# The Carnegie-Chicago Hubble Program: Discovery of the Most Distant Ultra-faint Dwarf Galaxy in the Local Universe

## TL;DR

This paper reports the discovery of the most distant ultra-faint dwarf galaxy, Fornax UFD1, in the Fornax cluster, demonstrating that such faint galaxies are more common and detectable at greater distances than previously known.

## Contribution

The discovery of Fornax UFD1 as the most distant resolved ultra-faint dwarf galaxy, expanding understanding of their distribution and properties in galaxy clusters.

## Key findings

- Fornax UFD1 is at 19.0 Mpc, the most distant resolved UFD.
- Its properties are similar to Virgo UFD1, with Mv ~ -7.6 and radius ~146 pc.
- UFDs are more ubiquitous and detectable in galaxy clusters than previously thought.

## Abstract

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.