  Nearby galactic nuclei are observed to be very much dimmer than active
galactic nuclei in distant galaxies. The Chandra X-ray Observatory has provided
a definitive explanation for why this is so. With its excellent angular
resolution, Chandra has imaged hot X-ray-emitting gas close to the
gravitational capture radius of a handful of supermassive black holes,
including Sgr A* in the nucleus of our own Galaxy. These observations provide
direct and reliable estimates of the Bondi mass accretion rate in these nuclei.
It is found that the Bondi accretion rate is significantly below the Eddington
mass accretion rate, but this alone does not explain the dimness of the
accretion flows. In all the systems observed so far, the accretion luminosity
is orders of magnitude less than Mdot_{Bondi} times c^2, which means that the
accretion must occur via a radiatively inefficient mode. This conclusion, which
was strongly suspected for many years, is now inescapable. Furthermore, if the
accretion in these nuclei occurs via either a Bondi flow or an
advection-dominated accretion flow, the accreting plasma must be
two-temperature at small radii, and the central mass must have an event
horizon. Convection, winds and jets may play a role, but observations do not
yet permit definite conclusions.
