On the relation between X-ray absorption and optical extinction in AGN

According to the Unified Model of Active Galactic Nuclei (AGN), an X-ray unabsorbed AGN  should appear as unobscured in the optical (Type-1) and viceversa (Type-2). However, there is an important fraction (10-30%) of AGN whose optical and X-ray classifications do not match. To  provide insight into the origin of such apparent discrepancies, we have conducted two  analyses: 1) a detailed study of the UV-to-near-IR emission of two X-ray unabsorbed Type-2 AGN  drawn from the Bright Ultra-Hard XMM-Newton Survey (BUXS); 2) a statistical analysis of the  optical obscuration and X-ray absorption properties of 159 Type-1 AGN drawn from BUXS to determine the distribution of dust-to-gas ratios in AGN over a broad range of luminosities and redshifts. In our works we have also determined the impact of contamination from the AGN hosts in the optical classification of AGNs. Our studies are already provided very exciting results such as the detection of objects with extreme dust-to-gas ratios, between 300-10000 times below the Galactic dust-to-gas ratio.