Epoch of Reionization

1
Epoch of Reionization
2
History of Hydrogen

• z gt 1000 hot universe, hydrogen is ionized,
  universe is opaque to radiation
• z 1000 hydrogen recombines, becomes
  transparent to radiation, CMB decouples from
  matter
• 15 lt 1000 lt z dark ages, CMB is essentially
  free, galactic halos have not yet formed, little
  star formation
• 3 lt z lt 15 star formation, AGN, UV radiation,
  reionization of hydrogen

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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 gt 10 solar mass, producing UV
  radiation
• Shock waves from Supernovae

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QSO

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

6
Massive Star Formation

• Hydrogen may ionized by UV radiation from massive
  (gt10 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

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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 (zgt6) also responsible
  for reionization or late enrichment (z3) 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

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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