What Are the Pillars of Reionization? Revising the AGN Luminosity Function at z ∼ 5

Abstract

In the past, high-z active galactic nuclei (AGNs) were given a minor role as possible drivers of reionization, despite initial evidence in favor of their large space densities at low luminosities by Chandra and the Hubble Space Telescope. Recent observations from JWST are finding relatively large numbers of faint AGNs at z > 4, convincingly confirming these early results. We present a sample of z ∼ 5 AGNs, both from wide, shallow ground-based surveys and from deep, pencil-beam observations from JWST, allowing us to estimate their space densities with unprecedented accuracy. The bright end (M 1450 < −26) of the z ∼ 5 AGN luminosity function is well constrained, with a rather steep slope. The faint end (M 1450 ≥ −22) indicates a high space density, the scatter is significant, and the knee (M 1450 ∼ −24) is mostly undetermined. Comparisons with state-of-the-art models find reasonable agreement with the observed AGN luminosity function at z = 5, while the predicted space density evolution at higher redshifts appears to be too fast with respect to observational constraints. Given the large variance at the faint end, we consider different options in fitting the luminosity functions and deriving the ionizing emissivity. Even in the most conservative scenario, the photoionization rate produced by z ∼ 5 AGNs is consistent with the ultraviolet background measurements. A slow evolution of the space density of faint AGNs is observed, indicating that active SMBHs are probably producing large amounts of ionizing photons at z > 6, well into the Epoch of Reionization. This is an important indication that high-z AGNs could be major contributors to the reionization of the Universe.