CANDELS: CONSTRAINING THE AGN-MERGER CONNECTION WITH HOST MORPHOLOGIES AT<i>z</i>∼ 2

Abstract

Using Hubble Space Telescope/WFC3 imaging taken as part of the Cosmic&#13;\nAssembly Near-infrared Deep Extragalactic Legacy Survey, we examine the&#13;\nrole that major galaxy mergers play in triggering active galactic&#13;\nnucleus (AGN) activity at z ~ 2. Our sample consists of 72&#13;\nmoderate-luminosity (L &lt;SUB&gt;X&lt;/SUB&gt; ~ 10&lt;SUP&gt;42-44&lt;/SUP&gt; erg&#13;\ns&lt;SUP&gt;-1&lt;/SUP&gt;) AGNs at 1.5 &lt; z &lt; 2.5 that are selected using the&#13;\n4 Ms Chandra observations in the Chandra Deep Field South, the deepest&#13;\nX-ray observations to date. Employing visual classifications, we have&#13;\nanalyzed the rest-frame optical morphologies of the AGN host galaxies&#13;\nand compared them to a mass-matched control sample of 216 non-active&#13;\ngalaxies at the same redshift. We find that most of the AGNs reside in&#13;\ndisk galaxies (51.4&lt;SUP&gt;+5.8&lt;/SUP&gt; &lt;SUB&gt;- 5.9&lt;/SUB&gt;%), while a smaller&#13;\npercentage are found in spheroids (27.8&lt;SUP&gt;+5.8&lt;/SUP&gt; &lt;SUB&gt;-&#13;\n4.6&lt;/SUB&gt;%). Roughly 16.7&lt;SUP&gt;+5.3&lt;/SUP&gt; &lt;SUB&gt;- 3.5&lt;/SUB&gt;% of the AGN&#13;\nhosts have highly disturbed morphologies and appear to be involved in a&#13;\nmajor merger or interaction, while most of the hosts&#13;\n(55.6&lt;SUP&gt;+5.6&lt;/SUP&gt; &lt;SUB&gt;- 5.9&lt;/SUB&gt;%) appear relatively relaxed and&#13;\nundisturbed. These fractions are statistically consistent with the&#13;\nfraction of control galaxies that show similar morphological&#13;\ndisturbances. These results suggest that the hosts of&#13;\nmoderate-luminosity AGNs are no more likely to be involved in an ongoing&#13;\nmerger or interaction relative to non-active galaxies of similar mass at&#13;\nz ~ 2. The high disk fraction observed among the AGN hosts also appears&#13;\nto be at odds with predictions that merger-driven accretion should be&#13;\nthe dominant AGN fueling mode at z ~ 2, even at moderate X-ray&#13;\nluminosities. Although we cannot rule out that minor mergers are&#13;\nresponsible for triggering these systems, the presence of a large&#13;\npopulation of relatively undisturbed disk-like hosts suggests that the&#13;\nstochastic accretion of gas plays a greater role in fueling AGN activity&#13;\nat z ~ 2 than previously thought.