Diffuse Ionized Gas in the Andromede Galaxy
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Session 19 -- Gas and Star Formation in Spiral Galaxies
Display presentation, Wednesday, January 12, 9:30-6:45, Salons I/II Room (Crystal Gateway)

## [19.06] Diffuse Ionized Gas in the Andromede Galaxy

R.A.M. Walterbos (NMSU), R. Braun (NFRA)

Diffuse Ionized Gas (DIG), also referred to as the Warm Ionized Medium (WIM) has been observed especially in edge-on galaxies in recent years, to investigate the disk-halo interface, and determine the vertical distribution of the low-density ionized gas. However, several other relevant issues concerning the properties of DIG in spiral galaxies can be best addressed in more face-on systems. In particular, questions concerning morphology, correlation with other population I material, especially OB stars, total H$\alpha$ luminosity, and fractional H$\alpha$ luminosity are best answered by observing face-on systems. We have started emission-line imagery of the DIG in more face-on spirals, and report results on the DIG in our best known neighbor, Andromeda, or M31. We find that DIG contributes 40\% of the total H$\alpha$ luminosity of M31, substantially more than the 10 to 15\% estimated for the Galaxy. DIG in the main spiral arms reaches average emission measures of EM=15 pc cm$^{-6}$, again substantially higher than found in the solar neighborhood; there are, however, several weaker spiral arms in M31 where the DIG reaches an EM similar to that in the solar neighborhood. The total H$\alpha$ luminosity of M31 is quite low, and implies a star formation rate of about 0.35 solar masses per year. DIG is strongly concentrated near HII regions, so that only a relatively small fraction of the disk is covered by it. This latter point explains why the high EM of DIG in the main spiral arms is not in contradiction with the low overall star formation rate. The DIG in M31 shows the by now well-established spectral signature of strongly enhanced [SII] emission compared to discrete HII regions. We derive an average [SII] over H$\alpha$ intensity ratio of 0.5, with evidence for even higher values at low emission measures. The discrete source population in M31 also shows a trend towards higher [SII] over H$\alpha$ flux ratios as sources become more diffuse. However, few sources reach similar ratios as found in the DIG.