Two Record-Breaking Compact Stellar Systems

Following the discovery of the densest known galaxy, M60-UCD1, Sandoval et al. (2015) present the discovery of M59-UCD3 and M85-HCC1 - two record-breaking compact stellar systems. Since the density of stars in a galaxy decreases gradually away from the center, one way to express the size of a galaxy is by its half-light radius. The half-light radius is basically the size of the volume of space that is contributing to half the galaxy’s brightness and it can also apply to other stellar systems.

M59-UCD3 is an ultracompact dwarf (UCD) galaxy similar in size to M60-UCD1. It has a half-light radius of roughly 70 light years, but it is 40 percent more luminous than M60-UCD1. This makes M59-UCD3 the new densest known galaxy. M59-UCD3 is estimated to contain roughly ~180 million times the Sun’s mass and this means an average density of roughly 30 solar masses per volume of space one light year across. For comparison, an observer on Earth can see ~6,000 stars with unaided eyes under “typical” dark sky conditions. However, an observer in the core of M59-UCD3 would see roughly a million stars in the sky.

M85-HCC1 is an extremely compact stellar system with a half-light radius of roughly 6 light years, similar in size to a typical globular cluster. It is estimated to contain ~12 million times the Sun’s mass and has an average density of roughly 3,000 solar masses per volume of space one light year across. This is ~100 times the density of M59-UCD3. For comparison, the nearest star to the Sun is Proxima Centauri, located 4.24 light years away.

M59-UCD3 and M85-HCC1 are estimated to be ~9 billion and ~3 billion years old, respectively. They are most likely the remnant cores of what were once much larger galaxies that got tidally-stripped. This scenario can be tested for M59-UCD3 by looking to see if it contains an “overweight” supermassive black hole (SMBH) since almost all massive galaxies host SMBHs. M60-UCD1, the previous record holder for the densest known galaxy, is known to host an “overweight” SMBH comprising a whopping ~15 percent of the galaxy’s total mass.

References:
- Sandoval et al. (2015), “Hiding in plain sight: record-breaking compact stellar systems in the Sloan Digital Sky Survey”, arXiv:1506.08828 [astro-ph.GA]
- 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]