A New Milky Way Companion: Unusual Globular Cluster or Extreme Dwarf Satellite?

We report the discovery of SDSSJ1049+5103, an overdensity of resolved bluestars at (\alpha_{2000}, \delta_{2000}) = (162.343, 51.051). This objectappears to be an old, metal-poor stellar system at a distance of 45 +/- 10 kpc,with a half-light radius of 23$\pm 10$ pc and an absolute magnitude of M_V =-3.0^{+2.0}_{-0.7}. One star that is likely associated with this companion hasan SDSS spectrum confirming it as a blue horizontal branch star at 48 kpc. Thecolor-magnitude diagram of SDSSJ1049+5103 contains few, if any, horizontal orred giant branch stars, similar to the anomalously faint globular cluster AM 4.The size and luminosity of SDSSJ1049+5103 places it at the intersection of thesize-luminosity relationships followed by known globular clusters and by MilkyWay dwarf spheroidals. If SDSSJ1049+5103 is a globular cluster, then itsproperties are consistent with the established trend that the largest radiusGalactic globular clusters are all in the outer halo. However, the five knownglobular clusters with similarly faint absolute magnitudes all have half-massradii that are smaller than SDSSJ1049+5103 by a factor of $\gtrsim$ 5. If it isa dwarf spheroidal, then it is the faintest yet known by two orders ofmagnitude, and is the first example of the ultra-faint dwarfs predicted by sometheories. The uncertain nature of this new system underscores the sometimesambiguous distinction between globular clusters and dwarf spheroidals. A simplefriends-of-friends search for similar blue, small scalesize star clustersdetected all known globulars and dwarfs closer than 50 kpc in the SDSS area,but yielded no other candidates as robust as SDSSJ1049+5103.Comment: Replaced with AJ accepted versio