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 History z ~ 1000z ~ 30z ~ 6z ~ 0 ?

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 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 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 probe Furlanetto, Sokasian, Hernquist 2003 Probe the reionization process with 21cm tomography ( Madau, Meiksen & Rees 1997 )

Ongoing 21cm projects Carilli 2005 MWA 21CMA/PAST

LOFAR prototype MWA prototype interferometers clustered dipoles typical baseline of a few km, collecting area 10 5 m 2

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 CMB Ly  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

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

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 Model Expansion of ionized (HII) region photon production rate: calculate the bound fraction of baryons in star forming halos (M>M min ), then assume each baryon produce a number of photons Evolution of ionization fraction

The 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 CMB gas spin star formation z>150: T k =T cmb =T s 50<z<150: T s =T k <T cmb collisional coupling 25<z<50: T k <T s = T cmb no coupling 15<z<25: T k <T s <T cmb Ly alpha coupling 10 T s >T cmb 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

The 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 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 stars halo gravity exceeds gas pressure (Jeans mass) gas in the halo can cool molecule H cooling ~ K atomic cooling ~ 10 4 K Star could form in a dark halo only if Simulations (Abel 2000, Bromm 2000) indicate first star may form in halos of solar mass, one or a few per halo, with masses of a few hundred solar. H H2H2

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 ~ 10 8 yr light propagation time ~ size of Lya sphere (10 kpc) halo virial radius ~ 0.1 kpc Chen & Miralda-Escude 2006

Radiative Transfer For stellar mass of 25, 50, 100, 200, 400, 800 Msun.

continuum 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 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 profile with injected Lyman photons very strong absorption with only continuum Ly photons, weak absorption

Formation Rate of first stars Minimal mass requirement: T vir > 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) PS ST

The effect of heating no heatingwith 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

Foreground X. Wang et al astro-ph/

Observablity measurement error system temperature dominated by galactic foreground: covering factor signal to noise ratio

Observablity difficult to achieve high SNR: require almost-filled array

Signal To Noise Ratio Assume: z=20, M=400, t=1.5 Myr, 1 year intergration covering factor=1

Some Numbers baselinebandwidthbeamwidthSNR 45 km30 kHz20 arcsec5 65 km30 kHz14 arcsec10 91 km30 kHz1020 Very Challenging, far beyond the capability of current generation 21cm experiments. But Not Impossible!

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

Thanks

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