Discovery of an Ultra-Faint Dwarf Galaxy in the Intracluster Field of
  the Virgo Center : A fossil of the First Galaxies?
  Ultra-faint dwarf galaxies (UFDs) are newcomers among galaxies, and are the
faintest galaxies in the observed universe. To date, they have only been found
around the Milky Way Galaxy and M31 in the Local Group. We present the
discovery of an UFD in the intracluster field in the core of the Virgo cluster
(Virgo UFD1), which is far from any massive galaxies. The color-magnitude
diagram of the resolved stars in this galaxy shows a narrow red giant branch,
similar to those of metal-poor globular clusters in the Milky Way. We estimate
its distance by comparing the red giant branch with isochrones, and we obtain a
value 16.4 +/- 0.4 Mpc. This shows that it is indeed a member of the Virgo
cluster. From the color of the red giants we estimate its mean metallicity to
be very low, [Fe/H]= -2.4 +/- 0.4. Its absolute V-band magnitude and effective
radius are derived to be M_V = -6.5 +/- 0.2 and r_eff = 81 +/- 7 pc, much
fainter and smaller than the classical dwarf spheroidal galaxies. Its central
surface brightness is estimated to be as low as u_V,0 = 26.37 +/- 0.05 mag
arcsec^-2. Its properties are similar to those of the Local Group analogs. No
evidence of tidal features are found in this galaxy. Considering its narrow red
giant branch with no asymptotic giant branch stars, low metallicity, and
location, it may be a fossil remnant of the first galaxies.
