Epoch of Reionization

1. Epoch of Reionization

2. History of Hydrogen • z > 1000: hot universe, hydrogen is ionized, universe is opaque to radiation • z = 1000: hydrogen recombines, becomes transparent to radiation, CMB decouples from matter • 15 < 1000 < z: dark ages, CMB is essentially free, galactic halos have not yet formed, little star formation • 3 < z < 15: star formation, AGN, UV radiation, reionization of hydrogen

3. Why Do We Care? • Reionization plays key role in formation of stars and galaxies • Tests cosmological models which suppress halo formation at early times

4. Possible sources for Reionization • Radiation from QSO at UV wavelengths and shorter • Star formation, M > 10 solar mass, producing UV radiation • Shock waves from Supernovae

5. QSO • Radio surveys and Optical surveys find few bright Quasi Stellar Objects at z > 3 • Observations of Lyman alpha emission indicate ionization at z > 6 • QSO are a very unlikely candidate for reionization

6. Massive Star Formation • Hydrogen may ionized by UV radiation from massive (>10 M0) stars • High degree of ionization if radiation releases ~25 eV per atom in the IGM • Nuclear burning from 0 to solar metallicity, energy radiated per baryon = 0.02 x 0.007 x mH • 1/3 energy goes into UV: 5.0 x 10-5 x mH per atom

7. Mechanical Energy from Supernovae • Same massive stars will eventually explode as supernovae • Collisional ionization from shock waves • 25 eV per IGM atom produces ionization of only a few parts in 100,000 • SN produce approximately 4x10-6 x mH per atom • Much less energy produced than UV radiation from nuclear burning

8. Nuclear Burning vs SNe • Nuclear burning in massive stars appears to be more efficient at reionizing the IGM • Important is efficiency of the two mechanisms at escaping into IGM • SNe in low mass halos produce enough energy to escape the halo and shock IGM • Star formation efficiency may be to slow? P.3

9. Metal Enrichment of IGM • IGM appears uniformly enriched at 10-3.2 -10-2.5 relative to solar, caused by SNe • May be early enrichment (z>6) also responsible for reionization or late enrichment (z=3) by star forming galaxies • Still too early to use measurements of metal enrichment to determine timescale

10. 21-cm Line • Hyperfine transition in neutral hydrogen at 21-cm wavelengths • Mechanism such as scattering by Lyman alpha photons is required to knock 21-cm emission out of equilibrium with CMB photons creating an observable signal • Signal would of course disappear up reionization

11. Effect of 21-cm radiation • 21-cm radiation would cause an increase in the apparent temperature of the CMB at wavelengths shorter than those corresponding to the reshifted wavelength of the 21-cm radiation at reionization

12. Direct Measurements • LOFAR (Low frequency Array), 10 – 240 MHz range, 100 sq km, hoping to be operational by 2008 • One of main Science drivers will be to probe epoch of reionization, sensitive to redshifted 21-cm line

13. WMAP Measurement • WMAP measures CMB power spectrum and TE power spectrum • At large angular scales, measurements are sensitive to reionization • Expect amplitude of temperature anisotropy to be suppressed due to increased optical depth • Expect increased TE correlation • Measurements: reionization at z = 17 +/- 5