Colour gradients within SDSS DR7 galaxies: hints of recent evolution
  The evolutionary path followed by a galaxy shapes its internal structure,
and, in particular, its internal colour variation. We present a study of the
internal colour variation within galaxies from the Seventh Data Release of the
Sloan Digital Sky Survey (SDSS DR7). We statistically study the connection
between the internal colour variation and global galactic properties, looking
for hints of the recent galactic evolution. Considering only galaxies with good
photometry and spectral measurements, we define four luminosity-threshold
samples within the redshift range 0.01<z<0.17, each containing more than 48000
galaxies. Colour gradients are calculated for these galaxies from the surface
brightness measurements provided by the SDSS DR7. Possible systematic effects
in their determination have been analysed. We find that, on average, galaxies
have redder cores than their external parts. We also find that it is more
likely to find steep colour gradients among late-type galaxies. This result
holds for a range of classifications based on both morphological and spectral
characteristics. In fact, our results relate, on average, steep colour
gradients to a higher presence of young stars within a galaxy. Our results also
suggest that nuclear activity is a marginal driver for creating steep colour
gradients in massive galaxies. We have selected pairs of interacting galaxies,
with a separation of 5 arcsec, in projected radius, and a difference in
redshift of 100 km/s, finding that they present steeper gradients than the
average population, skewed towards bluer cores. Our analysis implies that
colour gradients can be useful for selecting galaxies that have suffered a
recent (minor) burst of star formation.
