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

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 M V ≈ −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.

D1005+68: A New Faint Dwarf Galaxy in the M81 Group

“…rent stellar mass, determined from isochrone fitting, is M * = 2.5 Figure 4 shows d1005+68 in context of Local Group and M81 Group members. d1005+68 is among the faintest confirmed galaxies discovered outside of the Local Group -similar in brightness to M81 group member d0944+69 (Chiboucas et al 2013; M V = −8.05 with no claimed uncertainty), NGC 2403 member MAD-CASH J074238+652501-dw (Carlin et al 2016; M V = −7.7 ± 0.7), Centaurus group member Dw5 (Crnojević et al 2016; M V = −7.2 ± 1.0), and Fornax cluster member Fornax UFD1 (Lee et al 2017; M V = −7.6 ± 0.2) -and probes the very faintest end of the known M81 satellite luminosity function.…”

The Faintest Dwarf Galaxies

“…Since faint dwarfs contain few stars near the tip of the red giant branch, imaging to fainter than 26th magnitude is necessary to identify an ultra-faint dwarf at these distances as an overdensity of resolved stars. In dedicated deep surveys and HST imaging of nearby galaxy clusters, several objects near or below our magnitude limit separating UFDs from dSphs have recently been discovered, including d0944+69 (MV = −6.4; Chiboucas, Karachentsev & Tully 2009;Chiboucas et al 2013) Lee et al 2017) in the Fornax cluster. Low-surface brightness dwarfs in the Local Volume with luminosities in the UFD regime can also be identified via their diffuse light (e.g., Bennet et al 2017;Danieli, van Dokkum & Conroy 2018).…”

An observer's guide to the (Local Group) dwarf galaxies: predictions for their own dwarf satellite populations

“…If the goal is simply to discover a lot of lowluminosity, low-surface-brightness objects with a small number of pointing, targeting the central regions of galaxy clusters is the way to go if the required depth can be achieved. Indeed, this is the approach taken by a number of groups, although the number of M * a few ×10 5 M dwarfs discovered so far is a literal handful, in part on account of the large distances to even nearby clusters like Virgo (Jang & Lee 2014;Grossauer et al 2015;Muñoz et al 2015;Lee et al 2017). If the goal is, as we argue, to discover small dwarfs relatively untouched by environmental processes, it is more efficient to target the inner regions of low-mass hosts.…”