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