Papers › Constraining the contribution of active galactic nuclei to reionisation
Constraining the contribution of active galactic nuclei to reionisation
Sultan Hassan, Romeel Davé, Sourav Mitra, Kristian Finlator, Benedetta Ciardi, Mario G. Santos
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Recent results have suggested that active galactic nuclei (AGN) could provide enough photons to reionise the Universe. We assess the viability of this scenario using a semi-numerical framework for modeling reionisation, to which we add a quasar contribution by constructing a Quasar Halo Occupation 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 ionising emissivity. Such a model predicts τₑ too low by ∼2σ relative to Planck constraints, and reionises the Universe at z≲5. Arbitrarily increasing the AGN emissivity to match these results yields a strong mismatch with the observed ionising 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 reionisation, 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 halos than typical reionising galaxies, resulting in stronger clustering and later formation times. AGN-dominated models produce larger ionising bubbles that are reflected in ∼×2 more 21cm power on all scales. A model with equal parts galaxies and AGN contribution is still (barely) consistent with observations, but could be distinguished using next-generation 21cm experiments HERA and SKA-low. We conclude that, even with recent claims of more faint AGN than previously thought, AGN are highly unlikely to dominate the ionising photon budget for reionisation.
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