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 ($V_{\rm R, \star}=2433\pm8$~km s$^{-1}$), confirming the previous assumptions that it is both part of the NGC~502 group and has an associated HI-reservoir ($V_{\rm R,HI}=2423\pm 15$~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 hypothesise that it is likely not a low redshift analogue of the progenitor to a ``failed galaxy'' UDGs.

Figures (7)

• Figure 1: The projected distribution of globular cluster candidates around MATLAS-42 on the sky. The cutout is 1.5$\times$1.5' (14.6$\times$14.6 kpc)
• Figure 2: GC system candidate colour vs projected radius from the galaxy centre. Kernel density estimates of each axis property are plotted alongside each axis. We take our GC positions and colours from the study of Marleau2024, which used Vega magnitudes. Note that a significant fraction of the 32 plotted candidates, likely about a third, are statistical contaminants. While GC candidates do not show a strong trend with radius, there is a bimodality in colour.
• Figure 3: Left: A 2'$\times$2' cutout centred around MATLAS-42 taken from the DECaLS Legacy Skyviewer (available https://www.legacysurvey.org/viewer?ra=20.6258&dec=8.7614&layer=ls-dr9&zoom=16). In red, we show the positioning of KCWI observations. In cyan, we show the offset sky. Included in the offset sky is a small background galaxy. North is up and east is left as indicated. At MATLAS-42 distance, the 16"$\times$20" medium slicer of KCWI corresponds to $\sim2.8\times3.5$ kpc. Right: Our spectrum for MATLAS-42 (black) with best fitting pPXF template (red) and with residuals of the fit also shown (grey). A clear Balmer series is evident (blue vertical lines) making it likely that this is a young galaxy -- as confirmed by our pPXF fitting.
• Figure 4: The stellar mass--metallicity relationship. We include MATLAS-42 (star) along with UDGs from the catalogue of Gannon2024b where most come from the study of FerreMateu2023. The stellar mass -- metallicity relationship for classical dwarfs at $z=0$ is plotted from Kirby2013 along with the simulated relationship at $z=2.2$ from Ma2016. Points are colour-coded by their relative GC richness with their style corresponding to their environment (circles are in clusters, triangles are in groups and squares are located in the field). UDGs where no GC count is available are plotted in grey. MATLAS-42 is slightly more metal-poor than the stellar mass -- metallicity relationship; however, it is not as metal-poor as the expectation at high redshift. It is a crucial addition to the number of lower-density environment UDGs studied so far.
• Figure 5: Mass-weighted metallicity vs mass-weighted mean stellar age. Point and colour styles follow Figure \ref{['fig:mass_met']}. In comparison to other UDGs known to be GC-rich, MATLAS-42 is noticeably younger. The young age may be attributable to it being in a less-dense group environment, while the other UDGs studied tend to reside in galaxy clusters.