A stellar content survey of NGC 2403 and M81

We report on a survey of the stellar content of the nearby pair of spiral galaxies NGC 2403 (ScIII) and M81 (SbI-II) from photographic UBVR_ck plates. The slopes of the luminosity functions we derived for these two galaxies show no significant differences. For the magnitude range 19.5m<V<21m the slope of the visual luminosity function is 0.55 ± 0.10 and 0.51 ± 0.08 for NGC 2403 and M81, respectively. The slopes also agree well with previous results for M31 (SbI-II) and M33 (ScIII) obtained using similar color criteria for the selection of blue stars. Our new results thus confirm the proposed similarity of the slopes of the luminosity functions among Sb- and Sc-type galaxies. Stellar and nonstellar objects have been separated for a catalog of the brightest stars in the fields of NGC 2403 and M81. The catalogs include positions, magnitudes (V) and colors B - V, U - B, and V - R_ck. For NGC 2403 we found the two brightest blue stars to have M_v = -9.5m and -9.3m. Both are spectroscopically confirmed members. The three brightest blue candidates in M81 appear to have visual luminosities of M_v= -9.8m, -9.5m, and 9.1m. They still must be confirmed spectroscopically as single star members. The visually brightest red supergiants appear to have visual luminosities around M_v= -8m in both galaxies, similar to Local Group galaxies. Based on the results for the luminosity functions and the properties of the brightest stars it appears that the stellar populations in NGC 2403 and M81 are rather similar, unless the brightest blue candidates in M81 are not single star members. In this case, M81 would also show a lack of the brightest blue supergiants compared to Sc-type spirals like the morphologically similar galaxy M31.