CANDELS: CONSTRAINING THE AGN-MERGER CONNECTION WITH HOST MORPHOLOGIES ATz∼ 2

Abstract

Using HST /WFC3 imaging taken as part of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), we examine the role that major galaxy mergers play in triggering active galactic nuclei (AGN) activity at z ∼ 2. Our sample consists of 72 moderate-luminosity (L X ∼ 10 42−44 erg s −1 ) AGN at 1.5 < z < 2.5 that are selected using the 4 Msec Chandra observations in the Chandra Deep Field South, the deepest X-ray observations to date. Employing visual classifications, we have analyzed the rest-frame optical morphologies of the AGN host galaxies and compared them to a massmatched control sample of 216 non-active galaxies at the same redshift. We find that most of the AGN reside in disk galaxies (51.4 +5.8 −5.9 %), while a smaller percentage are found in spheroids (27.8 +5.8 −4.6 %). Roughly 16.7 +5.3 −3.5 % of the AGN hosts have highly disturbed morphologies and appear to be involved in a major merger or interaction, while most of the hosts (55.6 +5.6 −5.9 %) appear relatively relaxed and undisturbed. These fractions are statistically consistent with the fraction of control galaxies that show similar morphological disturbances. These results suggest that the hosts of moderate-luminosity AGN are no more likely to be involved in an ongoing merger or interaction relative to non-active galaxies of similar mass at z ∼ 2. The high disk fraction observed among the AGN hosts also appears to be at odds with predictions that merger-driven accretion should be the dominant AGN fueling mode at z ∼ 2, even at moderate X-ray luminosities. Although we cannot rule out that minor mergers are responsible for triggering these systems, the presence of a large population of relatively undisturbed disk-like hosts suggests that secular processes play a greater role in fueling AGN activity at z ∼ 2 than previously thought.

Herschel Observed Stripe 82 Quasars and Their Host Galaxies: Connections Between Agn Activity and Host Galaxy Star Formation

“…It seems that AGN evolution is dominated by a nonmerger process. Other studies about AGN hosts reach similar conclusions, such as Kocevski et al (2012), Santini et al (2012), and Schawinski et al (2011). Their results provide evidence that the starburst galaxies only account for 10% of the cosmic SFR density at z ∼ 2 (Rodighiero et al 2011;Lamastra et al 2013).…”

Using AGN Variability Surveys to explore the AGN-Galaxy Connection

“…The SEDS fields are also covered by CANDELS (Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey; Grogin et al 2011), which has obtained deep, highresolution HST imaging at 1 to 1.6 microns with HST WFC3 in portions of these fields. Recent CANDELS results have found that, for X-ray selected AGN, the fraction of irregular and disturbed host galaxies is no greater than that of a control sample (Kocevski et al 2012). In contrast to the X-ray selected sample, our SEDS variability survey finds a significant number of mid-IR variability-selected AGN in disturbed galaxy hosts or those having irregular morphology compared to a control sample of galaxies at similar mass and redshift (Figure 2).…”

“…The rest frame U -V colors of the mid-IR variable host galaxies are distributed through the "green valley" and, similar to the optical variables, several lie in the blue, star-forming region of the galaxy color-magnitude diagram. Kocevski et al 2012) while the magenta bars represent mid-IR variability selected AGN (non-shaded regions) compared to a control sample of galaxies with the same mass distribution for each sample (gray bars in each section). While overall morphologies of the hosts are largely similar, mid-IR variability detected AGN are more likely to reside in irregular/disturbed host galaxies than the control group contrasting with the X-ray detected AGN sample.…”

“…These results are based on a sample of ∼20 mid-IR variability-selected AGN in the EGS field that fall within the current coverage of the CANDELS images. The visual analysis was done in the same way as that of Kocevski et al (2012) but our initial sample covers a much broader redshift and mass range and contains fewer sources. Our control group was chosen to reflect the same demographics as our AGN sample for proper comparison.…”

Do we expect most AGN to live in discs?

“…Nevertheless, recent observations of AGN host morphologies and colors have suggested that major mergers probably do not fuel most low and intermediate-luminosity AGN, as a large fraction appear in "normal" disks (Gabor et al 2009;Schawinski et al 2011;Cisternas et al 2011;Kocevski et al 2012;Rosario et al 2011;Civano et al 2012;Santini et al 2012;Mullaney et al 2012;Treister et al 2012). This should perhaps not be surprising.…”