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Session 48 - Spiral & Field Galaxies.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center

[48.11] He\,I \lambda5876 Recombination Emission from the Diffuse Ionized Gas in NGC 891: Constraints on Ionization Models

R. J. Rand (Univ. of Maryland)

Recent constraints on helium recombination emission in the Warm Ionized Medium (WIM) of the Milky Way have implied that helium is primarily neutral, much more so than previously believed (Reynolds amp; Tufte 1995, ApJ, 439, L17; Heiles et al. 1995, preprint). This presents a challenge to models of ionization of the diffuse gas by massive stars by implying that the spectrum of ionizing radiation is unexpectedly soft (T_*\alt 36,000 K). Either the most massive stars, which dominate the H ionizing output, contribute little to the diffuse ionization, or else their harder photons are efficiently absorbed relatively close to the stars. Regardless, it may be hard to explain the various ratios of forbidden line to H\alpha in the diffuse gas with such a soft spectrum. We present a very deep long-slit spectrum of the diffuse ionized gas in NGC 891 from the KPNO 4-m telescope in an attempt to detect the He\,I \lambda5876 line. The slit runs perpendicular to the midplane in a region of strong diffuse emission. This line is seen up to > 1 kpc above the plane, well into the diffuse layer. An analysis of the implications for the ionizing spectrum will be presented. H\alpha and [NII]\lambda6583 are seen up to > 5 kpc above the plane, much higher than in previous observations.

Program listing for Tuesday