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 \pm 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 ] \approx - 2.4 ) .
Total magnitude and effective radius of Fornax UFD1 are M _ { V } \approx - 7.6 \pm 0.2 mag and r _ { eff } = 146 \pm 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 .