Color and metallicity gradients in E and S0 galaxies

TL;DR: In this paper, the metal-to-hydrogen ratio (Z) was used to calibrate the (V-K) color indices of the galaxies NGC 3115 (S0) and NGC 4762 (S 0) for four filter bandpasses: U, B, V, and K. The observed color gradients are greatest along the minor axes of the flattened systems.

Abstract: Photometric maps of the galaxies NGC 3115 (S0), NGC 3377 (E5), NGC 3379 (E0), and NGC 4762 (S0) are presented for four filter bandpasses: U, B, V, and K. In each of the galaxies, the color indices (U-B), (B-V), and (V-K) tend to decrease outward from the nucleus. The observed color gradients are greatest along the minor axes of the flattened systems. Color changes in these and other galaxies of like morphological type appear to be dominated by a single parameter which most logically seems to be the metal-to-hydrogen ratio Z. Because the (V-K) color index is dominated by light from K and M stars of luminosity class III, it is readily amenable to calibration in terms of Z. Grasdalen's (1975a) recent observations of (V-K) indices for globular cluster giants form the basis for this calibration. The metallicity gradients derived from the data are compared with those predicted from theoretical models recently reported by Larson (1975a); the comparison outside the nuclear regions is remarkably good. The (V-K) index places rather severe restrictions on the contribution of M dwarf stars to the integrated colors. If such stars are the major low-luminosity constituent of the galactic mass, it is difficult to justifymore » M/L ratios much in excess of 50. (AIP)« less