Sources of Reionization

Jordi Miralda Escudé

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

Beijing, 10-7-2008

Quasar Lyα absorption spectra

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• 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

A question you may wonderwhen you are swimming in the sea

this summer.• Were 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?

• An atom may have already been part of a proto-galaxy at reionization, from which it went on to a circumstellar disk and then on to a planet.

• While reionization had to ionize all the matter in low-density regions it didn’t have to ionize the high-density gas in Lyman-limit and damped Lyα systems.

Model for density distribution, ionized gas clumping factor and reionization

• Assume only gas with density Δ < Δi is ionized.

• This is only a rough approximation for the unsmoothed density distribution. On large-scales, the gas is actually ionized first in the denser regions, where there are more sources, because the sources are highly biased to high-density regions.

Inferred emission of ionizing photons in the post-overlap phase

• Lyα forest opacity yields intensity of ionizing background (photon density), with known baryon density.

• Mean free path is deduced from observed Lyman limit systems at z < 4 (models need to be used at higher redshift).

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

Result: only ~ 10 ionizing photons per baryon and per Hubble time are being emitted at z = 4, and only ~ 3 at z=6.

Reionization is photon-starved: few recombinations take place, and reionization occurs over an extended epoch (Miralda-Escudé 2003; Bolton & Haehnelt 2007)

Ionizing emissivity (Bolton & Haehnelt 2007)

Only models with emissivity not falling with redshift at z above 6 are consistent with completion of reionization by z = 6.

Reionization model based on two source populations (Onken & JM, Font-Ribera & JM)

• Assume population A emits from halos with σ > σ0 at all times, and population B from all halos with σ > σmin while matter is not reionized. The emissivities are proportional to the mass fraction in halos.

• Adjust the emissivities to reproduce the measured value at z=4, and reionization ending at z=6.

History of ionized fraction

Reionization started with the first

metal-free stars in the universe

• Cooling of gas first occurs through from molecular hydrogen, at z~30 in halos of mass ~ 106 Msun , making massive (M~100 MSun) stars.

• Binary stars might also be formed, which might result in X-ray binaries.

Effect of X-ray sources• Where the X-rays are absorbed:

– E < 0.1 keV: absorbed locally (near HII regions)– 0.1 keV < E < 1 keV: absorbed far away, heat atomic medium– E > 1 keV: redshifted (universe is effectively transparent)

• High-energy electrons produced by X-rays give rise to secondary ionizations, excitations, and heating by Coulomb interactions.

• If X-ray binaries are present, then a fraction y of the volume is totally, ionized by UV sources, and the fraction 1-y has ionization fraction x due to the X-ray sources. The total ionized fraction is y + (1-y)x, and recombinations are slower.

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• WMAP measurement:

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Evolution of the optical depth

Conclusions

• The measured emissivity at redshifts z=4 to 6 implies that reionization is photon-starved, so it occurred over an extended period of time and the ionized gas clumping factor was small during reionization.

• The most simple models extrapolating the ionizing emissivity from halos in CDM to z > 6 can easily agree with the optical depth measured by WMAP. A value twice as large or twice as small would be very difficult to reconcile.