THE BULK OF THE BLACK HOLE GROWTH SINCEz∼ 1 OCCURS IN A SECULAR UNIVERSE: NO MAJOR MERGER-AGN CONNECTION

Abstract

What is the relevance of major mergers and interactions as triggering\nmechanisms for active galactic nuclei (AGNs) activity? To answer this\nlong-standing question, we analyze 140 XMM-Newton-selected AGN host\ngalaxies and a matched control sample of 1264 inactive galaxies over z ~\n0.3-1.0 and M * < 1011.7 M sun with\nhigh-resolution Hubble Space Telescope/Advanced Camera for Surveys\nimaging from the COSMOS field. The visual analysis of their morphologies\nby 10 independent human classifiers yields a measure of the fraction of\ndistorted morphologies in the AGN and control samples, i.e., quantifying\nthe signature of recent mergers which might potentially be responsible\nfor fueling/triggering the AGN. We find that (1) the vast majority\n(>85%) of the AGN host galaxies do not show strong distortions and\n(2) there is no significant difference in the distortion fractions\nbetween active and inactive galaxies. Our findings provide the best\ndirect evidence that, since z ~ 1, the bulk of black hole (BH) accretion\nhas not been triggered by major galaxy mergers, therefore arguing that\nthe alternative mechanisms, i.e., internal secular processes and minor\ninteractions, are the leading triggers for the episodes of major BH\ngrowth. We also exclude an alternative interpretation of our results: a\nsubstantial time lag between merging and the observability of the AGN\nphase could wash out the most significant merging signatures, explaining\nthe lack of enhancement of strong distortions on the AGN hosts. We show\nthat this alternative scenario is unlikely due to (1) recent major\nmergers being ruled out for the majority of sources due to the high\nfraction of disk-hosted AGNs, (2) the lack of a significant X-ray signal\nin merging inactive galaxies as a signature of a potential buried AGN,\nand (3) the low levels of soft X-ray obscuration for AGNs hosted by\ninteracting galaxies, in contrast to model predictions.\n\nBased on observations with the NASA/ESA Hubble Space Telescope, obtained\nat the Space Telescope Science Institute, which is operated by AURA\nInc., under NASA contract NAS 5-26555; the XMM-Newton, an ESA science\nmission with instruments and contributions directly funded by ESA Member\nStates and NASA; European Southern Observatory under Large Program\n175.A-0839; and the Subaru Telescope, which is operated by the National\nAstronomical Observatory of Japan.

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