An Unusually Clumpy Ultra-Diffuse Galaxy in the Coma Cluster: A Missing Link for Globular Cluster Formation?

Abstract

We present a study of Coma-B122, an unusually clumpy star-forming ultra-diffuse galaxy (UDG) in the Coma cluster outskirts, using deep Keck Cosmic Web Imager integral-field spectroscopy and Subaru Hyper Suprime-Cam imaging. B122 hosts $\sim10$ luminous star-forming clumps contributing $\sim25$% of its luminosity and $\sim7$% of its stellar mass, with typical clump masses of $\sim10^6\ M_\odot$ comparable to the most massive super star clusters known in nearby starbursting dwarfs. The clumps form a distinctive ring configuration $\sim2$ kpc from the center. Full-spectrum fitting shows that $\sim12.5$% of the diffuse body's stellar mass formed during an extended episode of enhanced star formation over the past $\sim300$ Myr, within which all the clumps assembled. B122 shows evidence of weak ram-pressure stripping (RPS), including a diffuse light distribution truncated on the side facing the Coma center and extended to the opposite side, along with an asymmetric H I velocity profile. Its ordered stellar and ionized-gas rotation, cold gas fraction, and phase-space position are consistent with an early stage of infall. Spatial variations in clump star formation histories and gas-phase abundances further indicate that RPS-driven metal-poor gas inflow may have triggered and sustained the recent star formation. B122 may offer a rare present-day view of how UDGs assemble their globular cluster (GC) systems during cluster infall, although it remains uncertain if the clumps could survive as GCs. Future observations with higher spatial resolution and of more analogs are important to provide a complete picture of the formation and evolution of clumpy low-surface-brightness dwarf galaxies.