Radial Distribution of ISM Gas-phase Metallicity in CLASH Clusters at z ~ 0.35: A New Outlook on Environmental Impact on Galaxy Evolution

We present the first observation of cluster-scale radial metallicity gradients from star-forming galaxies. We use DEIMOS on the Keck II telescope to observe two CLASH clusters at z ∼ 0.35: MACS J1115+0129 and RX J1532+3021. Based on our measured interstellar medium properties of star-forming galaxies out to a radius of 2.5 Mpc from the cluster center, we find that the galaxy metallicity decreases as a function of projected cluster-centric distance (-0.15 ± 0.08 dex/Mpc) in MACS 1115+01. On the mass-metallicity relation (MZR), star-forming galaxies in MACS J1115+01 are offset to higher metallicity (∼0.2 dex) than the local Sloan Digital Sky Survey galaxies at a fixed mass range. In contrast, the MZR of RX J1532+30 is consistent with the local comparison sample. RX J1532+30 exhibits a bimodal radial metallicity distribution, with one branch showing a similar negative gradient to MACS J1115+01 (-0.14 ± 0.05 dex/Mpc) and the other branch showing a positive radial gradient. The positive gradient branch in RX J1532+30 is likely caused by either interloper galaxies or an in-plane merger, indicating that cluster-scale abundance gradients probe cluster substructures and thus the dynamical state of a cluster. Most strikingly, we discover that neither the radial metallicity gradient nor the offset from the MZR is driven by the stellar mass. We compare our observations with Rhapsody-G cosmological hydrodynamical zoom-in simulations of relaxed galaxy clusters and find that the simulated galaxy cluster also exhibits a negative abundance gradient, albeit with a shallower slope (-0.04 ± 0.03 dex/Mpc). Our observations suggest that the negative radial gradient originates from ram pressure stripping and/or strangulation processes in the cluster environments.