Accretion and Nuclear Activity of Quiescent Supermassive Black Holes. II. Optical Study and Interpretation

Our X-ray study of the nuclear activity in a new sample of six quiescentearly-type galaxies, and in a larger sample from the literature, confirmed(Soria et al., Paper I) that the Bondi accretion rate of diffuse hot gas is nota good indicator of the supermassive black hole (SMBH) X-ray luminosity. Herewe suggest that a more reliable estimate of the accretion rate must include thegas released by the stellar population inside the sphere of influence of theSMBH, in addition to the Bondi inflow of hot gas across that surface. We useoptical surface-brightness profiles to estimate the mass-loss rate from starsin the nuclear region: we show that for our sample of galaxies it is an orderof magnitude higher (~ 10^{-4} - 10^{-3} M_sun/yr) than the Bondi inflow rateof hot gas, as estimated from Chandra (Paper I). Only by taking into accountboth sources of fuel can we constrain the true accretion rate, the accretionefficiency, and the power budget. Radiatively efficient accretion is ruled out,for quiescent SMBHs. For typical radiatively inefficient flows, the observedX-ray luminosities of the SMBHs imply accretion fractions ~ 1 - 10% (ie, ~ 90 -99% of the available gas does not reach the SMBH) for at least five of our sixtarget galaxies, and most of the other galaxies with known SMBH masses. Wediscuss the conditions for mass conservation inside the sphere of influence, sothat the total gas injection is balanced by accretion plus outflows. We showthat a fraction of the total accretion power (mechanical plus radiative) wouldbe sufficient to sustain a self-regulating, slow outflow which removes from thenuclear region all the gas that does not sink into the BH (``BH feedback'').The rest of the accretion power may be carried out in a jet, or advected. Wealso discuss scenarios that would lead to an intermittent nuclear activity.Comment: 15 pages, accepted by ApJ. See also our Paper I, astro-ph/051129