WHAT MAKES A RADIO-AGN TICK? TRIGGERING AND FEEDING OF ACTIVE GALAXIES WITH STRONG RADIO JETS

Abstract

Although the link between activity in the nuclei of galaxy and galactic mergers has been under scrutinyfor several years, it is still unclear to what extent and for which populations of active galaxies merger-triggered activity is relevant. The environments of AGN allow an indirect probe of the past merger historyand future merger probability of these systems, su ering less from sensitivity issues when extended tohigher redshifts than traditional morphological studies of AGN host galaxies. Here we present results fromour investigation of the environment of radio selected sources out to a redshift z=2. We employ the rstdata release J-band catalog of the new near-IR Infrared Medium-Deep Survey (IMS), 1.4 GHz radio datafrom the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey and a deep dedicated VLA survey ofthe VIMOS eld, covering a combined total of 20 sq. degrees. At a ux limit of the combined radio catalogof 0.1 mJy, we probe over 8 orders of magnitude of radio luminosity. Using the second closest neighbordensity parameters, we test whether active galaxies inhabit denser environments. We nd evidence for asub-population of radio-selected AGN that reside in signi cantly overdense environments at small scales,although we do not nd signi cant overdensities for the bulk of our sample. We show that radio-AGN inthe most underdense environments have vigorous ongoing star formation. We interpret these results interms of the triggering and fuelling mechanism of radio-AGN.Key words: galaxies: active { galaxies: jets { galaxies: star formation { galaxies: evolution1. THE TRIGGERING OF RADIO-AGNThe recent decades have revolutionized our understand-ing of how galaxies and the supermassive black holes attheir centers evolve (e.g., Springel et al. 2005, Hopkinset al. 2006). However, an ongoing debate concerns theway that active galactic nuclei (AGN) in galaxies aretriggered and sustained. The temporal coincidence ofthe peak of cosmic nuclear and star-formation activity(e.g., Richards et al. 2006, Aird et al. 2010), as well asthe predominantly very luminous AGN discovered be-yond our local universe, have led to the merger-drivenparadigm of AGN triggering (e.g., Sanders et al. 1988).However, with the advent of deeper extragalactic sur-veys this paradigm has come in question. Moderateluminosity X-ray AGN at intermediate redshifts werenot found to be preferentially associated with merg-ers (e.g., Cisternas et al. 2011), nor to inhabit denserenvironments than their non-active counterparts (e.g.,Karouzos et al. 2014a). While undoubtedly a fractionof AGN are triggered by mergers, the question still re-mains: how are di erent species of AGN triggered?Here we use the population of radio-AGN as a case studyto address this question. We present results about thehttp://pkas.kas.orgenvironments and host galaxy properties of radio-AGNthat help elucidate the di erent AGN feeding mecha-nisms within the broader context of galaxy evolution.2. THE SA22 FIELD AND THE RADIO-AGN WITHIN ITWe use data from the Infrared Medium-Deep Survey(IMS; see this proceedings) at J band and the Deep eX-tragalactic Survey (DXS; Lawrence et al. 2007). In ad-dition, we utilize radio data at 1.4 GHz from the FIRSTsurvey (Becker et al. 1995) and a deep VLA survey ofpart of SA22 (Chapman et al. 2004a). The combined ra-dio ux density limit of our sample is 0.1 mJy. Finally,we also employ the full coverage of the WISE telescope(Wright et al. 2010). In total we select 913 radio-AGNover the ˘ 20 deg