The clearest examples of host–AGN interaction are arguably found in nearby brightest cluster galaxies. The AGN in these systems have been shown to deposit vast amounts of energy into the surrounding intracluster medium via heating and (mega-parsec scale) jets both observationally and by means of modelling (e.g. Binney 2004; Scannapieco, Silk & Bouwens 2005; Gitti, Brighenti & McNamara 2012, for a review; English, Hardcastle & Krause 2016), which maintain the hot-gas reservoirs in these systems, prevent cooling flows, and thus suppress star formation (e.g. Binney & Tabor 1995; Li et al. 2015). In lower density environments (where the majority of galaxies live), the empirical picture is much less clear. Direct observational evidence for AGN feedback on galactic scales in such environments remains sparse. More specifically, while outflows have been observationally detected in a number of instances around AGN in various gas phases, most of these detections have been made in ultra-luminous infrared galaxies or some of the closest quasar–host galaxies (e.g. Rupke, Veilleux & Sanders 2005b; Nyland et al. 2013; Harrison et al. 2014), with only a few examples where the AGN have been shown to couple with kpc-scale outflows that are capable of impacting star-formation on galactic scales (for a review Cicone et al. 2014; Harrison 2017). Thus, it is still unclear to what extent the general AGN population could drive kpc-scale galactic outflows capable of possibly limiting or quenching star-formation in the local Universe. Indeed, recent observational work has cast doubt on the ability of AGN to directly regulate star formation in the nearby Universe. For example, Schawinski et al. (2014) found that black hole accretion occurs preferentially in quenched galaxies that experienced a rapid decay of their star-formation rates. Based on stellar population analysis, such a time delay between the peak of star formation and the onset of AGN activity has been reported to be of at least several dynamical time-scales (e.g. Kaviraj et al. 2015; Shabala et al. 2017, for radio and e.g. Schawinski et al. 2007; Kaviraj 2009; Wild, Heckman & Charlot 2010 for optical). AGN may therefore not play a significant direct role in regulating their associated star formation episodes as they would not couple directly to the cold-gas reservoir (see, e.g. the models of Kaviraj et al. 2011). Using observations for cold-gas outflows, this has been directly confirmed for radio AGN, which are found to couple mainly to residual gas in galaxies where the gas reservoir is already significantly depleted (e.g. Sarzi et al. 2016). A fuller understanding of the role of AGN in regulating star formation demands a direct study of whether outflows of neutral material (which are ultimately responsible for quenching star formation) are more likely launched in AGN hosts. Most importantly, a quantitative statement about the putative role of AGN in influencing the evolution of their host galaxy requires a study that employs a complete sample of such AGN in the local Universe. Performing such an analysis is the purpose of this paper.
Citation: Cicone et al. 2014
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