Discovery of a Faint Old Stellar System at 150 kpc

Abstract

We report the detection of a faint old stellar system at (SDSS J1257ϩ3419), based on (a, d) p (194Њ .29, 34Њ .32) the spatial distribution of bright red giant branch stars in the Sloan Digital Sky Survey Data Release 4. SDSS J1257ϩ3419 has a half-light radius of pc and an absolute integrated V magnitude of mag ϩ1.4 38 ‫ע‬ 12 M p Ϫ4.8 V Ϫ1.0 at a heliocentric distance of kpc. A comparison between SDSS J1257ϩ3419 and known Galactic halo 150 ‫ע‬ 15 objects suggests that SDSS J1257ϩ3419 is either (1) a faint and small dwarf galaxy or (2) a faint and widely extended globular cluster. In the former case, SDSS J1257ϩ3419 could represent an entity of a postulated subhalo of the Milky Way. Further photometric and dynamical study of this stellar system is vital to distinguish these possibilities.