Rxj0142.0+2131: I. the Galaxy Content of an X-ray-luminous Galaxy Cluster at Z = 0.28

We present a photometric and spectroscopic study of stellar populations in the X-ray-luminous cluster of galaxies RXJ0142.0+2131 at z = 0.280. This paper analyses the results of high signal-to-noise spectroscopy, as well as g-, r-, and i-band imaging, using the Gemini Multi-Object Spectrograph on Gemini North. Of 43 spectroscopic targets, we find 30 cluster members over a range in color. Central velocity dispersions and absorption-line strengths for lines in the range 3700Å . λrest . 5800Å are derived for cluster members, and are compared with a low-redshift sample of cluster galaxies, and single stellar population (SSP) models. We use a combination of these indicators to estimate luminosity-weighted mean ages, metallicities ([M/H]), and α-element abundance ratios ([α/Fe]). RXJ0142.0+2131 is a relatively poor cluster and lacks galaxies with high central velocity dispersions. Although the red sequence and the Faber-Jackson relation are consistent with pure passive evolution of the early-type population with a formation redshift of zform ≃ 2, the strengths of the 4000Å break and scaling relations between metal line indices and velocity dispersion reject this model with high significance. By inverting SSP models for the HβG, Mgb, and 〈Fe〉 line indices, we calculate that, at a given velocity dispersion and metallicity, galaxies in RXJ0142.0+2131 have luminosity-weighted mean ages 0.14±0.07 dex older than the low-redshift sample. We also find that [α/Fe] in stellar populations in RXJ0142.0+2131 is 0.14 ± 0.03 greater than at low redshift. All scaling relations are consistent with these estimated offsets. We speculate that the older luminosity-weighted mean ages and [α/Fe] enhancement can be brought about by a rapidly-curtailed burst of star formation in RXJ0142.0+2131, such as may be experienced in a cluster–cluster merger. We note that the cluster’s velocity dispersion, 1278 ± 134 km s, is larger than expected both from its X-ray luminosity and richness. However, the velocity distribution of galaxies in RXJ0142.0+2131 is consistent with being drawn from a Gaussian distribution and no sign of substructure is found. We conclude that stellar populations in RXJ0142.0+2131 cannot evolve into stellar populations similar to those seen in our low-redshift sample through passive evolution. This study provides further evidence that a more complex model, possibly involving ongoing or intermittent star formation and galaxy mergers, is required to describe the evolution of cluster galaxies. Subject headings: galaxies: clusters: individual: RXJ0142.0+2131 – galaxies: evolution – galaxies: stellar content