Abstract
The lack of a strong correlation between AGN X-ray luminosity ($L_X$; a proxy for AGN power) and the star formation rate (SFR) of their host galaxies has recently been attributed to stochastic AGN variability. Studies using population synthesis models have incorporated this by assuming a broad, universal (i.e. does not depend on the host galaxy properties) probability distribution for AGN specific X-ray luminosities (i.e. the ratio of $L_X$ to host stellar mass; a common proxy for Eddington ratio). However, recent studies have demonstrated that this universal Eddington ratio distribution fails to reproduce the observed X-ray luminosity functions beyond z$\sim$1.2. Furthermore, empirical studies have recently shown that the Eddington ratio distribution may instead depend upon host galaxy properties, such as SFR and/or stellar mass. To investigate this further we develop a population synthesis model in which the Eddington ratio distribution is different for star-forming and quiescent host galaxies. We show that, although this model is able to reproduce the observed X-ray luminosity functions out to z$\sim$2, it fails to simultaneously reproduce the observed flat relationship between SFR and X-ray luminosity. We can solve this, however, by incorporating a mass dependency in the AGN Eddington ratio distribution for star-forming host galaxies. Overall, our models indicate that a relative suppression of low Eddington ratios ($\lambda_{\rm Edd}\lesssim$0.1) in lower mass galaxies (M<$10^{10-11}$Msun) is required to reproduce both the observed X-ray luminosity functions and the observed flat SFR/X-ray relationship.
Highlights
Most galaxies host a central super massive black hole, the masses of which display a tight correlation with their host bulge masses, implying a co-evolution between SMBHs and their host galaxies (e.g. Kormendy & Richstone 1995; Magorrian et al 1998; Merritt 2000; Kormendy et al 2011)
It is argued that this randomness is the reason why most studies that have explored the relationship between star formation rate (SFR) and X-ray luminosity have reported no evidence of a strong correlation between these parameters, at least for moderate luminosity AGNs which form the majority of the population (i.e. 1042 < LX < 1045 erg s−1; e.g. Lutz et al 2010; Harrison et al 2012; Mullaney et al 2012; Rosario et al 2012; Rovilos et al 2012; Santini et al 2012; Azadi et al 2015; Stanley et al 2015)
Motivated by recent results reporting a different Eddington ratio distribution for star-forming and quiescent galaxies (Wang et al 2017; Aird et al 2017a), we attempt to constrain these distributions by using an analytical model to fit the observed X-ray luminosity functions of Aird et al (2015), assuming the galaxy mass functions of Davidzon et al (2017)
Summary
Most galaxies host a central super massive black hole (hereafter SMBH), the masses of which display a tight correlation with their host bulge masses, implying a co-evolution between SMBHs and their host galaxies (e.g. Kormendy & Richstone 1995; Magorrian et al 1998; Merritt 2000; Kormendy et al 2011). One means of connecting SMBH and galaxy growth that has gained popular support over the past two decades is for accreting SMBHs (observed as active galactic nuclei, AGNs) to directly influence the star formation rates of their hosts (hereafter SFRs) via a variety of feedback mechanisms (see the review of Fabian 2012, for details on the feedback mechanisms). Using optically selected AGNs, Stanley et al (2017) reported an enhancement of SFR among the highest luminosity AGNs (i.e. LX > 1045 erg s−1) They demonstrated that this is a direct consequence of the most luminous AGNs residing in more massive host galaxies, meaning the enhanced SFR is a consequence of the relationship between stellar mass and SFR (hereafter referred to as the main sequence, MS; e.g. Daddi et al 2007; Rodighiero et al 2011; Schreiber et al 2015)
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