Hiding in plain sight: record-breaking compact stellar systems in the
  Sloan Digital Sky Survey
  Motivated by the recent, serendipitous discovery of the densest known galaxy,
M60-UCD1, we present two initial findings from a follow-up search, using the
Sloan Digital Sky Survey, Subaru/Suprime-Cam and Hubble Space Telescope
imaging, and SOuthern Astrophysical Research (SOAR)/Goodman spectroscopy. The
first object discovered, M59-UCD3, has a similar size to M60-UCD1 (half-light
radius of r_h ~ 20 pc) but is 40% more luminous (M_V ~ -14.6), making it the
new densest-known galaxy. The second, M85-HCC1, has a size like a typical
globular cluster (GC; r_h ~ 1.8 pc) but is much more luminous (M_V ~ -12.5).
This hypercompact cluster is by far the densest confirmed free-floating stellar
system, and is equivalent to the densest known nuclear star clusters. From
spectroscopy, we find that both objects are relatively young (~9 Gyr and ~3
Gyr, respectively), with metal-abundances that resemble those of galaxy
centers. Their host galaxies show clear signs of large-scale disturbances, and
we conclude that these dense objects are the remnant nuclei of recently
accreted galaxies. M59-UCD3 is an ideal target for follow-up with
high-resolution imaging and spectroscopy to search for an overweight central
supermassive black hole as was discovered in M60-UCD1. These findings also
emphasize the potential value of ultra-compact dwarfs and massive GCs as
tracers of the assembly histories of galaxies.
