Optical Spectroscopy of Diffuse Ionized Gas in M31
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Abstract
We have obtained sensitive long-slit spectra of di use ionized gas (DIG) in the Andromeda galaxy, M31, covering the wavelength range of 35506850 By co-adding extracted DIG spectra, we reached a A . 1 p 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 rst measurements of [O II] j3727 and [O III] j5007 of the truly di use ionized medium in the disk of an external spiral galaxy. We nd that The I *O II+ /I Ha \ 0.91.4. [O III] line is weak but it is stronger than found for the Galactic DIG. Measurements (I *O III+ /I Hb \ 0.5), of [N II] j6583 and [S II] (j6717 ] j6731) are also presented. The [S II] lines are clearly stronger than typical H II regions compared to 0.2), conrming various imaging studies of spiral gal-(I *S II+