  Ultra-faint dwarf galaxies (UFDs) are the faintest known galaxies and due to
their incredibly low surface brightness, it is difficult to find them beyond
the Local Group. We report a serendipitous discovery of an UFD, Fornax UFD1, in
the outskirts of NGC 1316, a giant galaxy in the Fornax cluster. The new galaxy
is located at a projected radius of 55 kpc in the south-east of NGC 1316. This
UFD is found as a small group of resolved stars in the Hubble Space Telescope
images of a halo field of NGC 1316, obtained as part of the Carnegie-Chicago
Hubble Program. Resolved stars in this galaxy are consistent with being mostly
metal-poor red giant branch (RGB) stars. Applying the tip of the RGB method to
the mean magnitude of the two brightest RGB stars, we estimate the distance to
this galaxy, 19.0 +- 1.3 Mpc. Fornax UFD1 is probably a member of the Fornax
cluster. The color-magnitude diagram of these stars is matched by a 12 Gyr
isochrone with low metallicity ([Fe/H] ~ -2.4). Total magnitude and effective
radius of Fornax UFD1 are Mv ~ -7.6 +- 0.2 mag and r_eff = 146 +- 9 pc, which
are similar to those of Virgo UFD1 that was discovered recently in the
intracluster field of Virgo by Jang & Lee (2014).Fornax UFD1 is the most
distant known UFD that is confirmed by resolved stars. This indicates that UFDs
are ubiquitous and that more UFDs remain to be discovered in the Fornax
cluster.
