  Low-mass compact galaxies (ultracompact dwarfs [UCDs] and compact ellipticals
[cEs]) populate the stellar size-mass plane between globular clusters and
early-type galaxies. Known to be formed either in-situ with an intrinsically
low mass or resulting from the stripping of a more massive galaxy, the presence
of a supermassive or an intermediate-mass black hole (BH) could help
discriminate between these possible scenarios. With this aim, we have performed
a multiwavelength search of active BH activity, i.e. active galactic nuclei
(AGN), in a sample of 937 low-mass compact galaxies (580 UCDs and 357 cEs).
This constitutes the largest study of AGN activity in these types of galaxies.
Based on their X-ray luminosity, radio luminosity and morphology, and/or
optical emission line diagnostic diagrams, we find a total of 11 cEs that host
an AGN. We also study for the first time the location of both low-mass compact
galaxies (UCDs and cEs) and dwarf galaxies hosting AGN on the BH-galaxy scaling
relations, finding that low-mass compact galaxies tend to be overmassive in the
BH mass-stellar mass plane but not as much in the BH mass-stellar velocity
dispersion correlation. This, together with available BH mass measurements for
some of the low-mass compact galaxies, supports a stripping origin for the
majority of these objects that would contribute to the scatter seen at the
low-mass end of the BH-galaxy scaling relations. However, the differences are
too large to be explained solely by this scatter, and thus our results suggest
that a flattening at such low-masses is also plausible, happening at a velocity
dispersion of ~20-40 km/s.
