A search for active galactic nuclei in low-mass compact galaxies

Abstract

ABSTRACT 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−1.