Radial Color Gradients in K + A Galaxies in Distant Clusters of Galaxies

Galaxies in rich clusters with z 0.3 are observed to have a higher fraction of photometrically blue galaxies than their nearby counterparts. This raises the important question of what environmental effects can cause the termination of star formation between z ≈ 0.3 and the present. The star formation may be truncated because of ram pressure stripping, or the gas in the disk may be depleted by an episode of star formation caused by some external perturbation. To help resolve this issue, surface photometry was carried out for a total of 70 early-type galaxies in the cluster Cl 1358+62, at z ~ 0.33, using two-color images from the Hubble archive. The galaxies were divided into two categories based on spectroscopic criteria: 24 are type K + A (e.g., strong Balmer lines, with no visible emission lines), while the remaining 46 are in the control sample, with normal spectra. Radial color profiles were produced to see whether the K + A galaxies show bluer nuclei in relation to their surrounding disks. Specifically, a linear gradient was fitted to the radial color profile of each galaxy. We find that the K + A galaxies on average tend to have slightly bluer gradients toward the center than the normal galaxies. A Kolmogorov-Smirnov two-sample test has been applied to the two sets of color gradients. The result of the test indicates that there is only a ~2% probability that the K + A and normal samples are drawn from the same parent distribution. There is a possible complication from a trend in the apparent magnitude-versus-color gradient relation, but overall our results favor the centralized star formation scenario as an important process in the evolution of galaxies in dense clusters.