Growing supermassive black holes in the late stages of galaxy mergers
  are heavily obscured
  Mergers of galaxies are thought to cause significant gas inflows to the inner
parsecs, which can activate rapid accretion onto supermassive black holes
(SMBHs), giving rise to Active Galactic Nuclei (AGN). During a significant
fraction of this process, SMBHs are predicted to be enshrouded by gas and dust.
Studying 52 galactic nuclei in infrared-selected local Luminous and
Ultra-luminous infrared galaxies in different merger stages in the hard X-ray
band, where radiation is less affected by absorption, we find that the amount
of material around SMBHs increases during the last phases of the merger. We
find that the fraction of Compton-thick (CT, $N_{\rm\,H}\geq
10^{24}\rm\,cm^{-2}$) AGN in late merger galaxies is higher
($f_{\rm\,CT}=65^{+12}_{-13}\%$) than in local hard X-ray selected AGN
($f_{\rm\,CT}=27\pm 4\%$), and that obscuration reaches its maximum when the
nuclei of the two merging galaxies are at a projected distance of
$D_{12}\simeq0.4-10.8$ kiloparsecs ($f_{\rm\,CT}=77_{-17}^{+13}\%$). We also
find that all AGN of our sample in late merger galaxies have $N_{\rm\,H}>
10^{23}\rm\,cm^{-2}$, which implies that the obscuring material covers
$95^{+4}_{-8}\%$ of the X-ray source. These observations show that the material
is most effectively funnelled from the galactic scale to the inner tens of
parsecs during the late stages of galaxy mergers, and that the close
environment of SMBHs in advanced mergers is richer in gas and dust with respect
to that of SMBHs in isolated galaxies, and cannot be explained by the classical
AGN unification model in which the torus is responsible for the obscuration.
