Author ’ s version of paper submitted to Nature June 21st , accepted July 29th , to appear in Sept. 18th issue at http : //dx.doi.org/10.1038/nature13762

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 [ 1 ] .
Dynamical mass estimates show that many UCDs are more massive than expected from their luminosity [ 2 ] .
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 [ 3 , 4 ] .
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 .