Inhomogeneous recombinations during cosmic reionization

Emanuele Sobacchi, Andrei Mesinger

Research output: Contribution to journalArticlepeer-review

128 Scopus citations


By depleting the ionizing photon budget available to expand cosmic HII regions, recombining systems (or Lyman limit systems) can have a large impact during (and following) cosmic reionization. Unfortunately, directly resolving such structures in large-scale reionization simulations is computationally impractical. Instead, here we implement a subgrid prescription for tracking inhomogeneous recombinations in the intergalactic medium. Building on previous work parametrizing photoheating feedback on star formation, we present large-scale, seminumeric reionization simulations which self-consistently track the local (subgrid) evolution of both sources and sinks of ionizing photons. Our simple, single-parameter model naturally results in both an extended reionization and a modest, slowly evolving emissivity, consistent with observations. Recombinations are instrumental in slowing the growth of largeHII regions, and damping the rapid rise of the ionizing background in the late stages of (and following) reionization. As a result, typical HII regions are smaller by factors of ̃2 to 3 throughout reionization. The large-scale (k ≤ 0.2 Mpc-1) ionization power spectrum is suppressed by factors of ≥2-3 in the second half of reionization. Therefore properly modelling recombinations is important in interpreting virtually all reionization observables, including upcoming interferometry with the redshifted 21cm line. Consistent with previous works, we find the clumping factor of ionized gas to be CHII ~ 4 at the end of reionization.

Original languageEnglish
Pages (from-to)1662-1673
Number of pages12
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - 1 Jan 2014
Externally publishedYes


  • Cosmology:theory
  • Dark ages, reionization, first stars
  • Early Universe
  • Galaxies:formation
  • Galaxies:high:
  • Intergalactic medium
  • Redshift

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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