The Central Regions of M81

Abstract

High-angular resolution near-infrared images obtained with the Canada-France-Hawaii Telescope adaptive optics system are combined with archival Hubble Space Telescope data to investigate the central regions of the nearby Sb galaxy M81 (NGC 3031). The spectral-energy distribution of the circumnuclear region, which extends out to 1.5" (~24 pc if mu_0=27.5) from the nucleus, can be modeled as a combination of an old metal-rich population and emission from hot dust. Thermal emission, which has been attributed to hot dust, has been detected near other active galactic nuclei, and simple models indicate that hot dust can account for ~20% of the light in K within 0.5" of the M81 nucleus. An elongated structure with M_V~-7, which may be an area of active star formation, is detected 0.45" from the nucleus. At distances in excess of 1.5" from the nucleus, the J-K color of the M81 bulge is not significantly different from what is seen in M31. The HST data are also used to search for bright globular clusters within 2 kpc of the center of M81. The area within 0.26 kpc of the M81 nucleus is largely devoid of bright globular clusters, in agreement with what is seen in the central regions of the Galaxy and M31. However, our survey indicates that there may be ~45+/-12 globular cluster candidates with M_V<=-7 within 2 kpc of the galaxy center, which is consistent with what would be infrared from the Milky Way cluster system after adjusting for differences in the total number of clusters.