Optical Spectroscopy of Diffuse Ionized Gas in M31

Abstract

We have obtained sensitive long-slit spectra of Diffuse Ionized Gas (DIG) in the Andromeda Galaxy, M31, covering the wavelength range of 3550-6850 Angs. By co-adding extracted DIG spectra, we reached a 1 sigma uncertainty of 9.3E-19 ergs/s/cm^{2}/arcsec^{2} corresponding to .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 [OII]\lambda3727 and [OIII]\lambda5007 of the truly diffuse ionized medium in the disk of an external spiral galaxy. We find that I_[OII]/I_H\alpha=.9-1.4. The [OIII] line is weak (I_[OIII]/I_H\beta = .5), but stronger than in the Galactic DIG. Measurements of [NII]\lambda6583 and [SII](\lambda6717+\lambda6731) are also presented. The [SII] lines are clearly stronger than typical HII regions (I_[SII]/I_H\alpha = .5 compared to .2). 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. We have also detected HeI\lambda5876 emission from the brightest DIG in M31. The HeI 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.