X-rays across the galaxy population – III. The incidence of AGN as a function of star formation rate

J Aird,A L Coil,A Georgakakis

doi:10.1093/mnras/stz125

Abstract

We map the co-eval growth of galaxies and their central supermassive black holes in detail by measuring the incidence of Active Galactic Nuclei (AGN) in galaxies as a function of star formation rate (SFR) and redshift (to z~4). We combine large galaxy samples with deep Chandra X-ray imaging to measure the probability distribution of specific black hole accretion rates (LX relative to stellar mass) and derive robust AGN fractions and average specific accretion rates. First, we consider galaxies along the main sequence of star formation. We find a linear correlation between the average SFR and both the AGN fraction and average specific accretion rate across a wide range in stellar mass ($M_* \sim 10^{8.5-11.5}M_\odot$) and to at least z~2.5, indicating that AGN in main-sequence galaxies are driven by the stochastic accretion of cold gas. We also consider quiescent galaxies and find significantly higher AGN fractions than predicted, given their low SFRs, indicating that AGN in quiescent galaxies are fuelled by additional mechanisms (e.g. stellar winds). Next, we bin galaxies according to their SFRs relative to the main sequence. We find that the AGN fraction is significantly elevated for galaxies that are still star-forming but with SFRs below the main sequence, indicating further triggering mechanisms enhance AGN activity within these sub-main-sequence galaxies. We also find that the incidence of high-accretion-rate AGN is enhanced in starburst galaxies and evolves more mildly with redshift than within the rest of the galaxy population, suggesting mergers play a role in driving AGN activity in such high-SFR galaxies.

Highlights

The growth of galaxies via star formation and the growth of the supermassive black holes that lie at their centres via periods of accretion that produce an Active Galactic Nucleus (AGN) appear to have proceeded in a broadly coherent manner over cosmic time, at least in a global sense
We present measurements of the AGN fraction to two different limits: (i) f, roughly corresponding to black holes growing above ∼1 per cent of their Eddington limit and indicating the fraction of galaxies with a moderate-to-high accretion rate, X-ray selected AGN; and (ii) f, corresponding to the fraction of galaxies with a high-accretion-rate AGN, growing their black holes at 10 per cent of their Eddington limit and hereafter referred to as the “high-λsBHAR AGN fraction”
For Starburst galaxies, we find an enhancement in the incidence of AGN compared to Main Sequence galaxies, seen most clearly in the high-λsBHAR AGN fractions and the average specific accretion rates and reflecting the excess of higher λsBHAR seen in our measurements of p(λsBHAR) (Figure 9)

Summary

Introduction

The growth of galaxies via star formation and the growth of the supermassive black holes that lie at their centres via periods of accretion that produce an Active Galactic Nucleus (AGN) appear to have proceeded in a broadly coherent manner over cosmic time, at least in a global sense. Both the total star formation rate (SFR) density and the total AGN accretion rate density peak at a redshift z ∼ 1−3 and decline at roughly the same rate towards more recent cosmic times AGN with low-tomoderate luminosities are found to have average SFRs consistent with star-forming galaxies at the same redshift, once matched in stellar mass (e.g. Rosario et al 2013b; Stanley et al 2015, 2017)

Methods

Findings

Discussion

Conclusion