Black Holes and AGN
I have extensively investigated the properties of circumnuclear medium in accreting supermassive black hoes (Active Galactic Nuclei, AGNs) especially in the context of the Unified Model, according to which the observational properties of the main classes of AGNs can be explained in terms of orientation of our line of sight relative to an obscuring (~axysimmetric, possibly toroidal) structure.
By analyzing the properties of a well defined sample of local AGN, we proposed for the first time the need for two absorbing structures, one on small scales (~1 pc), unrelated relative to the host galaxy, responsible for the heavy obscuration of the nucleus, and a second one on large scales (~100 pc), aligned with the galaxy disc, and responsible for milder absorption and obscuration (Maiolino & Rieke 1995).
By exploiting some of the first sensitive hard-X surveys, we could establish for the distribution of absorbing column densities around AGNs, revealing for the first time a large population AGNs absorbed by extremely large column densities ("Compton thick AGNs", Maiolino et al. 1998, Risaliti, Maiolino & Salvati 1999, Bassani, Dadina, Maiolino et al. 1999).
By comparing the obscuration in the optical with the X-ray absorption, I found that the dust absorption in the optical is much lower than expected from the column of gas inferred from the X-rays. I inferred that either the absorbing gas must be dust-poor or that the dust properties must be different than in the diffuse intestellar medium (Maiolino et al. 2001a, Maiolino et al. 2001b). Subsequent studies found evidence for both scenarios. In particular, X-ray monitoring of AGNs revealing variable absorption has revealed that a significant fraction of the absorbing medium must reside within the "dust sublimation radius", close to the accretion disc, where dust cannot survive. By analyzing in detail the variation of the X-ray absorption I could study in detail the morphology and the physics of the clouds orbiting and eclipsing the black hole accretion disc. The data have revealed that such clouds have a cometary shape and that, by loosing large amount of matter in the tail, they have a lifetime of only a few months and dissolve completely afterwards (Maiolino et al. 2010).
On larger scales I could measure the distribution of dust (outside the sublimation radius) and in particular its covering factor relative to the accreting black hole, by measuring the emission of hot dust (observed in the mid-infrared) relative to the primary radiation emitted by the accretion disc; I inferred that the dust covering factor decreases with AGN luminosity (Maiolino et al. 2007). By using hard X-ray observations I also found evidence for a population of AGNs that do not show any signature of nuclear activity at optical wavelengths, probably because completely embedded and covered by dust; such objects were dubbed "Elusive AGNs" (Maiolino et al. 2003), and subsequent surveys have found several additional examples of this population, also at high redshift.