Central X-ray point-sources found to be abundant in low-mass, late-type
  galaxies predicted to contain an intermediate-mass black hole
  Building upon three late-type galaxies in the Virgo cluster with both a
predicted black hole mass of less than $\sim$10$^5$ M$_{\odot}$ and a
centrally-located X-ray point-source, we reveal 11 more such galaxies, more
than tripling the number of active intermediate-mass black hole candidates
among this population. Moreover, this amounts to a 36$\pm$8% X-ray detection
rate (despite the sometimes high, X-ray-absorbing, HI column densities),
compared to just 10$\pm$5% for (the largely HI-free) dwarf early-type galaxies
in the Virgo cluster. The expected contribution of X-ray binaries from the
galaxies' inner field stars is negligible. Moreover, given that both the spiral
and dwarf galaxies contain nuclear star clusters, the above inequality appears
to disfavor X-ray binaries in nuclear star clusters. The higher occupation, or
rather detection, fraction among the spiral galaxies may instead reflect an
enhanced cool gas/fuel supply and Eddington ratio. Indeed, four of the 11 new
X-ray detections are associated with known LINERs or LINER/HII composites. For
all (four) of the new detections for which the X-ray flux was strong enough to
establish the spectral energy distribution in the Chandra band, it is
consistent with power-law spectra. Furthermore, the X-ray emission from the
source with the highest flux (NGC 4197: $L_X \approx 10^{40}$ erg s$^{-1}$)
suggests a non-stellar-mass black hole if the X-ray spectrum corresponds to the
`low/hard state'. Follow-up observations to further probe the black hole
masses, and prospects for spatially resolving the gravitational
spheres-of-influence around intermediate-mass black holes, are reviewed in some
detail.
