PPAK Wide field Integral Field Spectroscopy of NGC 628 III. Stellar
  population properties
  We present a stellar population analysis of the nearby, face-on, SA(s)c
galaxy, NGC628, which is part of the PPAK IFS Nearby Galaxies Survey (PINGS).
The data cover a field of view of ~6 arcmin in diameter with a sampling of
$\sim$2.7 arcsec per spectrum and a wavelength range (3700-7000A). We apply
spectral inversion methods to derive 2-dimensional maps of star formation
histories and chemical enrichment. We present maps of the mean (luminosity- and
mass-weighted) age and metallicity that reveal the presence of structures such
as a nuclear ring, previously seen in molecular gas. The disk is dominated in
mass by an old stellar component at all radii sampled by our data, while the
percentage of young stars increase with radius. The mean stellar age and
metallicity profiles have a two defined regions, an inner one with flatter
gradients (even slightly positive) and an external ones with a negative,
steeper one, separated at $\sim$60 arcsec. This break in the profiles is more
prominent in the old stellar component. The young component shows a metallicity
gradient that is very similar to that of the gas, and that is flatter in the
whole disc. The agreement between the metallicity gradient of the young stars
and the gas, and the recovery of the measured colours from our derived star
formation histories validate the techniques to recover the age-metallicity and
the star formation histories in disc galaxies from integrated spectra. We
speculate about the possible origin of the break and conclude that the most
likely scenario is that we are seeing, in the center of NGC 628, a dissolving
bar, as predicted in some numerical simulations.
