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