Optical-Near Infrared Color Gradients And Merging History Of Elliptical Galaxies

Abstract

It has been suggested that merging plays an important role in the formation and the evolution of elliptical galaxies(EGs). In order to understand the past merging history of EGs, we studied the optical-NIR color gradients of 204 EGs. The use of the optical and the NIR data provides large wavelength baselines, and breaks the age-metallicity(Z) degeneracy, allowing us to derive age and Z gradients. The use of the deep SDSS Stripe 82 images make it possible for us to examine how the color/age/Z gradients are related to merging features. We find that the optical-NIR color and the age/Z gradients of EGs with tidal features are consistent with those of relaxed EGs suggesting that the two populations underwent a similar merging history on average and that mixing of stars was more or less completed before the tidal features disappear. EGs with dust features have steeper color gradients than the other two types, even after masking out dust features during the analysis, which can be due to a process involving wet merging. More importantly, we find that the scatter in the color/age/Z gradients of the relaxed and merging feature types, decreases as their luminosities increase at M > 10^{11.4} M_{\odot} but stays to be large at lower luminosities. Mean Z gradients appear nearly constant over the explored mass range, but a possible flattening is observed at the massive end. According to our toy model that predicts how the distribution of Z gradients changes as a result of major dry merging, the mean Z gradient should flatten by 40% and its scatter become smaller by 80% per a mass doubling scale if EGs evolve only through major dry merger. Our result, although limited by a number statistics at the massive end, is consistent with the picture that major dry merging is an important mechanism for the evolution for EGs at M > 10^{11.4} M_{\odot}, but less important at the lower mass range.