Nuclear Accretion in Galaxies of the Local Universe: Clues from Chandra Observations

In order to find an explanation for the radiative quiescence of supermassive black holes in the local Universe, for a sample of nearby galaxies the most accurate estimates are collected for the mass of a central black hole M_BH, the nuclear X-ray luminosity L_X,nuc and the circumnuclear hot gas density and temperature, by using Chandra data. L_X,nuc varies by \sim 3 orders of magnitude and does not show a relationship with M_BH or with the Bondi mass accretion rate \dotM_B. L_X,nuc is always much lower than expected if \dotM_B ends in a standard accretion disc with high radiative efficiency (this instead can be the case of the active nucleus of Cen A). Radiatively inefficient accretion as in the standard ADAF modeling may explain the low luminosities of a few cases; for others, the predicted luminosity is still too high and, in terms of Eddington-scaled quantities, it is increasingly higher than observed, for increasing \dotM_B. Variants of the simple radiatively inefficient scenario including outflow and convection may reproduce the low emission levels observed, since the amount of matter actually accreted is reduced considerably. However, the most promising scenario includes feedback from accretion on the surrounding gas: this has the important advantages of naturally explaining the observed lack of relationship between L_X,nuc, M_BH and \dotM_B, and of evading the problem of the fate of the material accumulating in the central galactic regions over cosmological times.