperature variation induced by first galaxies
Barkana & Loeb 2004
Cosmology with 21 cm fluctuation
AP test peculiar velocity
• AP test• better measurement of power spectrum• but: ionization-induced fluctuation is astrophysical• higher redshift: difficult due to stronger foreground
Summary
• Current information come from: Ly alpha, high redshift galaxy, CMB• Future: 21cm
• Dark matter decay and annihilation during dark age
• Model of Reionization
• Cosmology with 21cm
Merci!