Hiding in plain sight: record-breaking compact stellar systems in the Sloan Digital Sky Survey

TL;DR

This paper reports the discovery of two ultra-compact stellar systems in the Sloan Digital Sky Survey, including the densest known galaxy and the densest free-floating stellar system, shedding light on galaxy assembly and nuclear remnants.

Contribution

The study presents the identification of two extreme stellar systems, expanding the catalog of such objects and providing insights into galaxy evolution and nuclear star cluster properties.

Findings

M59-UCD3 is the densest known galaxy with a half-light radius of ~20 pc and higher luminosity.

M85-HCC1 is the densest free-floating stellar system, comparable to nuclear star clusters.

Both objects are relatively young and show signs of recent galaxy interactions.

Abstract

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.