Dark-ages reionization and galaxy formation simulation – X. The small contribution of quasars to reionization

Abstract

Motivated by recent measurements of the number density of faint AGN at high redshift, we investigate the contribution of quasars to reionization by tracking the growth of central supermassive black holes in an update of the Meraxes semi-analytic model. The model is calibrated against the observed stellar mass function at z ∼ 0.6 − 7, the black hole mass function at z 0.5, the global ionizing emissivity at z ∼ 2 − 5 and the Thomson scattering optical depth. The model reproduces a Magorrian relation in agreement with observations at z < 0.5 and predicts a decreasing black hole mass towards higher redshifts at fixed total stellar mass. With the implementation of an opening angle of 80 deg for quasar radiation, corresponding to an observable fraction of ∼23.4 per cent due to obscuration by dust, the model is able to reproduce the observed quasar luminosity function at z ∼ 0.6 − 6. The stellar light from galaxies hosting faint AGN contributes a significant or dominant fraction of the UV flux. At high redshift, the model is consistent with the bright end quasar luminosity function and suggests that the recent faint z ∼ 4 AGN sample compiled by Giallongo et al. (2015) includes a significant fraction of stellar light. Direct application of this luminosity function to the calculation of AGN ionizing emissivity consequently overestimates the number of ionizing photons produced by quasars by a factor of 3 at z ∼ 6. We conclude that quasars are unlikely to make a significant contribution to reionization.

High-redshift Galaxies and Black Holes Detectable with the JWST: A Population Synthesis Model from Infrared to X-Rays

“…We are unable to find an acceptable fit for this LF for our standard BH-galaxy relationship, although when we limit the comparison only to the magnitude range of the data of Giallongo et al 2015 (from −19 to −21), i.e., we do not try to fit the full LF, the model parameters of our reference case produce an LF that is compatible with the LF of Giallongo et al 2015. We confirm the results by Qin et al (2017) that the UV luminosity is dominated by the galaxy stellar population in this magnitude range, with the AGN-only luminosity function, that is, without including stars in the source luminosity, 0.04 what we obtain including both stellar and AGN light in the "AGN" luminosity.…”

Dark-ages reionization and galaxy formation simulation – XVIII. The high-redshift evolution of black holes and their host galaxies

“…The model outputs a range of properties for each galaxy in the simulation, including the mass of hot gas, cold gas and stars, its star formation rate, and the mass of its central black hole. For a full description of the processes modelled in Meraxes, see Mutch et al (2016), Qin et al (2017) (herein Q17) and Marshall et al (2019) (herein M19). We outline the physical processes most relevant to this work in Sections 2.3 and 2.4 below.…”

Dark-age reionization and galaxy formation simulation – XIX. Predictions of infrared excess and cosmic star formation rate density from UV observations

“…This work investigates the IRX -β relation and cosmic SFRD at z ∼ 4 − 7 by combining the the meraxes semi-analytic galaxy formation model Qin et al 2017) Table 3. Tabular data of predicted cosmic star formation rate density (SFRD) for the three different dust models.…”

“…A detailed description of the model can be found in Mutch et al (2016), hereafter M16. The active galactic nuclei (AGN) feedback of the model is later introduced by Qin et al (2017). This work also applies several updates to the model, aiming to improve the predicted gas phase metallicity, which is an input of galaxy SEDs.…”