AGN evolution from galaxy evolution viewpoint - II

TL;DR

This paper presents a co-evolution model linking galaxy star formation and AGN activity, emphasizing the last phases of galaxy evolution, and shows it aligns well with observational data, especially favoring the quenching scenario.

Contribution

It introduces a new co-evolution model connecting galaxy quenching with AGN activity, matching observed quasar properties and galaxy relations, and distinguishes itself from previous models.

Findings

The black hole to galaxy mass ratio during quenching evolves as (1+z)^{1.5}.

The characteristic Eddington ratio increases with redshift up to z~2, then stabilizes near unity.

The quenching scenario better fits observational data than previous models.

Abstract

In order to relate the observed evolution of the galaxy stellar mass function and the luminosity function of active galactic nuclei (AGN), we explore a co-evolution scenario in which AGN are associated only with the very last phases of the star-forming life of a galaxy. We derive analytically the connections between the parameters of the observed quasar luminosity functions and galaxy mass functions. The $(m_{\rm bh}/m_{*})_{Qing}$ associated with quenching is given by the ratio of the global black hole accretion rate density (BHARD) and star-formation rate density (SFRD) at the epoch in question. Observational data on the SFRD and BHARD suggests $(m_{\rm bh}/m_{*})_{Qing} \propto (1+z)^{1.5}$ below redshift 2. This evolution reproduces the observed mass-luminosity plane of SDSS quasars, and also reproduces the local $m_{\rm bh}/m_{*}$ relation in passive galaxies. The characteristic Eddington ratio, $\lambda^*$, is derived from both the BHARD/SFRD ratio and the evolving $L^*$ of the AGN population. This increases up to $z \sim 2$ as $\lambda^* \propto (1+z)^{2.5}$ but at higher redshifts, $\lambda^*$ stabilizes at the physically interesting Eddington limit, $\lambda^* \sim 1$. The new model may be thought of as an opposite extreme to our earlier co-evolution scenario in Caplar et al. 2015. The main observable difference between the two co-evolution scenarios, presented here and in Caplar et al. 2015, is in the active fraction of low mass star-forming galaxies. We compare the predictions with the data from deep multi-wavelength surveys and find that the "quenching" scenario developed in the current paper is much to be preferred.