CLASH-VLT: Enhancement of (O/H) in z=0.35 RXJ 2248-4431 cluster galaxies

Aims. Gas-phase metallicities offer insight into the chemical evolution of galaxies as they reflect the recycling of gas through star formation and galactic inflows and outflows. Environmental effects such as star-formation quenching mechanisms play an important role in shaping the evolution of galaxies. Clusters of galaxies at z < 0.5 are expected to be the sites where environmental effects can be clearly observed with present-day telescopes.

Methods. We explored the Frontier Fields cluster RXJ2248-443 at z = 0.348 with VIMOS/VLT spectroscopy from CLASH-VLT, which covers a central region corresponding to almost 2 virial radii. The fluxes of [OII] λ3727, Hβ, [OIII] λ5007, Hα and [NII] λ6584 emission lines were measured allowing the derivation of (O/H) gas metallicities, star formation rates based on extinction-corrected Hα fluxes, and contamination from active galactic nuclei. We compared our sample of cluster galaxies to a population of field galaxies at similar redshifts.

Results. We use the location of galaxies in projected phase-space to distinguish between cluster and field galaxies. Both populations follow the star-forming sequence in the diagnostic diagrams, which allow the ionising sources in a galaxy to be disentangled, with only a low number of galaxies classified as Seyfert II. Both field and cluster galaxies follow the ’main sequence’ of star-forming galaxies, with no substantial difference observed between the two populations. In the mass–metallicity (MZ) plane, both high-mass field and cluster galaxies show comparable (O/H)s to the local SDSS MZ relation, with an offset of low-mass galaxies (log(M/M⊙) < 9.2) towards higher metallicities. While both the metallicities of "accreted" (R < R500) and "infalling" (R > R500) cluster members are comparable at all masses, the cluster galaxies from the ’mass complete’ bin (which is the intermediate mass bin in this study: 9.2 < log(M/M⊙) < 10.2), show more enhanced metallicities than their field counterparts by a factor of 0.065 dex with a ∼ 1.8σ significance. The intermediate-mass field galaxies are in accordance with the expected (O/H)s from the fundamental metallicity relation, while the cluster members deviate strongly from the model predictions, namely by a factor of ∼ 0.12 dex. The results of this work are in accordance with studies of other clusters at z < 0.5 and favour the scenario in which the hot halo gas of low- and intermediate-mass cluster galaxies is removed due to ram pressure stripping, leading to an increase in their gas-phase metallicity.