Colour and stellar population gradients in galaxies: correlation with mass

We analyse the colour gradients (CGs) of ∼50 000 nearby Sloan Digital Sky Survey galaxies estimated by their photometrical parameters (Sersic index, total magnitude and effective radius). From synthetic spectral models based on a simplified star formation recipe, we derive the mean spectral properties and explain the observed radial trends of the colour as gradients of the stellar population age and metallicity. CGs have been correlated with colour, luminosity, size, velocity dispersion and stellar mass. Distinct behaviours are found for early- and late-type galaxies (ETGs and LTGs), pointing to slightly different physical processes at work in different morphological types and at different mass scales. In particular, the most massive ETGs (M * ≳ 10 11 M ⊙ ) have shallow (even flat) CGs in correspondence of shallow (negative) metallicity gradients. In the stellar mass range (10 10.3 -10 10.5 ) ≲ M * ≲ 10 11 M ⊙ , the metallicity gradients reach their minimum of ∼ - 0.5 dex -1 . At M * ∼ 10 10.3 - 10 10.5 M ⊙ , colour and metallicity gradient slopes suddenly change. They turn out to anticorrelate with the mass, becoming highly positive at the very low masses, the transition from negative to positive occurring at M * ∼ 10 9-9.5 M ⊙ . These correlations are mirrored by similar trends of CGs with the effective radius and the velocity dispersion. We have also found that age gradients anticorrelate with metallicity gradients, as predicted by hierarchical cosmological simulations for ETGs. On the other side, LTGs have colour and metallicity gradients which systematically decrease with mass (and are always more negative than in ETGs), consistently with the expectation from gas infall and supernovae feedback scenarios. Metallicity is found to be the main driver of the trend of CGs, especially for LTGs, but age gradients are not negligible and seem to play a significant role too. Owing to the large data set, we have been able to highlight that older galaxies have systematically shallower age and metallicity gradients than younger ones. The emerging picture is qualitatively consistent with the predictions from hydrodynamical and chemodynamical simulations. In particular, our results for high-mass galaxies are in perfect agreement with predictions based on the merging scenario, while the evolution of LTGs and younger and less massive ETGs seems to be mainly driven by infall and supernovae feedback.