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For refcode 1988A&A...203..217V:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1988A&A...203..217V Three-color surface photometry of a selected sample of early-type galaxies II. Color gradients J.P. Vader, L. Vigroux, M. Lachieze-Rey and J. Souviron C.E.N. Saclay, Service d'Astrophysique, DPhG/SAp, F-91191 Gif sur Yvette Cedex, France Department of Astronomy, Yale University, Box 6666, New Haven, CT 06511, USA Received November 5, 1987; accepted February 10, 1988 Summary. Results of three- or two-color CCD surface photometry for a sample of 35 early-type galaxies are discussed. The galaxies are selected to cover a range in both morphological type, from ellipticals to early-type spirals, and in absolute magnitude (M_B_= -16 to -23). The data, taken with a blue, red, and MgIb filter, are presented in a previous paper (Vigroux et al., 1988a). The subject of this paper is a systematic study of color gradients and their dependence on various parameters such as morphological type, luminosity, and rotation velocity. The brighter galaxies (M_B_ < -18) are characterized by a systematic reddening in B-R over the main body toward the center, and generally have a red nucleus. With one exception, the nuclei of the nucleated dwarf ellipticals in our sample do not differ in color from their surroundings. The two most interesting results of this work are a dependence of color gradient on absolute magnitude and on rotation velocity for early type galaxies. Color gradients in intrinsically luminous and faint galaxies are of opposite sign: in contrast to brighter galaxies, most dwarf ellipticals (M_B_ > -18) tend to redden with increasing radius. Since no plausible scenario of galaxy formation predicts positive metallicity gradients, the color gradients in dwarf ellipticals are best explained as an age effect, with the absence of metallicity gradients resulting from galactic winds. For brighter galaxies they can be interpreted as metallicity gradients. Color gradients are largest around M_B_ = -20.5 for our sample, the same luminosity above which ellipticals tend to be slow rotators. Remarkably, faster rotating galaxies tend to have stronger color gradients. The latter result is partly due to the presence of lenticular galaxies in our sample and needs to be better established for ellipticals as a class. We propose that M_B_ = -20.5 is a critical luminosity separating galaxies whose evolution is dominated by internal (dissipation, galactic winds) and external (mergers) processes. In this picture the outward reddening of dwarf galaxies is the signature of galactic winds, the correlation between color gradient and rotation velocity is a result of dissipation which is preserved in mergers, and the decrease of both these quantities with luminosity exceeding the critical one is the signature of mergers between galaxies of comparable mass. Key words: galaxies: surface photometry - evolution
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