Constraining the contribution of active galactic nuclei to reionization

Sultan Hassan, Romeel Davé, Sourav Mitra, Kristian Finlator, Benedetta Ciardi, Mario G. Santos

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

Recent results have suggested that active galactic nuclei (AGN) could provide enough photons to reionize the Universe. We assess the viability of this scenario using a semi-numerical framework for modelling reionization, to which we add a quasar contribution by constructing a Quasar Halo Occupancy Distribution (QHOD) based on Giallongo et al. observations. Assuming a constant QHOD, we find that an AGN-only model cannot simultaneously match observations of the optical depth τe, neutral fraction and ionizing emissivity. Such a model predicts τe too low by ∼2σ relative to Planck constraints, and reionizes the Universe at z ≲ 5. Arbitrarily increasing the AGN emissivity to match these results yields a strong mismatch with the observed ionizing emissivity at z ∼ 5. If we instead assume a redshift-independent AGN luminosity function yielding an emissivity evolution like that assumed in Madau & Haardt model, then we can match τe albeit with late reionization; however, such evolution is inconsistent with observations at z ∼ 4-6 and poorly motivated physically. These results arise because AGN are more biased towards massive haloes than typical reionizing galaxies, resulting in stronger clustering and later formation times. AGN-dominated models produce larger ionizing bubbles that are reflected in ∼×2 more 21 cm power on all scales. A model with equal part galaxies and AGN contribution is still (barely) consistent with observations, but could be distinguished using next-generation 21 cm experiments such as Hydrogen Epoch of Reionization Array and SKA-low. We conclude that, even with recent claims of more faint AGN than previously thought, AGN are highly unlikely to dominate the ionizing photon budget for reionization.
Original languageEnglish
Pages (from-to)227-240
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Publication statusPublished - 28 Aug 2017

Keywords / Materials (for Non-textual outputs)

  • galaxies: active
  • galaxies: high-redshift
  • intergalactic medium
  • quasars: general
  • quasars: supermassive black holes
  • dark ages
  • reionization
  • first stars


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