THE<i>XMM-NEWTON</i>WIDE FIELD SURVEY IN THE COSMOS FIELD: REDSHIFT EVOLUTION OF AGN BIAS AND SUBDOMINANT ROLE OF MERGERS IN TRIGGERING MODERATE-LUMINOSITY AGNs AT REDSHIFTS UP TO 2.2

Abstract

We present a study of the redshift evolution of the projected&#13;\ncorrelation function of 593 X-ray selected active galactic nuclei (AGNs)&#13;\nwith I &lt;SUB&gt;AB&lt;/SUB&gt; &lt; 23 and spectroscopic redshifts z &lt; 4,&#13;\nextracted from the 0.5-2 keV X-ray mosaic of the 2.13 deg&lt;SUP&gt;2&lt;/SUP&gt;&#13;\nXMM- Cosmic Evolution Survey (COSMOS). We introduce a method to estimate&#13;\nthe average bias of the AGN sample and the mass of AGN hosting halos,&#13;\nsolving the sample variance using the halo model and taking into account&#13;\nthe growth of the structure over time. We find evidence of a redshift&#13;\nevolution of the bias factor for the total population of XMM-COSMOS AGNs&#13;\nfrom \\overline{b}(\\overline{z}=0.92)=2.30 +/- 0.11 to&#13;\n\\overline{b}(\\overline{z}=1.94)=4.37 +/- 0.27 with an average mass of&#13;\nthe hosting dark matter (DM) halos log M &lt;SUB&gt;0&lt;/SUB&gt;(h &lt;SUP&gt;-1&lt;/SUP&gt; M&#13;\n&lt;SUB&gt;sun&lt;/SUB&gt;) ~ 13.12 ± 0.12 that remains constant at all z&#13;\n&lt; 2. Splitting our sample into broad optical line AGNs (BL), AGNs&#13;\nwithout broad optical lines (NL), and X-ray unobscured and obscured&#13;\nAGNs, we observe an increase of the bias with redshift in the range&#13;\n\\overline{z}=0.7{--}2.25 and \\overline{z}=0.6{--}1.5 which corresponds&#13;\nto a constant halo mass of log M &lt;SUB&gt;0&lt;/SUB&gt;(h &lt;SUP&gt;-1&lt;/SUP&gt; M&#13;\n&lt;SUB&gt;sun&lt;/SUB&gt;) ~ 13.28 ± 0.07 and log M &lt;SUB&gt;0&lt;/SUB&gt;(h&#13;\n&lt;SUP&gt;-1&lt;/SUP&gt; M &lt;SUB&gt;sun&lt;/SUB&gt;) ~ 13.00 ± 0.06 for BL/X-ray&#13;\nunobscured AGNs and NL/X-ray obscured AGNs, respectively. The&#13;\ntheoretical models, which assume a quasar phase triggered by major&#13;\nmergers, cannot reproduce the high bias factors and DM halo masses found&#13;\nfor X-ray selected BL AGNs with L &lt;SUB&gt;BOL&lt;/SUB&gt; ~ 2 ×&#13;\n10&lt;SUP&gt;45&lt;/SUP&gt; erg s&lt;SUP&gt;-1&lt;/SUP&gt;. Our work extends up to z ~ 2.2 the z&#13;\n&lt;~ 1 statement that, for moderate-luminosity X-ray selected BL AGNs,&#13;\nthe contribution from major mergers is outnumbered by other processes,&#13;\npossibly secular ones such as tidal disruptions or disk instabilities.