We report the discovery of SDSSJ1049+5103 , an overdensity of resolved blue stars at ( \alpha _ { 2000 } , \delta _ { 2000 } ) = ( 162.343 , 51.051 ) .
This object appears to be an old , metal-poor stellar system at a distance of 45 \pm 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 has an SDSS spectrum confirming it as a blue horizontal branch star at 48 kpc .
The color-magnitude diagram of SDSSJ1049+5103 contains few , if any , horizontal or red 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 the size-luminosity relationships followed by known globular clusters and by Milky Way dwarf spheroidals .
If SDSSJ1049+5103 is a globular cluster , then its properties are consistent with the established trend that the largest radius Galactic globular clusters are all in the outer halo .
However , the five known globular clusters with similarly faint absolute magnitudes all have half-mass radii that are smaller than SDSSJ1049+5103 by a factor of \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 5 .
If it is a dwarf spheroidal , then it is the faintest yet known by two orders of magnitude , and is the first example of the ultra-faint dwarfs predicted by some theories .
The uncertain nature of this new system underscores the sometimes ambiguous distinction between globular clusters and dwarf spheroidals .
A simple friends-of-friends search for similar blue , small scalesize star clusters detected all known globulars and dwarfs closer than 50 kpc in the SDSS area , but yielded no other candidates as robust as SDSSJ1049+5103 .