2011AJ....142...16Z

Panchromatic spectral energy distribution from the ultraviolet, optical to infrared photometry of NGC 628, combined with evolutionary stellar population synthesis, is used to derive the spatially resolved age, metallicity, and reddening maps. These parameter distributions show that the bulge of this galaxy is a disk-like pseudobulge, which has a Sérsic index close to the exponential law, rich gas, and a young circumnuclear ring structure. We also discover that the disk has two distinct regions with different radial age and metallicity gradients. The inner region is older and has a much steeper age gradient than the outer region of the disk. Both these two regions and the central young structure can be seen in the radial profile of the optical color. Based on the age and reddening distributions, we conclude that the pseudobulge and disk are likely to have grown via secular evolution, which is the redistribution of mass and energy through the angular momentum transport caused by the non-axisymmetric potential of the spirals. However, possible gas accretion events could affect the outer region of the disk due to abundant H I gas accumulating in the outer disk.