2010ApJ...717..640P

Nuclear hard X-ray luminosities (L_X,nuc) for a sample of 112 early-type galaxies within a distance of 67 Mpc are used to investigate their relationship with the central galactic black hole mass M_BH(coming from direct dynamical studies or the M_BH-σ relation), the inner galactic structure (using the parameters describing its cuspiness), the hot gas content, and the core radio luminosity. For this sample, L_X,nuc ranges from 10³⁸ to 10⁴² erg./s, and the Eddington ratio L_X,nuc/L_Edd from 10^–9 to 10^–4, with the largest values belonging to four Seyfert galaxies. Together with a trend for L_X,nuc to increase on average with the galactic luminosity L_Band M_BH, there is a wide variation of L_X,nuc(and L_X,nuc/L_Edd), by up to 4 orders of magnitude, at any fixed L_B> 6x10⁹ L_B,☉or M_BH> 10⁷ M_☉. This large observed range should reflect a large variation of the mass accretion rate {dot}M_BH, and possible reasons for this difference are searched for. On the circumnuclear scale, in a scenario where accretion is (quasi) steady, {dot}M_BH at fixed L_B(or M_BH) could vary due to differences in the fuel production rate from stellar mass return linked to the inner galactic structure; a trend of L_X,nuc with cuspiness is not observed, though, while a tendency for L_X,nuc/L _Edd to be larger in cuspier galaxies is present. In fact, {dot}M_BH is predicted to vary with cuspiness by a factor exceeding a few only in hot gas-poor galaxies and for large differences in the core radius; for a subsample with these characteristics the expected effect seems to be present in the observed L_X,nucvalues. L_X,nuc does not show a dependence on the age of the stellar population in the central galactic region, for ages >3 Gyr; less luminous nuclei, though, are found among the youngest galaxies or galaxies with a younger stellar component. On the global galactic scale, L_X,nuc shows a trend with the total galactic hot gas cooling rate (L_ X,ISM_): it is detected both in gas-poor and gas-rich galaxies, and on average increases with L_X,ISM, but again with a large scatter. The observed lack of a tight relationship between L_X,nuc and the circumnuclear and total gas content can be explained if accretion is regulated by factors overcoming the importance of fuel availability, as (1) the gas is heated by black hole feedback and {dot}M_BH varies due to an activity cycle, and (2) the mass effectively accreted by the black hole can be largely reduced with respect to that entering the circumnuclear region, as in radiatively inefficient accretion with winds/outflows. Finally, differently from L_X,nuc, the central 5 GHz VLA luminosity shows a clear trend with the inner galactic structure that is similar to that shown by the total soft X-ray emission; therefore, it is suggested that they could both be produced by the hot gas.