A census of X-ray nuclear activity in nearby galaxies

We have studied the X-ray nuclear activity of 187 nearby (distance < 15 Mpc) galaxies observed with Chandra/ACIS. We found that 86 of them have a point-like X-ray core, consistent with an accreting BH. We argue that the majority of them are nuclear BHs, rather than X-ray binaries. The fraction of galaxies with an X-ray detected nuclear BH is higher (≈ 60 per cent) for ellipticals and early-type spirals (E to Sb), and lower (≈ 30 per cent) for late-type spirals (Sc to Sm). There is no preferential association of X-ray cores with the presence of a large-scale bar; in fact, strongly barred galaxies appear to have slighly lower detection fraction and luminosity for their nuclear X-ray sources, compared with non-barred or weakly barred galaxies of similar Hubble types. The cumulative luminosiy distribution of the nuclear sources in the 0.3–8 keV band is a power-law with slope ≈ −0.5, from ≈ 2×10 erg s to ≈ 10 erg s. The Eddington ratio is lower for ellipticals (LX/LEdd ∼ 10 ) and higher for late-type spirals (up to LX/LEdd ∼ 10 ), but in all cases, the accretion rate is low enough to be in the radiatively-inefficient regime. The intrinsic absorbing column density is generally low, especially for the less luminous sources: there appear to be no Type-2 nuclear BHs at luminosities . 10 erg s. The lack of a dusty torus or of other sources of intrinsic absorption (e.g., an optically-thick disk wind) may be directly related to the lack of a standard accretion disk around those faint nuclear BHs. The fraction of obscured sources increases with the nuclear BH luminosity: two thirds of the sources with LX > 10 40 erg s have a fitted column density > 10 cm. This is contrary to the declining trend of the obscured fraction with increasing luminosities, observed in more luminous AGN and quasars. Subject headings: X-rays: galaxies—galaxies: nuclei—galaxies: statistics—galaxies: active