Epoch of Reionization History of Hydrogen z 1000

Epoch of Reionization

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

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

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

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

Massive Star Formation • Hydrogen may ionized by UV radiation from massive (>10 M 0) stars • High degree of ionization if radiation releases ~25 e. V per atom in the IGM • Nuclear burning from 0 to solar metallicity, energy radiated per baryon = 0. 02 x 0. 007 x m. H • 1/3 energy goes into UV: 5. 0 x 10 -5 x m. H per atom

Mechanical Energy from Supernovae • Same massive stars will eventually explode as supernovae • Collisional ionization from shock waves • 25 e. V per IGM atom produces ionization of only a few parts in 100, 000 • SN produce approximately 4 x 10 -6 x m. H per atom • Much less energy produced than UV radiation from nuclear burning

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

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

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

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

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

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