A Compact Galaxy and its Supermassive Black Hole

An ultra-compact dwarf (UCD) is a type of galaxy whose size and mass is between that of globular clusters and compact elliptical galaxies. Two mechanisms have been proposed to explain the formation of UCDs. In the first scenario, UCDs are actually the most massive globular clusters. In the second scenario, UCDs are either extremely compact galaxies that formed in dense dark matter halos or the leftover compact cores of more massive galaxies that have been tidally-stripped.

M60-UCD1 is an extraordinary UCD located near the massive elliptical galaxy M60, about 54 million light years from Earth. It was discovered from imagery taken by NASA’s Hubble Space Telescope, with follow-up observations by NASA’s Chandra X-ray Observatory and ground-based telescopes. M60-UCD1 is the most luminous UCD known and also one of the most massive. What is remarkable about M60-UCD1 is its compactness. M60-UCD1 contains 200 million times the Sun’s mass. Half of this mass is concentrated within a central sphere only 80 light years in radius.

Figure 1: An artist impression of the supermassive black hole located at the center of M60-UCD1. Credit: NASA, ESA, D. Coe, G. Bacon (STScI).

Figure 2: An image taken by NASA’s Hubble Space Telescope showing M60-UCD1 near the giant elliptical galaxy M60. Credit: NASA/Space Telescope Science Institute/ESA.

The density of stars in M60-UCD1 is ~20,000 times greater than that found in the Sun’s stellar neighbourhood, which implies stars in this UCD are spaced ~30 times closer to one another. M60-UCD1 is the densest galaxy known in the local Universe and is believed to be the leftover core of a galaxy that underwent tidal stripping. The progenitor galaxy of M60-UCD1 was a much larger galaxy. A couple of billion years ago, it came too close to M60 and got tidally torn apart, leaving behind only its dense core behind.

M60-UCD1 also hosts a supermassive black hole at its core. Its presence is inferred from observations of the velocities of stars at the center of the galaxy. These stars are moving with such high velocities that only a supermassive black hole in the vicinity can keep these stars bound. From dynamical modelling, the supermassive black hole at the center of M60-UCD1 is estimated to have 21 million times the Sun’s mass. This is a remarkable ~15 percent of the total mass of the galaxy’s stars. The large mass of its supermassive black hole is consistant with M60-UCD1 being the tidally-stripped nucleus of what was once a much bigger galaxy.

References:
- Strader et al. (2014), “The Densest Galaxy”, arXiv:1307.7707 [astro-ph.CO]
- Seth et al. (2014), “A Supermassive Black Hole in an Ultracompact Dwarf Galaxy”, arXiv:1409.4769 [astro-ph.GA]