The Hubble Missing Globular Cluster Survey. IV. Ultra-faint compact satellites of the Milky Way: The case of Koposov 2

In recent decades, large panoramic surveys have discovered a number of extremely faint and compact Galactic satellites, known as ultra faint compact satellites (UFCS). The nature of these stellar systems is uncertain, both because they occupy a region of the M_V- _ plane where dwarf galaxies and star clusters overlap, and because of the observational challenges inherent to their faintness and distance. Here we show how deep HST photometry from the Missing Globular Clusters Survey (MGCS) can provide new insight into the nature of these satellites, especially when coupled with spectroscopic metallicity measurements, by providing unrivalled accurate estimates of their distance and age. As an example, we considered the intriguing case of Koposov 2, currently the most metal-poor bound star cluster known in the entire Milky Way, or an extreme case of an ultra-faint dwarf galaxy. By performing a spectroscopically informed Bayesian isochrone fit on the MGCS data we find $(m-M)_0=16.85± 0.06$ (D=23.4± 0.6 kpc) and age=13.7^ +0.9 -1.3 Gyr, showing that, contrary to previous age estimates, Koposov 2 is as old as the oldest Galactic GCs. The luminosity function, corrected for incompleteness, is well reproduced by a model with the same age and metallicity, and a slope of the mass function x=-0.35, suggesting a significant depletion of faint stars. We also modelled the surface stellar density field, deriving new robust estimates of the half-light radius (Rh=0.39^ +0.06 _ -0.04 arcmin, corresponding to Rh=2.7^ +0.4 _ -0.3 pc), the absolute integrated magnitude (M_V=-0.95 ± 0.22), and the stellar mass (M_⋆=371.8± 41.6 M_ ), showing that Koposov 2 is much more compact than confirmed dwarf galaxies of similar stellar mass. In summary, the new observational evidence provides significant support for the hypothesis that Koposov 2 is a star cluster that may have lost a large fraction of its original mass. Finally, we show that most UFCSs lie in the same locus of the M_⋆ - plane as Galactic open clusters, hinting at a possible additional channel for their formation.