# Presentation: 576120

## 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 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

## 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

## 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