We report on the discovery of a faint ( M _ { V } \sim - 10.6 \pm 0.2 ) dwarf spheroidal galaxy on deep F606W and F814W Hubble Space Telescope images of a Virgo intracluster field .
The galaxy is easily resolved in our images , as our color magnitude diagram ( CMD ) extends \gtrsim 1 magnitude beyond the tip of the red giant branch ( RGB ) .
Thus , it is the deepest CMD for a small dwarf galaxy inside a cluster environment .
Using the colors of the RGB stars , we derive a metal abundance for the dwarf of [ M/H ] = -2.3 \pm 0.3 , and show that the metallicity dispersion is less than 0.6 dex at 95 % confidence .
We also use the galaxy ’ s lack of AGB stars and the absence of objects brighter than M _ { bol } \sim - 4.1 \pm 0.2 to show that the system is old ( t \gtrsim 10 Gyr ) .
Finally , we derive the object ’ s structural parameters , and show that the galaxy displays no obvious evidence of tidal threshing .
Since the tip of the red giant branch distance ( ( m - M ) _ { 0 } = 31.23 \pm 0.17 or D = 17.6 \pm 1.4 Mpc ) puts the galaxy near the core of the Virgo cluster , one might expect the object to have undergone some tidal processing .
Yet the chemical and morphological similarity between the dwarf and the dSph galaxies of the Local and M81 Group demonstrates that the object is indeed pristine , and not the shredded remains of a much larger galaxy .
We discuss the possible origins of this galaxy , and suggest that it is just now falling into Virgo for the first time .