Iv. Using Color Profiles to Study Stellar and Dust Content of Galaxies

The stellar and dust content of spiral galaxies as function of radius has been investigated using nearinfrared and optical broadband surface photometry of 86 face-on spiral galaxies. Colors of galaxies correlate with the azimuthally averaged local surface brightness both within and among galaxies, with the lower surface brightness regions being bluer. The colors formed from different passband combinations correlate strongly indicating that they probably arise from the same physical process. A 3D radiative transfer model was developed to calculate the effect of dust absorption and scattering on the luminosity and color profiles of galaxies. Stellar synthesis models were used to investigate the effects of the star formation history and the metallicity on the broadband color profiles. Combining all optical and near-infrared data shows that the color gradients in this sample of face-on galaxies are best explained by a combined stellar age and metallicity gradient across the disk, with the outer regions being on average younger and of lower metallicity. Dust reddening probably plays only a minor role, as the dust models cannot produce reddening profiles that are compatible with the observations. The observed color differences implicate substantial M/Lλ differences, both within galaxies and among galaxies. The variations are such that the “missing light” problem derived from rotation fitting becomes even worse. Late-type galaxies (T≥6) have lower metallicities and are often of younger average age than earlier types and have therefore an entirely different M/Lλ in most passbands. The near-infrared passbands are recommended for studies where the M/Lλ ratios should not vary too much.