The origin of the U shape of the color profile's galaxies

The color radial profile of a disk galaxy has a U shape, yet it is unclear why this is the case. The disk usually gets bluer at increasing galactocentric distance, irrespective of the galaxy type. Because of the low star formation in the bulge and the stellar halo, these regions are redder than the disk and it might cause the U shape for the color radial profile. But in some galaxies after becoming progressively bluer in the beginning, the color of the galaxies gets redder before the truncation radius. We studied the radial color profile of NGC1042 and aimed to find the origin of the U shape of the color profile.