We report the discovery of a new ultra-faint dwarf satellite companion of the Milky Way based on the early survey data from the Hyper Suprime-Cam Subaru Strategic Program .
This new satellite , Virgo I , which is located in the constellation of Virgo , has been identified as a statistically significant ( 5.5 \sigma ) spatial overdensity of star-like objects with a well-defined main sequence and red giant branch in their color-magnitude diagram .
The significance of this overdensity increases to 10.8 \sigma when the relevant isochrone filter is adopted for the search .
Based on the distribution of the stars around the likely main sequence turn-off at r \sim 24 mag , the distance to Virgo I is estimated as 87 kpc , and its most likely absolute magnitude calculated from a Monte Carlo analysis is M _ { V } = -0.8 \pm 0.9 mag .
This stellar system has an extended spatial distribution with a half-light radius of 38 ^ { +12 } _ { -11 } pc , which clearly distinguishes it from a globular cluster with comparable luminosity .
Thus , Virgo I is one of the faintest dwarf satellites known and is located beyond the reach of the Sloan Digital Sky Survey .
This demonstrates the power of this survey program to identify very faint dwarf satellites .
This discovery of Virgo I is based only on about 100 square degrees of data , thus a large number of faint dwarf satellites are likely to exist in the outer halo of the Milky Way .