Optical Spectroscopy of Diffuse Ionized Gas in M31

We have obtained sensitive long-slit spectra of diffuse ionized gas (DIG) in the Andromeda galaxy, M31, covering the wavelength range of 3550-6850 Å. By co-adding extracted DIG spectra, we reached a 1 σ uncertainty of 9.3 × 10^-19 ergs s^-1 cm^-2 arcsec^-2 corresponding to 0.46 pc cm^-6 in emission measure. We present average spectra of DIG at four brightness levels with emission measures ranging from 9 to 59 pc cm^-6. We present the first measurements of [O II] λ3727 and [O III] λ5007 of the truly diffuse ionized medium in the disk of an external spiral galaxy. We find that I_{[O II]}/I_Hα = 0.9-1.4. The [O III] line is weak (I_{[O III]}/I_Hβ = 0.5), but it is stronger than found for the Galactic DIG. Measurements of [N II] λ6583 and [S II] (λ6717 + λ6731) are also presented. The [S II] lines are clearly stronger than typical H II regions (I_{[S II]}/I_Hα = 0.5 compared to 0.2), confirming various imaging studies of spiral galaxies. Overall, the line ratios are in agreement with predictions of photoionization models for diffuse gas exposed to a dilute stellar radiation field, but the line ratios of the DIG in M31 are somewhat different than observed for Galactic DIG. The differences indicate a less diluted radiation field in the DIG of M31's spiral arms compared to DIG in the Solar Neighborhood of the Milky Way. Turbulent mixing layers can contribute at most 20% of the ionization budget of the DIG, with lower percentages producing better fits to the observed line ratios. We have also detected He I λ5876 emission from the brightest DIG in M31. The He I line appears to be stronger than in the Galactic DIG, possibly indicating that most of the Helium in the bright DIG in M31 is fully ionized. However, this result is somewhat tentative since bright night sky lines hamper an accurate measurement of the He I line strength.