Radio emission from ultra-diffuse galaxies residing in galaxy clusters

Abstract

Ultra-diffuse galaxies (UDGs), defined by their extremely low surface brightness ($g$-band $μ\gtrsim 24$ mag arcsec$^{-2}$) and large effective radii (3--10 arcsec), remain one of the most puzzling galaxy populations in the nearby Universe \citep{vanDokkum2015}. Predominantly found in dense environments, UDGs in the Coma cluster show a preferential alignment of their major axes toward the cluster centre, suggesting strong environmental influence on their formation and evolution. Using high-sensitivity, low-frequency radio data from the upgraded Giant Metrewave Radio Telescope (GMRT), a pathfinder instrument we examined all 854 UDGs cataloged in Coma cluster \citep{Yagi2016}. Despite the unprecedented depth of these observations, no individual detections were made. A median stacking analysis in the upgraded GMRT band-3 achieved a 5 $\times$ \textsc{rms} upper limit $\simeq$1.5~$μ$Jy, providing the most stringent constraint yet on the average (mean) radio emission from UDGs, corresponding to star-formation rates $\lesssim$10$^{-3}$~M$_\odot$~yr$^{-1}$ for Coma-cluster-like systems and $\lesssim$10$^{-1}$~M$_\odot$~yr$^{-1}$ at $z \sim 0.05$. Looking ahead, the Square Kilometre Array (SKA) will transform the study of such faint galaxies. While the early AA$^\star$ configuration will deliver sensitivities comparable to the upgraded GMRT, the AA4 design baseline will achieve sub-$μ$Jy \textsc{rms} levels at matched frequencies ($ν\sim 200$~MHz--1.4~GHz), enabling detections of UDGs with star formation rates as low as 10$^{-4}$--10$^{-3}$~M$_\odot$~yr$^{-1}$ within Virgo and Coma distances. Such capabilities will allow robust discrimination between quenched, dark-matter-dominated systems and those sustaining weak residual star formation or low-luminosity nuclear activity.