Epoch of reionization
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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

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History of hydrogen
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
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
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

Epoch of reionization

  • 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
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
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 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
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
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
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
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 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