Monday 19^th January 2009, 4:00 P.M.

Università "La Sapienza" Roma - Aula 7 (Dip. Fisica, Ed. Marconi - Nuovo Edificio)

Title: Partial obscuration of Super Massive Black Holes growing through accretion in Active Galactic Nuclei

Abstract: The X-ray emission (up to 1046 erg/s) from the central region of some galaxies and from the QSO’s is the neatest signature of matter accreting onto Super Massive Black Holes (SMBH: 106–1010 Msun) in the Active Galactic Nuclei (AGN). The Cosmic X-Ray Background (CXRB) represents the integral over the cosmic time of this activity, and contains the fraction of power emitted in this band during the growth of the SMBH currently found in the nuclei of nearby “normal” galaxies. Most of the energy in the CXRB falls between 20 and 30 keV, a fact which demands that a good fraction of the energy emitted at lower energies was photoelectrically absorbed by gas belonging to the “host” galaxies. The “obscuration” phenomenon is well known since the historical, spectroscopic classification in the optical of AGN into Type 1 (unobscured) and Type 2 (obscured). In the currently adopted Unification Model, the obscuration is attributed to molecular gas in a toroidal configuration around the nucleus, on a scale of the order of 1 parsec: the spectral difference between the two Types is attributed to the line-of-sight intercepting (Type 2) or not (Type 1) the “torus”. Observations in hard X-rays (first by BeppoSAX) have shown that, among Type 2 objects, a further distinction is needed: those where the column density of the absorbing gas is smaller (Compton-thin) or greater (Compton-thick) than NH = 1/σT = 1.5x1024 H cm-2. The numerous samples of AGN collected with deep and shallow observations by the satellite Chandra and XMM-Newton (sensitive only at E < 10 keV) contains both Type 1 and Type 2 (Compton-thin) objects: the Compton-thick ones are virtually absent because they are hard to detect below 10 keV. Recent results solidly confirms a finding, that remained controversial for some years: the Fraction of obscured, Compton-thin objects decreases with increasing X-ray Luminosity. In this seminar a model will be presented and discussed, where in these objects the obscuration is mainly attributed to molecular gas distributed in a disk extending out to hundreds of parsec, rather than to the abovemenioned “torus” (whose role remains essential for the Compton-thick objects). It is the principal merit of this model that the anti-correlation finds a natural explanation in the gravitational force exerted by the SMBH on the molecular disk: this force reduces the angular “covering factor” of the disk as a function of the SMBH mass, in such a way that this “factor” decreases as the mass increases. Thus the anti-correlation with the luminosity might not be due to the luminosity itself, as many believe, but might simply reflect the one with the BH mass. Some considerations will be devoted to observations that might resolve the ambiguity between SMBH Mass and Luminosity as the “leading” physical parameter.