Growing supermassive black holes in the late stages of galaxy mergers are heavily obscured

Abstract

Mergers of galaxies are thought to cause significant gas inflows to the inner\nparsecs, which can activate rapid accretion onto supermassive black holes\n(SMBHs), giving rise to Active Galactic Nuclei (AGN). During a significant\nfraction of this process, SMBHs are predicted to be enshrouded by gas and dust.\nStudying 52 galactic nuclei in infrared-selected local Luminous and\nUltra-luminous infrared galaxies in different merger stages in the hard X-ray\nband, where radiation is less affected by absorption, we find that the amount\nof material around SMBHs increases during the last phases of the merger. We\nfind that the fraction of Compton-thick (CT, $N_{\\rm\\,H}\\geq\n10^{24}\\rm\\,cm^{-2}$) AGN in late merger galaxies is higher\n($f_{\\rm\\,CT}=65^{+12}_{-13}\\%$) than in local hard X-ray selected AGN\n($f_{\\rm\\,CT}=27\\pm 4\\%$), and that obscuration reaches its maximum when the\nnuclei of the two merging galaxies are at a projected distance of\n$D_{12}\\simeq0.4-10.8$ kiloparsecs ($f_{\\rm\\,CT}=77_{-17}^{+13}\\%$). We also\nfind that all AGN of our sample in late merger galaxies have $N_{\\rm\\,H}>\n10^{23}\\rm\\,cm^{-2}$, which implies that the obscuring material covers\n$95^{+4}_{-8}\\%$ of the X-ray source. These observations show that the material\nis most effectively funnelled from the galactic scale to the inner tens of\nparsecs during the late stages of galaxy mergers, and that the close\nenvironment of SMBHs in advanced mergers is richer in gas and dust with respect\nto that of SMBHs in isolated galaxies, and cannot be explained by the classical\nAGN unification model in which the torus is responsible for the obscuration.\n