Program: GN-2021A-Q-213

Environmental quenching of star formation starts in the filaments that feed galaxy clusters. Understanding how this quenching occurs requires both knowledge of the cold gas and of the timescale over which star formation is quenched. To address this need, we have obtained ALMA CO(3-2) cold gas measurements for 27 galaxies that lie in the structures surrounding a z=0.52 cluster, which we have exquisitely characterized. These galaxies are markedly deficient in their cold gas, a hallmark that quenching is underway. Surprisingly, however, their UV-based star formation rates (SFR), which sample activity on 100 Myr timescales, are identical to field galaxies. We will use precise Halpha measurements with GMOS to reveal the instantaneous SFR of our ALMA sources and compare it both to their cold gas contents and to their UV-based SFRs. Our experiment is clear. If the Halpha SFRs are lower than those from the UV, then quenching is occuring on short timescales. If the SFRs are comparable, then it indicates that the molecular gas is being affected without affecting the SFR, possibly via stripping of diffuse molecular gas. Either of these findings will dramatically improve our knowledge of galaxy quenching in the cosmic web around clusters.