RXJ0152.7–1357: STELLAR POPULATIONS IN AN X-RAY LUMINOUS GALAXY CLUSTER AT z=0.83

We present a study of the stellar populations of galaxies in the cluster RXJ0152.7–1357 at a redshift of 0.83. The study is based on new high signal-to-noise spectroscopy of 29 cluster members covering the wavelength range 5000-10000Å as well as riz photometry of the cluster. We use scaling relations between the central velocity dispersions of the galaxies and their luminosities, Balmer line strengths and various metal line strengths to parameterize the differences between the members of RXJ0152.7–1357 and our low redshift comparison sample. The luminosities of the RXJ0152.7–1357 galaxies and the strengths of the higher order Balmer lines Hγ and Hδ (for non-emission line galaxies) appear to be in agreement with pure passive evolution of the stellar populations with a formation redshift zform ≈ 4. However, the strengths of the D4000 indices and the metal indices do not support this interpretation. Compared to our low redshift comparison sample, the metal indices (C4668, Fe4383, CN3883, G4300 and CN2) show that at least half of the non-emission line galaxies in RXJ0152.7–1357 have an α-element abundance ratio [α/Fe] of 0.2 dex higher, and about half of the galaxies have significantly lower metal content. X-ray data have previously shown that RXJ0152.7–1357 is in the process of merging from two sub-clumps. We find that differences in stellar populations of the galaxies are associated with the location of the galaxies relative to the X-ray emission. The galaxies with weak C4668 and G4300, as well as galaxies for which weak [O II] emission indicates a very recent star formation episode involving about 1 per cent of the mass, are located in areas of low X-ray luminosity, on the outskirts of the two sub-clumps. It is possible that these galaxies are experiencing the effect of the cluster merger as (short) episodes of star formation, while the galaxies in the cores of the sub-clumps are unaffected by the merger. The spectroscopy of the RXJ0152.7–1357 galaxies shows for the first time galaxies in a rich cluster at intermediate redshift that cannot evolve passively into the present day galaxy population in rich clusters. Additional physical processes may be at work and we speculate that merging with infalling (disk) galaxies in which stars have formed over an extended period might produce the required reduction in [α/Fe]. However, the merging could not be accompanied by star formation involving a substantial mass fraction. We note that our conclusions, in part, rely on stellar population models for which the predictions of the indices in the rest frame blue have not yet been tested extensively. Subject headings: galaxies: clusters: individual: RXJ0152.7–1357 – galaxies: evolution – galaxies: stellar