  Ultracompact dwarf galaxies (UCDs) are among the densest stellar systems in
the universe. These systems have masses up to 200 million solar masses, but
half light radii of just 3-50 parsecs. Dynamical mass estimates show that many
UCDs are more massive than expected from their luminosity. It remains unclear
whether these high dynamical mass estimates are due to the presence of
supermassive black holes or result from a non-standard stellar initial mass
function that causes the average stellar mass to be higher than expected. Here
we present the detection of a supermassive black hole in a massive UCD.
Adaptive optics kinematic data of M60-UCD1 show a central velocity dispersion
peak above 100 km/s and modest rotation. Dynamical modeling of these data
reveals the presence of a supermassive black hole with mass of 21 million solar
masses. This is 15% of the object's total mass. The high black hole mass and
mass fraction suggest that M60-UCD1 is the stripped nucleus of a galaxy. Our
analysis also shows that M60-UCD1's stellar mass is consistent with its
luminosity, implying many other UCDs may also host supermassive black holes.
This suggests a substantial population of previously unnoticed supermassive
black holes.
