A new milky way halo star cluster in the southern galactic sky

We report on the discovery of a new Milky Way (MW) companion stellar system located at (α_{J 2000}, δ_{J 2000}) = (22^h10^m43.^s15, 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 Canada–France–Hawaii Telescope/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 = log₁₀(r_t/r_c) = 1.55. Asystematic isochrone fit to its color–magnitude diagram resulted in log (age yr⁻¹) =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 95 ± 6 stars. The direct integration of its background decontaminated luminosity function leads to an absolute magnitude of M_V = −1.21 ± 0.66. The resulting surface brightness is μ_V = 25.90 mag arcsec⁻². Its position in the M_V versus 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.