Color Gradients in Early-Type Galaxies in Abell 2199

Abstract

We performed B- and R-band surface photometry for E/S0 galaxies in the nearby rich cluster Abell 2199 to investigate their B-R color gradients [d(B-R)/d log r]. Our aims are to study statistical properties of the color gradients and, by comparing them with those in less dense environments, to examine environmental dependence of color gradients in elliptical galaxies. We studied the distribution of the B-R color gradients in the cluster elliptical galaxies and found that the mean value of the color gradients is -0.09 ± 0.04 mag dex-1, which can be converted to a metallicity gradient (d log Z/d log r) of -0.3 ± 0.1. The gradient seems to be comparable to that expected by a recent monolithic collapse model. We further studied the relations between the B-R color gradients and global properties of the galaxies. Our data suggest that for the galaxies brighter than an R-band magnitude of ~15 mag, which is roughly equivalent to L* at the distance of the cluster, brighter galaxies tend to have steeper color gradients. In addition, for the galaxies with effective radii larger than ~3'', which nearly corresponds to L* considering the correlation between galaxy luminosity and effective radius for elliptical galaxies, the galaxies with larger effective radii seem to have steeper color gradients. These trends could appear if elliptical galaxies formed through the monolithic collapse scenario. On the contrary, it is found based on the published data that such trends are not clearly seen for elliptical galaxies in less dense environments, suggesting that elliptical galaxies in sparse environments formed by galaxy mergers, although the distribution of the color gradients is quite similar to that in the rich cluster. In other words, our data and those in the literature suggest that there is an environmental dependence in the relationship between color gradient and global properties of elliptical galaxies, while the distribution of the values of color gradients is nearly independent of galaxy environment. These results do not fully support the view that elliptical galaxies in rich clusters formed through monolithic collapse, while those in sparse environments formed through galaxy mergers, because the latter elliptical galaxies are expected to have color gradients shallower on average than the former. This apparent conflict may be reconciled by taking into account star formation and subsequent chemical enrichment induced by a galaxy merger, which may allow a merger remnant to acquire a metallicity gradient and thus compensate the dilution of the existing metallicity gradients in the progenitors by mixture of stars during the galaxy merger.