A Spectroscopic Survey of the Galaxy Cluster CL 1358+62 at Z = 0.328

Abstract

We present a spectroscopic survey of the rich X-ray-selected galaxy cluster CL 1358+6245 at z = 0.328. When our 173 new multislit spectra of cluster galaxies are combined with data from the literature, we produce a catalog of 232 cluster members in a region 10' × 11' (3.5 Mpc × 3.8 Mpc) surrounding the brightest cluster galaxy. These data are used to study the structure and dynamics of the cluster and to examine the radial and velocity distributions as a function of spectral type. We classify the spectral types of the cluster members according to the strengths of the Balmer absorption lines (Hδ, Hγ, and Hβ) and the [O II] 3727 Å emission line.

We derive a mean redshift of z = 0.3283 +/- 0.0003 and a velocity dispersion of 1027⁺⁵¹_-45 km s^-1 for the 232 cluster members. However, the cluster velocity distribution is non-Gaussian, and we identify at least two subgroups with 10-20 members and dispersions of <400 km s^-1. The fraction of spectroscopically active galaxies (poststarburst and emission-line) in the core of the velocity distribution (within 0.6 σ of the mean cluster velocity) is 16% +/- 4%, rising to 32% +/- 7% for galaxies in the tails of the velocity distribution (2.0 σ from the mean cluster velocity). In total, the cluster is composed of 19% +/- 3% emission-line and 5% +/- 1% poststarburst galaxies. The velocity dispersion of the poststarburst galaxies is close to 2^1/2 times that of the absorption-line galaxies, consistent with free-fall accretion of the poststarburst galaxies.

The changing mix of galaxy spectral types as a function of local galaxy density (or distance from the cluster center) in CL 1358+62 is similar to what is observed in nearby rich clusters. The percentage of emission-line galaxies increases steadily with radius from 9% +/- 3% within a radius of r_p < 0.7 Mpc to 41% +/- 9% in our outermost radial bin at ~1.7 Mpc. The percentage of absorption-line galaxies falls from 84% +/- 9% to 59% +/- 11% over the same radial intervals. These results are consistent with the idea that the cluster grows through the accretion of late-type field galaxies, a fraction of which are transformed into poststarburst galaxies during the accretion event.

Our high-S/N spectra of the ``E + A'' galaxies allow a detailed comparison with spectra of nearby merging and strongly interacting galaxy systems. We find that nearby mergers have stronger [O II] 3727 Å emission ([O II] EW > 5 Å) than that observed for our E + A galaxies. This implies either that the E + A galaxies in CL 1358+62 were not formed through major mergers, or that if they were formed via merging, their gas supply was quickly depleted, possibly by the intracluster medium.