The black hole population in low-mass galaxies in large-scale cosmological simulations

Abstract

Recent systematic searches for massive black holes (BHs) in local dwarf galaxies led to the discovery of a population of faint Active Galactic Nuclei (AGN). We investigate the agreement of the BH and AGN populations in the Illustris, TNG, Horizon-AGN, EAGLE, and SIMBA simulations with current observational constraints in low-mass galaxies. We find that some of these simulations produce BHs that are too massive, and that the BH occupation fraction at z = 0 is not inherited from the simulation seeding modeling. In simulations, the ability of BHs and their host galaxies to power an AGN depends on the subgrid modeling of the BH and of the galaxy. The fraction of AGN in low-mass galaxies is not used to calibrate the simulations, and thus is a true prediction. AGN fractions at z = 0 span two orders of magnitude at fixed galaxy stellar mass in simulations, similarly to observational constraints, but uncertainties and degeneracies affect both observations and simulations. The agreement is difficult to interpret due to differences in BH masses between simulations and observations, BH occupation fraction affected by numerical choices, and an unknown obscured fraction of AGN. Our work advocates for more thorough comparisons with observations in the low-mass regime to improve the modeling of cosmological simulations, and our understanding of BH and galaxy physics in this regime. The mass of BHs, their ability to efficiently accrete gas, and the AGN occupation fraction in low-mass galaxies have important implications for the build-up of the entire BH and galaxy populations with time.

RESOLVE and ECO: Finding Low-Metallicity $z\sim0$ Dwarf AGN Candidates Using Optimized Emission-Line Diagnostics

“…The overall RESOLVE and ECO dwarf AGN percentage of ∼1-2% (for the full mass and volume-limited surveys, not just for SELs) agrees with the dwarf AGN occupation fraction seen in the Horizon-AGN simulation in all X-ray luminosity bins, and with EAGLE, Illustris, and TNG100 in some X-ray bins (Figure 5 of Haidar et al 2022). However, Haidar et al (2022) highlight several sources of overestimation or underestimation of AGN percentages for both theory and observations. On the theoretical side, simulations generally neglect BH wandering and obscuration, thus overestimating accretion and its observability, and seeding mechanisms are unphysical.…”

“…We also compare our dwarf AGN percentage with AGN populations in various simulations described by Haidar et al (2022). The overall RESOLVE and ECO dwarf AGN percentage of ∼1-2% (for the full mass and volume-limited surveys, not just for SELs) agrees with the dwarf AGN occupation fraction seen in the Horizon-AGN simulation in all X-ray luminosity bins, and with EAGLE, Illustris, and TNG100 in some X-ray bins (Figure 5 of Haidar et al 2022). However, Haidar et al (2022) highlight several sources of overestimation or underestimation of AGN percentages for both theory and observations.…”

“…On the other hand, past SEL studies have generally reported the AGN frequency for SEL galaxies, which overestimates the SEL-detected AGN frequency of the full galaxy population. Haidar et al 2022 stress the need for full-population statistics for a fair comparison between simulations and observations. Our raw SEL AGN frequencies exceed the frequencies to which Haidar et al (2022) compare (e.g., from Reines et al 2013).…”

“…Haidar et al 2022 stress the need for full-population statistics for a fair comparison between simulations and observations. Our raw SEL AGN frequencies exceed the frequencies to which Haidar et al (2022) compare (e.g., from Reines et al 2013). Our fullpopulation normalized SEL AGN frequencies, which are by definition lower than our raw SEL AGN frequencies and more directly comparable to simulations, also exceed previous estimates (where possible, as for Reines et al 2013 andSartori et al 2015 in Table 3).…”

“…The high degree of completeness and volume-and mass-limited survey definitions of RESOLVE and ECO allow us to normalize statistics for the SEL samples to the full parent survey, yielding "completeness-corrected" statistics useful for comparisons to theory (e.g., Haidar et al 2022). RESOLVE and ECO also allow us to study the properties of AGN hosts using extensive supporting data (especially gas content and environment), providing added value to SDSS emission line fluxes.…”