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Session 43 - The Diffuse ISM: Milky Way and Beyond.
Display session, Tuesday, June 11
We have obtained long-slit spectra of emission-line nebulae in M31, using the RC spectrograph on the KPNO 4-m telescope. The spectra cover the wavelength range from 3500 to 7000ÅWe observed 16 different locations, which sample objects over a range of 4 to 16 kpc in radial distance from the center of M31. Each slit position included one or more HII regions and one supernova remnant candidate. In addition, most of the slits included regions of diffuse ionized gas, the low-density, Warm Ionized Medium (WIM) outside HII regions which contains most of the mass of ionized gas. In this paper we will discuss the results for the WIM. We measure forbidden line intensities of oxygen, nitrogen, and sulphur in both HII regions and the WIM, allowing a comparison of the excitation conditions in these two different environments. The WIM spectra provide constraints on the various ionization models that have been proposed over the past years. In emission-line images of the WIM in M31 we found strong evidence that the [SII] over H\alpha line-intensity ratios are significantly enhanced in the WIM as compared to HII regions, a property which has also been found for the Galactic WIM. Preliminary analysis of the spectra confirms the increase in [SII]/H\alpha line rations in the WIM compared to HII regions. In addition, we have measurements of [NII] and [OII] line strengths in several locations. In some regions we detect faint [OIII] emission from the WIM as well. We do not detect the HeI(5876Åline in individual spectra, but given the weak signal from the WIM this in itself does not imply that He is neutral. By co-adding spectra we may be able to detect this line, or place a significant upper limit on its strength. We will present the results of that analysis during the meeting.
Program listing for Tuesday