Discovery of a Faint Old Stellar System at 150 kpc

We report the detection of a faint old stellar system at (\, \) = (194. 29^\, ~34. 32^\) (SDSS J1257+3419), based on the spatial distribution of bright red-giant branch stars in the Sloan Digital Sky Survey Data Release 4. SDSS J1257+3419 has a half-light radius of 38\ 12 pc and an absolute integrated V-magnitude of MV=-4. 8^+1. 4-₁. ₀ mag at a heliocentric distance of 150\ 15 kpc. A comparison between SDSS J1257+3419 and known Galactic halo objects suggests that SDSS J1257+3419 is either (a) a faint and small dwarf galaxy or (b) a faint and widely extended globular cluster. In the former case, SDSS J1257+3419 could represent an entity of a postulated subhalo of the Milky Way. Further photometric and dynamical study of this stellar system is vital to discriminate these possibilities.