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, NGC 628, which is part of the PPAK Integral Field Spectroscopy Nearby Galaxies Survey. The data cover a field of view of similar to 6 arcmin in diameter with a sampling of similar to 2.7 arcsec per spectrum and a wavelength range (3700-7000 angstrom). We apply spectral inversion methods to derive 2D 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 disc 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 one with a negative, steeper one, separated at similar to 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 centre of NGC 628, a dissolving bar, as predicted in some numerical simulations.