  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.

Astrophysics