Modelling a bright z = 6 galaxy at the faint end of the AGN luminosity function

Abstract

Recent deep surveys have unravelled a population of faint active galactic nuclei (AGN) in the high redshift Universe, leading to various discussions on their nature and their role during the Epoch of Reionization. We use cosmological radiation-hydrodynamics simulations of a bright galaxy at z ∼ 6 (M 10 10 M ) hosting an actively growing super-massive black hole to study the properties of these objects. In particular, we study how the black hole and the galaxy co-evolve and what is the relative contribution of the AGN and of the stellar populations to the luminosity budget of the system. We find that the feedback from the AGN has no strong effect on the properties of the galaxy, and does not increase the total ionizing luminosity of the host. The average escape fraction of our galaxy is around f esc ∼ 5%. While our galaxy would be selected as an AGN in deep X-ray surveys, most of the UV luminosity is originating from stellar populations. This confirms that there is a transition in the galaxy population from star forming galaxies to quasar hosts, with bright Lyman-Break Galaxies (LBGs) with M UV around -22 falling in the overlap region. Our results also suggest that faint AGN do not contribute significantly to reionizing the Universe.

UV absorption lines and their potential for tracing the Lyman continuum escape fraction

“…5.5 we show the main results of the paper for Si ii λ1260. Here we use one galaxy from a zoom-in simulation, and in future work we will study a statistical sample of galaxies from various simulations like Sphinx 1 (Rosdahl et al 2018) or Obelisk (Trebitsch et al 2020).…”

“…The main candidates for the source of reionization are active galactic nuclei (AGNs) and massive stars. Although recent work suggests AGNs could be an important driver of reionization (Madau & Haardt 2015;Giallongo et al 2019), a consensus seems to be emerging on the idea that their contribution is negligible compared to that of stars (Grissom et al 2014;Parsa et al 2018;Trebitsch et al 2020).…”

Black Holes during the Epoch of Reionization

“…For this reason, the literature has shown contrasting results, with some works suggesting a significant contribution of AGN to reionization (e.g. Mitra et al 2018;Grazian et al 2018;Finkelstein et al 2019), while others found it to be largely subdominant (Onoue et al 2017;Qin et al 2017;Hassan et al 2018;Trebitsch et al 2018Trebitsch et al , 2020. Dayal et al (2020), using the Delphi semi-analytic model (SAM) presented in this thesis, found INTRODUCTION that AGN account for about 10´25% of the total amount of ionising photons at z ě 4.…”

“…Despite the fact that the spin parameter of the black hole is tracked, the radiative efficiency is fixed at 10%, and both the quasar and radio feedback modes are implemented. On a sub-volume of this same simulation, Trebitsch et al (2020) built the OBELISK simulation, an attempt to bridge the gap between large-volume cosmological simulations and high-resolution zoom-in studies, managing to study at the same time reionization evolution and halos just above the atomic-cooling limit. The EAGLE simulation (Crain et al 2015;Schaye et al 2015) is based on the SPH code GADGET (Springel 2005).…”

“…Nevertheless, as pointed out by Ono et al (2018), its bright end still suffers from contamination coming from misidentified AGN and strongly-lensed sources, as well as from inaccuracies given by photometric redshift determination and low statistics. Alongside the galaxy UVLF, the high-redshift AGN UVLF has been studied to gain an understanding of the evolution of supermassive black holes and about the contribution of quasars to reionisation with contrasting results (Madau et al 2004;Giallongo et al 2015;Grazian et al 2018;Dayal et al 2019;Trebitsch et al 2018Trebitsch et al , 2020, given that the AGN emission tracks the accretion rate of the central black holes. The study of the AGN UVLF, though, is complicated by the obscuration effects of the torus surrounding the central black hole, depending on its orientation with respect to the line of sight (Antonucci & Miller 1985;Urry & Padovani 1995), and this might result in incomplete samples.…”