The Role of Spiral Arms in Galaxies

Abstract: We test the influence of spiral arms on the star formation activity of disk galaxies by constructing and fitting multi-wavelength SEDs for the two nearby spiral galaxies NGC 628 and NGC 4321, at a spatial scale of 1-1.5kpc scale. Recent results in the literature support the 'gatherers' picture, i.e., that spiral arms gather material but do not trigger star formation. However, ambiguities in the diagnostics used to measure star formation rates (SFRs) and other quantities have hampered attempts at reaching definite conclusions. We approach this problem by utilizing the physical parameters output of the MAGPHYS fitting code, which we apply to the Ultraviolet-to-Far Infrared (UV-to-FIR) photometry, in $\geq$20 bands, of spatially-resolved regions in the two galaxies. We separate the regions in arm and interarm, and study the distributions of the specific SFRs (sSFRs=SFR/M${star}$), stellar ages and star formation efficiency (SFE=SFR/M${gas}$). We find that the distributions of these parameters in the arm regions are almost indistinguishable from those in the interarm regions, with typical differences of a factor 2 or less in the medians. These results support the 'gatherer' scenario of spiral arms, which we plan to test with a larger sample in the near future.