On the Properties of Spectroscopically-Confirmed Ultra-Diffuse Galaxies Across Environment

Abstract: We present new redshift measurements for 19 candidate, ultra-diffuse galaxies (UDGs) from the Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes) survey after conducting a long-slit, spectroscopic follow-up campaign on 23 candidates at the Large Binocular Telescope. We combine these results with redshift measurements from other sources for 29 SMUDGes and 20 non-SMUDGes candidate UDGs. Together, this sample yields 44 spectroscopically-confirmed UDGs ($r_e\geq1.5$ kpc and $\mu_g(0)\geq24$ mag arcsec$^{-2}$ within uncertainties) and spans cluster and field environments, with all but one projected on the Coma cluster and environs. We find no statistically significant differences in the structural parameters of cluster and non-cluster confirmed UDGs, although there are hints of differences among the axis ratio distributions. Similarly, we find no significant structural differences among those in locally dense or sparse environments. However, we observe a significant difference in color with respect to projected cluster-centric radius, confirming trends observed previously in statistical UDG samples. This trend strengthens further when considering whether UDGs reside in either cluster or locally dense environments, suggesting starkly different star formation histories for UDGs residing in high and low-density environments. Of the 16 large ($r_e \geq 3.5$ kpc) UDGs in our sample, only one is a field galaxy that falls near the early-type galaxy red sequence. No other field UDGs found in low density environments fall near the red sequence. This finding, in combination with our detection of GALEX NUV flux in nearly half of the UDGs in sparse environments, suggest that field UDGs are a population of slowly evolving galaxies.