Optical Spectroscopy and Ionization Models of the Diffuse Ionized Gas in M33, M51/NGC 5195, and M81

We report the results of a spectroscopic investigation of the diffuse ionized gas (DIG) in M33, M81, and M51. All three galaxies have enhanced [S II]/Hα and [N II]/Hα ratios in the DIG compared to H II regions, in agreement with the predictions of photoionization models. The DIG in all three galaxies also has [O III]/Hβ ratios equal to or greater than that in neighboring H II regions. The [S II]/[N II] ratio does not vary with Hα surface brightness and is similar in both H II regions and DIG. We have tentatively detected the He I λ5876 line in the bright DIG near NGC 604, indicating a contribution from O7 and hotter stars to the ionizing spectrum in the DIG at that location. The DIG in M33 has lower [N II]/Hα ratios than M51 or the Galaxy, most likely because of low N abundance in M33. We used CLOUDY models to test the feasibility of two photoionization sources for the DIG: (1) the diluted ionizing continua of massive stars (``standard'' models) and (2) the transmitted ionizing continua of density-bounded H II regions (``leaky H II region'' models). The leaky H II region models produce higher [S II]/Hα, [N II]/Hα, and [O III]/Hβ ratios than standard DIG models for the same stellar temperatures. This alleviates somewhat the problem of producing high [O III]/Hβ ratios in the DIG. However, no single model can simultaneously reproduce all three line ratios. We conclude that either a combination of models is necessary or other ionization or heating mechanisms play a role in powering the DIG. Finally, we report on the spectrum of ionized gas associated with the companion to M51, NGC 5195. The gas in and around NGC 5195 has very high forbidden-line ratios and broad line widths (~ 500 km s^-1 FWHM), indicative of shock-ionized gas. At some locations the emission lines split into two velocity components, suggesting an expanding structure or outflow. The line ratios, morphology, and kinematics show that the interstellar medium in NGC 5195 is very disturbed.

Based on observations obtained with the Apache Point Observatory 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium.