MATLAS-42, a globular cluster-rich ultra-diffuse galaxy that diverges from the ‘failed galaxy’ formation pathway

ABSTRACT To date, there has been significant interest in globular cluster (GC)-rich ultra-diffuse galaxies (UDGs) and the evidence that they have formed via an unexpected, ‘failed galaxy’ formation pathway. The majority of the evidence for ‘failed galaxy’ UDGs originates from spectroscopic observations targeting passive GC-rich UDGs, with a focus on those residing in galaxy clusters. In this work, we study the gas-rich, GC-rich group UDG MATLAS-42 and derive its stellar population properties using the Keck Cosmic Web Imager. We measure a redshift for the galaxy ($V_{\rm R, \star }=2433\pm 8$ km s$^{-1}$), confirming the previous assumptions that it is both part of the NGC 502 group and has an associated H i-reservoir ($V_{\rm R,H\, {\small I}}=2423\pm 15$ km s$^{-1}$). We measure integrated stellar populations and find the galaxy to be both young (mass-weighted age $=3.2^{+2.6}_{-1.5}$Gyr) and of average-to-low metallicity ($[M/H]=-1.19^{+0.42}_{-0.30}$ dex). When considering these properties in the context of the galaxy’s formation, we note it likely does not follow the ‘failed galaxy’ formation pathway commonly attributed to GC-rich, cluster UDGs, as it has experienced recent star formation. At most it started failed, however, it has recently rejuvenated its star formation. Finally, we build a toy model of the passive evolution of this galaxy, finding that its relative GC-richness (i.e. $M_{\rm GC}/M_\star$) will likely decrease with time as GCs slowly evaporate/disrupt to contribute to the stellar mass of the galaxy. Due to this, we hypothesize that it is likely not a low-redshift analogue of the progenitor to a ‘failed galaxy’ UDGs.