1. Sources of Reionization Jordi Miralda Escudé Institut de Ciències de l’Espai (IEEC-CSIC, ICREA), Barcelona. Beijing, 10-7-2008

2. Quasar Lyα absorption spectra

3. 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

4. A question you may wonder when 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.

5. Model for density distribution, ionized gas clumping factor and reionization Assume only gas with density Δ < Δ i 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.

6. 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)

7. 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.

8. Reionization model based on two source populations (Onken & JM, Font-Ribera & JM) Assume population A emits from halos with σ > σ 0 at all times, and population B from all halos with σ > σ min 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.

9. History of ionized fraction

10. 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 ~ 10 6 M sun, making massive (M~100 M Sun ) stars. Binary stars might also be formed, which might result in X-ray binaries.

11. 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.

12. 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:

13. Evolution of the optical depth

14. 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.