The First Measured Electron Temperatures for Metal-rich H ii Regions in M51

We present the first direct measurements of electron temperatures in two low-excitation, and thus assumed very metal-rich, H II regions, in the spiral galaxy M51. We detect the [N II] λ5755 line in both H II regions and derive T e from the λ5755/λ6583 intensity ratio. We find values T e ([N ]) = 6000 K for CCM 72 and 7400 K for CCM 10. Oxygen abundances based on these temperatures are approximately a factor of 3 smaller than those derived previously from photoionization models. The higher electron temperatures also lead to S/O abundance ratios that are consistent with the solar neighborhood ratio at all metallicities, rather than the declining S/O at high metallicities inferred from earlier studies. The results suggest that metallicities in the disks of spiral galaxies do not rise to values as high as 3 or 4 times the solar value, in contradiction with abundances inferred from model calibrations of O/H versus strong emission lines.