Spatially Resolved Spectrophotometry of M81: Age, Metallicity, and Reddening Maps

In this paper we present a multicolor photometric study of the nearby spiral galaxy M81, using images obtained with the Beijing Astronomical Observatory 60/90 cm Schmidt telescope in 13 intermediate-band filters from 3800 to 10000 Å. The observations cover the whole area of M81, with a total integration of 51 hr from 1995 to 1997 February. This provides a multicolor map of M81 in pixels of 1.″7 × 1.″7. Using theoretical stellar population synthesis models, we demonstrate that some BATC (Beijing-Arizona-Taiwan-Connecticut Multicolor Sky Survey) colors and color indices can be used to disentangle the age and metallicity effect. We compare in detail the observed properties of M81 with the predictions from population synthesis models and quantify the relative chemical abundance, age, and reddening distributions for different components of M81. We find that the metallicity of M81 is about Z = 0.03, with no significant difference over the whole galaxy. In contrast, an age gradient is found between stellar populations of the central regions and of the bulge and disk regions of M81: the stellar population in its central regions is older than 8 Gyr, while the disk stars are considerably younger (∼2 Gyr). We also give the reddening distribution in M81. Some dust lanes are found in the galaxy bulge region, and the reddening in the outer disk is higher than that in the central regions.