First Stars, Quasars, and the Epoch of Reionization

Jordi Miralda Escudé

Institut de Ciències de l’Espai(IEEC-CSIC, ICREA), Barcelona.

Instituto de Astrofísica de Canarias, 13-11-2007

History of recombination and reionization

• At redshift z ≈ 1000, hydrogen atoms were formed in a uniform medium at T ≈ 3000 K in equilibrium with the CMB radiation.

• Reionization starts with the first stars at z ≈ 20, and ends at z ≈ 6 with the overlap of HII regions created by luminous sources as increasingly massive halos collapse and form stars and quasars.

Observational Probes to Reionization

• Lyα forest, Gunn-Peterson trough– Reionization ends at z ~ 6.5

• Lyα-emission galaxy surveys

• CMB optical depth and polarization

• 21 cm emission and absorption by the atomic intergalactic medium.

Quasar Lyα absorption spectra

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The appearance of the

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Inferred emission of ionizing photons in the post-overlap phase

• Lyα forest opacity yields intensity of ionizing background (photon density).

• Mean free path is deduced from observed Lyman limit systems.

• Emissivity is the ratio of photon density over mean free path.

Result: only about 1 ionizing photon per baryon and per Hubble time are being emitted at z ≈ 6.

Reionization is photon-starved: not many recombinations take place, and it develops over an extended epoch (Miralda-Escudé 2003; Bolton & Haehnelt 2007)

• Evidence for a rapid decline in the intensity at

• z > 6 suggests we have reached the end of reionization at the highest redshift quasars observed.

Fan et al. 2006

The damped wing of the Gunn-Peterson trough indicates that a

source is being seen behind atomic intergalactic medium

• We may observe this on the spectra of a gamma-ray burst optical afterglow.

• We may also detect this effect in suppressing the Lyman alpha emission line of star-forming galaxies. The large-scale distribution of Lya emitting galaxies during the reionization epoch might then be modulated by the structure of cosmological HII regions.

Dawson et al. 2004

McQuinn et al. 2007

The Clustering of Lyα emittersincreases owing

to a patchy reionization

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The Thomson optical depth to the CMB depends on the whole history of reionization

• Up to z=6: • We expect more optical depth to be added from the era of partial

ionization of the universe• The optical depth can be measured from the temperature and

polarization fluctuations introduced in the CMB.

– WMAP measurement: (consistent with extended reionization epoch over 6 < z < 20)• Also, small-scale polarization on the CMB is proportional to the

electron column density along each line of sight.

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21 cm emission

• 21 cm emission on the CMB: )/1( sCMBHI TTT • The spin temperature must be coupled to the kinetic

temperature Tk to make HI observable in 21cm, either collisionally or through Lyman alpha photons (e.g., Madau, Meiksin, & Rees 1997).

• Initially, Tk < TCMB, HI is seen in absorption. Then, lyα photons from stars increase Tk-Ts coupling. Later, X-rays heat the kinetic temperature and HI is seen in emission. The structure of the atomic and ionized regions can be probed through this 21 cm emission.

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Evolution of kinetic temperature• Typical X-ray emission of

local starbursts: 1 keV per baryon.• Hard X-rays ( > 1keV) heat

the medium homogeneously; soft X-rays (such as the photospheric emission from metal-free stars) heat inhomogeneously.

• Early emission of hard X-rays depends on the presence of massive close binaries among the first stars.

Reionization was started by the first

stars in the universe• Cooling of gas first took place

from molecular hydrogen, at z~30 in halos of mass ~ 106

Msun . Gas cools only to ~ 200 K.

• Accretion rate ~ cs3/G

• Massive (M~100 MSun), metal-free stars were made.

• The most metal-poor stars show independent evidence that most first stars were massive (e.g., Tumlinson 2007).

What is a first star?• All metal-free stars? Stars forming from matter that has

never been in other stars.• Another possible definition: a star forming at a place and

time where no light from another star has yet reached.– For CDMΛ model: first stars form at z ~ 40 from 6-sigma

fluctuations.• Or: a star forming at a place and time where no light

from other stars is substantially affecting any of its observable properties.

How can we observe a first star?• Supernovae? Gamma-ray bursts?• 21 cm emission/absorption

on the CMB: )/1( sCMBHI TTT

What happens around one metal-free star?

• Lyα photons couple the spin and kinetic temperatures out to a radius much larger than the HII region.

• X-rays from the stellar photosphere heat the medium.

• X-ray ionizations also produce injected Lyα photons, which turn out to dominate for the surface temperatures of metal-free stars. These yield a dominant absorption signal from a ``Lyα sphere’’ around a metal-free star.

• Kinetic temperature is greatly heated just beyond the HII region, but further out it has been adiabatically cooled.

• 21cm absorption strongly dominates over the inner emission core.

Temperature and 21cm profiles

Detectability of single Lyα spheres• Angular size: θ ~ 10” (20 kpc at z=30)

– Required baseline: 100 km (at z=30)

– Signal temperature: δT ≈ 200 mK– Synchrotron background temperature: Tb≈4000 K (z=30)

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for t=1 year

• We need a large array of telescopes.• It may be better to look for clusters of Lyα spheres

on larger angular scales, or for a global signal.

Conclusions

• The Gunn-Peterson trough in quasar spectra informs us about how reionization ended.

• Reionization was photon-starved, and a long epoch of partial ionization existed before reionization ended, contributing to the CMB Thomson optical depth.

• Damped intergalactic absorption and 21cm emission can probe the evolution of the intergalactic medium during the epoch of partial ionization.

• The Lyα sphere of a metal-free star produces a 21cm absorption signal which is an unmistakable signature of a first star.

Typical history of reionization, contributed by

metal-free stars, and galaxies(Rozas et al. 2006)

Lyα background intensity• The coupling parameter yα

gets close to unity at z ≈ 25 everywhere because of the light background from all metal-free stars, so Lyα spheres lose their contrast.

• In addition, global temperature starts rising at z ≈ 25 due to X-rays, so absorption weakens, eventually turning to emission.

• 21 cm absorption must be searched at 30 – 40 MHz

Properties of first metal-free stars

• Central gas cools only to T ≈ 200 K. Molecular hydrogen lines can be collisionally deexcited at density n > 104 cm-3, making the cooling rate independent of density and inhibiting fragmentation.

• Jeans mass ≈ 300 Msun .

• Accretion rate ≈ cs3/G ≈ 10-3 Msun/yr

• The first metal-free stars were massive, with L ≈ LEdd and T ≈ 105 K (Abel etal 2002, Bromm etal 2002, Schaerer 2002). Their lifetime is ~ 3 million years.

A question you can wonder aboutwhen you are swimming in the sea

this summer.

• Where all of the hydrogen atoms around you ionized at some stage after they first formed at z ≈ 1000, or were some of them never ionized?

First ionized regions• Each metal-free star can produce about 105

ionizing photons per baryon it contains, creating an HII region of ~ 107 Msun of gas, of physical radius ~ 1 kpc at z=30. Probably only one metal-free star forms per halo.

• Star formation occurring after the HII region recombines and merges is probably from metal enriched gas.

Heating due to the scattering of Lyα photons itself is negligible

• Heating rate: IcnJ

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Injected photons:

Continuum photons:

Lyα spheres at z≈30 are strongly biased Average number of neighboring

star-forming halos

History of recombination and reionization

• The fraction of ionized gas in the universe gradually increased as more massive halos collapsed, forming galaxies and the first quasars. Initially, most photons escaping from the dense vicinity of sources were used to ionize the diffuse intergalactic matter.

• Reionization ends when all the low-density medium is ionized. At this time, most ionizing photons are absorbed in dense, self-shielded clumps (Lyman limit systems). An abrupt increase of the mean free path and intensity of the ionizing background is expected at the end of reionization.