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 a 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 mu(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.