STELLAR POPULATION GRADIENTS IN BRIGHT CLUSTER GALAXIES AT z=0.2

Abstract

Slit spectra, covering the rest frame near-ultraviolet and blue wavelength regions, are combined with moderately deep g and R images to investigate radial population gradients in the brightest components of six z=0.2 galaxy clusters selected by x-ray brightness. We conclude that the brightest cluster galaxies (BCGs) of the EMSS0440+02, EMSS0906+11 and EMSS1231+15 clusters have global stellar contents resembling nearby elliptical galaxies. The BCGs in the clusters EMSS0839+29, EMSS1455+22 and Abell2390 are complicated by ionized gas, which is detected over large portions of all three systems. Radial color variations are detected in the majority of galaxies and, for those systems not showing extended [OII] emission, the weakening of D4000 (which measures the 4000 Angstrom break in magnitudes) with radius falls in the upper range of what is seen among nearby ellipticals. However, there is no correlation between local D4000 values and (g-r), suggesting that these quantities are sensitive to different parameters. We estimate the metallicity sensitivity of D4000 using a combination of observational and theoretical data, and find metallicity gradients for BCGs which are relatively steep, and in qualitative agreement with predictions for galaxies which experience a single monolithic dissipative collapse. Line gradients measured with respect to g surface brightness show excellent galaxy-to-galaxy consistency, and are in good agreement with what has been measured in Coma cluster galaxies. The presence of significant stellar population gradients in the BCGs suggests that the material from which they formed experienced dissipation during the collapse phases, and it seems likely that these galaxies could not have formed solely from the merging of gas-poor galaxies.