M101: UV Leakage and Recent Star Formation
A comparative study of the spatial distribution of UV and far-infrared fluxes across the disk of M 101 has been undertaken by Popescu et al. . By comparing total UV emission from both GALEX bands with total far-infrared (FIR) flux from ISOPHOT (60, 100 and 170 µm) they discovered a very strong correspondence of the FIR/UV ratio with galactocentric radius (Figure 7). The ratio decreases monotonically from values somewhat in excess of 3 near the nucleus to values near zero in the outer regions of the galaxy. This they attribute to a large-scale distribution of diffuse dust decreasing in optical depth with radius and dominating over the more localized variations in opacity occurring between the arm and interarm regions. They also find a tendency for the FIR/UV ratio (at a given radius) to take on higher values in the regions of diffuse interarm emission and lower values in the spiral-arm regions. This is interpreted both in terms of the escape probability of UV photons from spiral arms and their subsequent scattering in the interarm regions, and in terms of the larger relative contribution of optical photons to the heating of the dust in the interarm regions.
Figure 7. The pixel values of the FIR/UV ratio map of M101 at the resolution of the 170µm image versus the galactocentric radius. The dots are for lines of sight towards interarm regions and the diamonds are towards the spiral arm regions. The solid line is an offset exponential fit to the data. (Adapted from Popescu et al. 
Another trend with galactocentric radius in M101 (and M51) is reported and discussed by Bianchi et al. . Using GALEX and SDSS photometry of compact stellar clusters in M101 they have used population synthesis models to derive ages, reddenings luminosities and current/initial masses. A galactocentric gradient in the FUV/NUV color index suggests younger clusters are preferentially found in the outer parts of the disks, with M101 showing the more pronounced gradient as compared to M51. The radial profiles in FUV/NUV for other galaxies (M31, M33 and M83) show the same trend; however, the magnitude of the effect is variable from galaxy to galaxy.