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Abstract:

We present a study of the stellar populations of galaxies in the cluster RX J0 152.7-1357 at a redshift of 0.83. The study is based on new high signal-to-noise ratio spectroscopy of 29 cluster members covering the wavelength range 5000-10000 Å, as well as r′i′z′ 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 RX J0152.7-1357 and our low-redshift comparison sample. The luminosities of the RX J0152.7-1357 galaxies and the strengths of the higher order Balmer lines H γ and Rδ (for non-emission-line galaxies) appear to be in agreement with pure passive evolution of the stellar populations with a formation redshift Z form ≈ 4. However, the strengths of the D4000 indices and the metal indices do not support this interpretation. Compared with our low-redshift comparison sample, the metal indices (C4668, Fe4383, CN3883, G4300, and CN 2) show that at least half of the non-emission-line galaxies in RX J0152.7-1357 have an Q-element abundance ratio [a/Fe] of 0.2 dex higher, and about half of the galaxies have significantly lower metal content. X-ray data have previously shown that RX J0152.7-1357 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 [0 II] emission indicates a very recent star formation episode involving about 1% of the mass, are located in areas of low X-ray luminosity, on the outskirts of the two subclumps. 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 subclumps are unaffected by the merger. The spectroscopy of the RX JO 152.7-1357 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 [a/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. © 2005. The American Astronomical Society. All rights reserved.

Abstract:

High-resolution N-body re-simulations of 15 massive (10^{14}-10^{15} Msun) dark matter haloes have been combined with the hybrid galaxy formation model GalICS (Hatton et al. 2003), to study the formation and evolution of galaxies in clusters, within the framework of the hierarchical merging scenario. New features in GalICS include a better description of galaxy positioning within dark matter haloes, a more reliable computation of the temperature of the inter-galactic medium as a function of redshift, and a description of the ram pressure stripping process. We focus on the luminosity functions, morphological fractions and colour distributions of galaxies in clusters and in cluster outskirts, at z=0. No systematic dependency on cluster richness is found either for the galaxy luminosity functions, morphological mixes, or colour distributions. Moving from higher density (cluster cores), to lower density environments (cluster outskirts), we detect a progressive flattening of the luminosity functions, an increase of the fraction of spirals and a decrease of that of ellipticals and S0s, and the progressive emergence of a bluer tail in the distributions of galaxy colours, especially for spirals. As compared to cluster spirals, early-type galaxies show a flatter luminosity function, and more homogeneous and redder colours. An overall good agreement is found between our results and the observations, particularly in terms of the cluster luminosity functions and morphological mixes. However, some discrepancies are also apparent, with too faint magnitudes of the brightest cluster members, especially in the B band, and galaxy colours tendentially too red (or not blue enough) in the model, with respect to the observations. Finally, ram pressure stripping appears to affect very little our results.