Revisiting the distance and the globular cluster system of the remarkable galaxy UDG1 in the NGC 5846 group

Abstract

Two studies that utilised the same HST/WFC3 imaging of NGC5846_UDG1 have reported quite different total counts for its globular cluster (GC) system, i.e. 54 $\pm$ 9 vs 33 $\pm$ 3 GCs. In both cases they counted all GCs, that met their selection criteria, down to the faintest magnitudes. They also disagree as to whether NGC5846_UDG1 lies in the NGC 5846 group or well outside the group, in the field. As an ultra diffuse galaxy with one of the richest GC systems known, and therefore implications for its halo mass, it is important to understand which of these is closer to the truth. Here we present a new SBF-based distance to NGC5846_UDG1 from HST/ACS imaging of 26.5 $\pm$ 2.7 Mpc, which places it squarely within the NGC 5846 group. Using this distance we adopt the standard approach of only counting GCs brighter than the turnover magnitude. This has the advantage of considering only the brighter GCs which are resolved in HST imaging and largely confirmed by spectroscopy, while also avoiding the fainter candidates for which contamination is potentially an issue. With this robust approach we find that the two studies are entirely consistent with each other. Both imply a total GC system of around 50 GCs and by inference a massive galaxy halo of greater than 10$^{11}$ M$_{\odot}$. We also revisit the two previous photometric studies focusing on half a dozen intermediate magnitude objects that are selected by one study but excluded by the other. These objects have GC-like magnitudes, sizes and are nearly round with GC-like appearances. They are very unlikely to be background galaxies or interloper GCs and thus bona fide GCs associated with NGC5846_UDG1.