Program: GN-2018B-FT-114

A blue ring encircling a red galaxy, which lies close to the center of a galaxy cluster at z~1.1, is discovered serendipitously in the Subaru Hyper Suprime-Cam survey. Archival GMOS IFU data are used to rule out strong gravitational lensing as the origin of the ring, and it is highly probable that the ring system belongs to the cluster. Moreover, available Spitzer data indicate that several galaxies in the cluster core are actively forming stars, with the ring system having a star formation rate of 200 Msun/ yr, making it among the most actively star-forming, distant cluster galaxies known. It is likely that we are catching the short-lived moment when the ring is powered by extremely young stars. To understand the origin of the star formation activity in this cluster, we request observing time for multiobject spectroscopy with GMOS, with the goal of measuring the mass and dynamical states of the cluster. These observations would allow us to probe the immediate environments of the ring system, as well as the initial conditions that cause the star formation in cores of distant clusters in general, providing precious data for understanding the stellar mass assembly processes of massive cluster galaxies.