A new Milk Way halo star cluster in the southern galactic sky

Abstract

We report on the discovery of a new Milky Way (MW) companion stellar system located at (αJ 2000, δJ 2000) = (22h10 m 43 s. 15,14º56'58".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 rh = 7.24+1.94 −1.29 pc and a concentration parameter of c = log10(rt/rc) = 1.55. A systematic isochrone fit to its color–magnitude diagram resulted in log (age yr-ˡ) = 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 MV = −1.21 ± 0.66. The resulting surface brightness is μV = 25.90 mag arcsec-². Its position in the MV versus rh 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.