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!