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 _ { H } \geq 10 ^ { 24 } cm ^ { -2 } ) AGN in late merger galaxies is higher ( f _ { CT } = 65 ^ { +12 } _ { -13 } \% ) than in local hard X-ray selected AGN ( f _ { 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 } \simeq 0.4 - 10.8 kiloparsecs ( f _ { CT } = 77 _ { -17 } ^ { +13 } \% ) .
We also find that all AGN of our sample in late merger galaxies have N _ { H } > 10 ^ { 23 } 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 can not be explained by the classical AGN unification model in which the torus is responsible for the obscuration .