A Census of X-ray Nuclear Activity in Nearby Galaxies

We have studied the X-ray nuclear activity of 187 nearby (distance less than 15 Mpc) galaxies observed with Chandra/ACIS. We found that 86 of them have a pointlike X-ray core, consistent with an accreting black hole (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%) for ellipticals and early type spirals (E to Sb), and lower (≈30%) 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 slightly lower detection fraction and luminosity for their nuclear X-ray sources, compared with nonbarred or weakly barred galaxies of similar Hubble types. The cumulative luminosity distribution of the nuclear sources in the 0.3-8 keV band is a power law with slope ≈ -0.5, from ≈2 × 10³⁸ erg s^-1 to ≈10⁴² erg s^-1. The Eddington ratio is lower for ellipticals (L _X/L _Edd ~ 10^-8) and higher for late-type spirals (up to L _X/L _Edd ~ 10^-4), 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 lsim10³⁹ erg s^-1. 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 L _X > 10⁴⁰ erg s^-1 have a fitted column density greater than 10²² cm^-2. This is in contrast to the declining trend of the obscured fraction with increasing luminosities, observed in more luminous active galactic nuclei and quasars.