Hiding in Plain Sight: Record-breaking Compact Stellar Systems in the Sloan Digital Sky Survey
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 rh ∼ 20 pc) but is 40% more luminous (MV ∼ -14.6), making it the new densest-known galaxy. The second, M85-HCC1, has a size like a typical globular cluster (GC; rh ∼ 1.8 pc) but is much more luminous (MV ∼ -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 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2015
- DOI:
- 10.1088/2041-8205/808/1/L32
- arXiv:
- arXiv:1506.08828
- Bibcode:
- 2015ApJ...808L..32S
- Keywords:
-
- galaxies: fundamental parameters;
- galaxies: nuclei;
- galaxies: star clusters: general;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- ApJ Letters, published, 7 pages, 4 figures. Very galaxies, such dense, wow. Press release with music video: http://www.noao.edu/news/2015/pr1504.php