Massive black holes in stellar systems: 'quiescent' accretion and
  luminosity
  Only a small fraction of local galaxies harbor an accreting black hole,
classified as an active galactic nucleus (AGN). However, many stellar systems
are plausibly expected to host black holes, from globular clusters to nuclear
star clusters, to massive galaxies. The mere presence of stars in the vicinity
of a black hole provides a source of fuel via mass loss of evolved stars. In
this paper we assess the expected luminosities of black holes embedded in
stellar systems of different sizes and properties, spanning a large range of
masses. We model the distribution of stars and derive the amount of gas
available to a central black hole through a geometrical model. We estimate the
luminosity of the black holes under simple, but physically grounded,
assumptions on the accretion flow. Finally we discuss the detectability of
'quiescent' black holes in the local Universe.
