The discovery of a correlation between the mass of supermassive black holes (SMBHs) and several properties of their host galaxies (e.g. Magorrian et al. 1998; Ferrarese & Merritt 2000; Gebhardt et al. 2000; Kormendy & Ho 2013) has suggested that the growth of SMBHs and their host galaxies are tightly connected. Mergers of galaxies are thought to be one of the most important mechanisms with which galaxies build up their stellar masses (White & Rees 1978). Both observational (e.g. Lonsdale, Persson & Matthews 1984; Joseph & Wright 1985; Armus, Heckman & Miley 1987; Clements et al. 1996; Alonso-Herrero et al. 2000; Ellison et al. 2008) and theoretical (e.g. Mihos & Hernquist 1996; Di Matteo et al. 2007) studies have shown that galaxy mergers enhance star formation (SF). Simulations have also shown that the interaction between two or more galaxies can reduce the angular momentum of the circumnuclear material (e.g. Barnes & Hernquist 1991; Blumenthal & Barnes 2018), thus providing an effective mechanism to trigger accretion on to SMBHs (e.g. Di Matteo, Springel & Hernquist 2005). Observationally, several works have confirmed this scenario. Koss et al. (2010) and Silverman et al. (2011) found a higher AGN fraction in pairs than in isolated galaxies with similar stellar masses. It has been shown that the fraction of AGN in mergers tends to increase as the separation between the two galaxies decreases (Ellison et al. 2011), and peaks after coalescence (Ellison et al. 2013). Koss et al. (2012) have shown that the average luminosity of dual AGN also increases with decreasing separation (see also Hou, Li & Liu 2020), and it is higher for the primary (i.e. more massive) component of the system (see also De Rosa et al. 2019, for a recent review). While AGNs with moderate X-ray luminosities are typically found in non-interacting disc galaxies (e.g. Koss et al. 2011; Kocevski et al. 2012; Schawinski et al. 2012), more luminous objects are commonly found in merging systems (e.g. Treister et al. 2012; Glikman et al. 2015; Hong et al. 2015). Treister et al. (2012) showed that, while for 2–10 keV AGN luminosities of $L_{2-10}\sim 10^{41}\rm \, erg\, s^{-1}$ only a small fraction (${\lt}1{{\ \rm per\ cent}}$) of AGNs are in mergers, at $L_{2-10}\sim 10^{46}\rm \, erg\, s^{-1}$ ∼ 70–80 per cent of the sources are found in interacting systems (see also Glikman et al. 2015). Recent evidence has suggested that hot dust obscured galaxies (Hot DOGs; Wu et al. 2012; Assef et al. 2015), which are some of the most luminous galaxies observed so far ($L_{\rm \, IR} \gt 10^{13} \mathrm{ L}_{\odot }$), are also found in mergers (e.g. Fan et al. 2016). These observations suggest that, while at low luminosities SMBH accretion is triggered by secular processes, at high luminosities mergers can play a dominant role. This is in agreement with the evolutionary scenario proposed by Sanders et al. (1988) for ultra-luminous [$L_{\rm \, IR}(8\!-\!1000\, \mu \rm m)\ge 10^{12}\,L_{\odot }$] infrared galaxies (ULIRGs; e.g. Sanders & Mirabel 1996; Pérez-Torres et al. 2021). In this scheme, two gas-rich disc galaxies collide, triggering SF and accretion on to the SMBH. The strong accretion on to the SMBH would lead the source to evolve first in a luminous red quasar (e.g. Urrutia, Lacy & Becker 2008; Glikman et al. 2015; LaMassa et al. 2016) and then in an unobscured blue quasar.
Citation: Ellison et al. 2011
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