CHAOS. II. GAS-PHASE ABUNDANCES IN NGC 5194

We have observed NGC 5194 (M51a) as part of the CHemical Abundances of Spirals project. Using the Multi Object Double Spectrographs on the Large Binocular Telescope we are able to measure one or more of the temperature-sensitive auroral lines ([O III] lambda 4363, [N II] lambda 5755, [S III] lambda 6312) and thus measure direct gas-phase abundances in 29 individual H II regions. [O III] lambda 4363 is only detected in two H II regions, both of which show indications of excitation by shocks. We compare our data to previous direct abundances measured in NGC 5194 and find excellent agreement (Delta[log(O/H)] approximate to 0.05) for all but one region. We find no evidence of trends in Ar/O, Ne/O, or S/O within NGC 5194 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.