M60-UCD1

M60-UCD1 is an ultracompact dwarf galaxy. It is 49 million light years from Earth, close to Messier 60 (M60, NGC 4649) in the Virgo Cluster. Half of its stellar mass is in the central sphere 160 light years in diameter.

Characteristics


M60-UCD1's dynamical mass is $0.004$ million solar masses. Colours and lack of colour gradients indicate a uniform stellar population about $1,278 km/s$ (indistinguishable from the age of the Universe). Their metallicity is similar to that of the Sun. The orbital velocity dispersion of the innermost stars exceeds 100 km/s, due to the gravity of a dense mass concentration. The galactic nucleus contains a bright and variable X-ray source, presumably a supermassive black hole with a mass of (10% of the dynamical mass of the entire galaxy). With this proportion of the mass of the black hole to that of the whole galaxy, it is one of the most black hole dominated galaxies known.

M60-UCD1 is believed to be the stripped core of a much more massive galaxy, whose mass was stripped in an encounter with M60 some 10 billion years ago. It may yet be absorbed completely by M60, its central black hole merging with M60's as well. The galaxy may have once had some 10 billion stars.

As of 2013, it is possibly the densest known galaxy with over one hundred stars per cubic light-year. As of 2014, it is the smallest and least massive galaxy known to host a central black hole. It was previously known as the most massive ultracompact dwarf galaxy known.