Outer-disk reddening and gas-phase metallicities: The CALIFA connection
  We study, for the first time in a statistically significant and well-defined
sample, the relation between the outer-disk ionized-gas metallicity gradients
and the presence of breaks in the surface brightness profiles of disk galaxies.
SDSS g'- and r'-band surface brightness, (g'- r') color, and ionized-gas oxygen
abundance profiles for 324 galaxies within the CALIFA survey are used for this
purpose. We perform a detailed light-profile classification finding that 84% of
our disks show down- or up-bending profiles (Type II and Type III,
respectively) while the remaining 16% are well fitted by one single exponential
(Type I). The analysis of the color gradients at both sides of this break shows
a U-shaped profile for most Type II galaxies with an average minimum (g'- r')
color of ~0.5 mag and a ionized-gas metallicity flattening associated to it
only in the case of low-mass galaxies. More massive systems show a rather
uniform negative metallicity gradient. The correlation between metallicity
flattening and stellar mass results in p-values as low as 0.01. Independently
of the mechanism having shaped the outer light profiles of these galaxies,
stellar migration or a previous episode of star formation in a shrinking
star-forming disk, it is clear that the imprint in their ionized-gas
metallicity was different for low- and high-mass Type II galaxies. In the case
of Type III disks, a positive correlation between the change in color and
abundance gradient is found (the null hypothesis is ruled out with a p-value of
0.02), with the outer disks of Type III galaxies with masses $\leq$10$^{10}$
M$_{\odot}$ showing a weak color reddening or even a bluing. This is
interpreted as primarily due to a mass down-sizing effect on the population of
Type III galaxies having recently experienced an enhanced inside-out growth.
