Environmental Quenching seen in CO Emitting Galaxies in a Massive Cluster SPT-CL J0615-5746

The cosmic star formation rate (SFR) density continues to decrease after it peaks at z ~ 2. Previous studies suggested that the environmental quenching of star formation is one of the plausible causes for this decrease. Though several scenarios have been proposed for the environmental quenching process, it is not clear what the dominant mechanism is for individual clusters and galaxies. In order to understand the mechanism of environmental quenching, we used ALMA to investigate the physical properties of 4 member galaxies located at the center (~ 0.2 R_200) of SPT-CL J0615-5746 (SPT0615) at z = 0.972. Since SPT0615 is considered to be filled with high temperature ICM, it is expected that environmental quenching will be observed in the center of the cluster. Four member galaxies are detected in dust continuum, and three of these are also detected in CO(J = 5 – 4) with the significance over 5σ. The inferred SFR and molecular mass are high (SFR ~ 10 – 10^2 M_Sun/yr and M_mol ~ 10^10 M_Sun), implying the existence of active star formation. In addition, a CO(J = 5 – 4) tail feature is detected in one of those galaxies with the significance 5.7σ, which is likely to be molecular gas stripped from this galaxy by interaction with the ICM. In this galaxy, HST rest–frame UV to NIR images show that star formation takes place even in the stripped gas. Here we show, for the first time, gas stripping in a cluster at z ~ 1.