CHAOS II: Gas-Phase Abundances in NGC 5194

Abstract: We have observed NGC5194 (M51a) as part of the CHemical Abundances of Spirals (CHAOS) project. Using the Multi Object Double Spectrographs (MODS) on the Large Binocular Telescope (LBT) we are able to measure one or more of the temperature-sensitive auroral lines ([O III] 4363, [N II] 5755, [S III] 6312) and thus measure "direct" gas-phase abundances in 29 individual HII regions. [O III] 4363 is only detected in two HII regions both of which show indications of excitation by shocks. We compare our data to previous direct abundances measured in NGC5194 and find excellent agreement for all but one region (Delta[log(O/H)] ~ 0.04). We find no evidence of trends in Ar/O, Ne/O, or S/O within NGC5194 or compared to other galaxies. We find modest negative gradients in both O/H and N/O with very little scatter (sigma < 0.08 dex), most of which can be attributed to random error and not to intrinsic dispersion. The gas-phase abundance gradient is consistent with the gradients observed in other interacting galaxies, which tend to be shallower than gradients measured in isolated galaxies. The N/O ratio (<log(N/O)> = -0.62) suggests secondary nitrogen production is responsible for a significantly larger fraction of nitrogen (e.g., factor of 8-10) relative to primary production mechanisms than predicted by theoretical models.