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arXiv · 1002.3424

Demography of SDSS early-type galaxies from the perspective of radial color gradients

Hyewon Suh·Hyunjin Jeong·Kyuseok Oh·Sukyoung K. Yi·Ignacio Ferreras·Kevin Schawinski
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Abstract

We have investigated the radial g-r color gradients of early-type galaxies in the Sloan Digital Sky Survey (SDSS) DR6 in the redshift range 0.00<z<0.06. The majority of massive early-type galaxies show a negative color gradient (red-cored) as generally expected for early-type galaxies. On the other hand, roughly 30 per cent of the galaxies in this sample show a positive color gradient (blue-cored). These "blue-cored" galaxies often show strong H beta absorption line strengths and/or emission line ratios that are indicative of the presence of young stellar populations. Combining the optical data with Galaxy Evolution Explorer (GALEX) UV photometry, we find that all blue-cored galaxies show UV-optical colors that can only be explained by young stellar populations. This implies that most of the residual star formation in early-type galaxies is centrally concentrated. Blue-cored galaxies are predominantly low velocity dispersion systems. A simple model shows that the observed positive color gradients (blue-cored) are visible only for a billion years after a star formation episode for the typical strength of recent star formation. The observed effective radius decreases and the mean surface brightness increases due to this centrally-concentrated star formation episode. As a result, the majority of blue-cored galaxies may lie on different regions in the Fundamental Plane from red-cored ellipticals. However, the position of the blue-cored galaxies on the Fundamental Plane cannot be solely attributed to recent star formation but require substantially lower velocity dispersion. We conclude that a low-level of residual star formation persists at the centers of most of low-mass early-type galaxies, whereas massive ones are mostly quiescent systems with metallicity-driven red cores.

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Hyewon Suh, Hyunjin Jeong, Kyuseok Oh, Sukyoung K. Yi, Ignacio Ferreras, Kevin Schawinski. 2010-02-18. Demography of SDSS early-type galaxies from the perspective of radial color gradients. https://doi.org/10.1088/0067-0049/187/2/374

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