2009ApJ...697.1439D

Wide-field images obtained with the 3.6 m Canada-France-Hawaii Telescope are used to investigate the spatial distribution and photometric properties of the brightest stars in the disk of M81 (NGC 3031). With the exception of the central ∼2 kpc of the galaxy and gaps between CCDs, the survey is spatially complete for stars with i' < 24 and major axis distances < 18 kpc. A more modest near-infrared survey detects stars with K < 20 over roughly one third of the disk. Bright main-sequence (MS) stars and RSGs are traced out to galactocentric distances of at least 18 kpc. The color of the red supergiant (RSG) locus suggests that Z = 0.008 when R_GC> 6 kpc, and this radially uniform RSG metallicity is consistent with [O/H] measurements from H II regions. The density of bright MS stars and RSGs drops when R_GC< 4 kpc, suggesting that star formation in the inner disk was curtailed within the past ∼100 Myr. The spatial distribution of bright MS stars tracks emission at far-ultraviolet, mid- and far-infrared wavelengths, although tidal features contain bright MS stars but have little or no infrared flux. The specific frequency of bright MS stars and RSGs, normalized to K-band integrated brightness, increases with radius, indicating that during the past ∼30 Myr the specific star formation rate (SSFR) has increased with increasing radius. Still, the SSFR of the M81 disk at intermediate radii is consistent with that expected for an isolated galaxy as massive as M81, indicating that the star formation rate in the disk of M81 has not been markedly elevated during the past few tens of millions of years. The stellar content of the M81 disk undergoes a distinct change near R_GC∼ 14 kpc; the K-band light profile, which is dominated by old and intermediate age stars, breaks downward at this radius, whereas the density profile of young stars flattens, but does not break downward. Thus, the luminosity-weighted mean age decreases with increasing radius in the outer regions of the M81 disk.