A new Milky Way halo star cluster in the Southern Galactic Sky

TL;DR

This paper reports the discovery of a new faint Milky Way star cluster in the southern sky, characterized by its old age, low mass, and typical globular cluster properties, identified through SDSS data and follow-up observations.

Contribution

The paper introduces a new faint star cluster in the Milky Way halo, identified via automated overdensity searches and confirmed with follow-up imaging, expanding the known low-mass cluster population.

Findings

Located at 31.9 kpc from the Sun.

Has a half-light radius of approximately 7.24 pc.

Estimated absolute magnitude of MV = -1.21.

Abstract

We report on the discovery of a new Milky Way companion stellar system located at (RA, Dec) = (22h10m43.15s, +14:56:58.8). The discovery was made using the eighth data release of SDSS after applying an automated method to search for overdensities in the Baryon Oscillation Spectroscopic Survey footprint. Follow-up observations were performed using CFHT-MegaCam, which reveal that this system is comprised of an old stellar population, located at a distance of 31.9+1.0-1.6 kpc, with a half-light radius of r_h = 7.24+1.94-1.29 pc and a concentration parameter of c = 1.55. A systematic isochrone fit to its color-magnitude diagram resulted in log(age) = 10.07+0.05-0.03 and [Fe/H] = -1.58+0.08-0.13 . These quantities are typical of globular clusters in the MW halo. The newly found object is of low stellar mass, whose observed excess relative to the background is caused by 96 +/- 3 stars. The direct integration of its background decontaminated luminosity function leads to an absolute magnitude of MV = -1.21 +/- 0.66. The resulting surface brightness is uV = 25.9 mag/arcsec2 . Its position in the M_V vs. r_h diagram lies close to AM4 and Koposov 1, which are identified as star clusters. The object is most likely a very faint star cluster - one of the faintest and lowest mass systems yet identified.