Ultraviolet through far-infrared spatially resolved analysis of the recent star formation in M81 (NGC 3031)

The recent star formation (SF) in the early-type spiral galaxy M81 is characterized using imaging observations from the far-ultraviolet to the far-infrared. We compare these data with models of the stellar, gas, and dust emission for subgalactic regions. Our results suggest the existence of a diffuse dust emission not directly linked to the recent star formation. We find a radial decrease of the dust temperature and dust mass density, and in the attenuation of the stellar light. The IR emission in M81 can be modeled with three components: ( 1) cold dust with a temperature < T-c > = 18 +/- 2 K, concentrated near the H II regions but also presenting a diffuse distribution; ( 2) warm dust with < T-w > = 53 +/- 7 K, directly linked with the H II regions; and (3) aromatic molecules, with diffuse morphology peaking around the H II regions. We derive several relationships to obtain total IR luminosities from IR monochromatic fluxes, and we compare five different star formation rate (SFR) estimators for H II regions in M81 and M51: the UV, H alpha, and three estimators based on Spitzer data. We find that the H alpha luminosity absorbed by dust correlates tightly with the 24 mu m emission. The correlation with the total IR luminosity is not as good. Important variations from galaxy to galaxy are found when estimating the total SFR with the 24 mu m or the total IR emission alone. The most reliable estimations of the total SFRs are obtained by combining the H alpha emission (or the UV) and an IR luminosity (especially the 24 mu m emission), which probe the unobscured and obscured SF, respectively. For the entire M81 galaxy, about 50% of the total SF is obscured by dust. The percentage of obscured SF ranges from 60% in the inner regions of the galaxy to 30% in the outer zones.