HeI lambda 5876 Recombination Emission from the Diffuse Ionized Gas in NGC 891: Constraints on Ionization Models

Details HeI lambda 5876 Recombination Emission from the Diffuse Ionized Gas in NGC 891: Constraints on Ionization Models HeI lambda 5876 Recombination Emission from the Diffuse Ionized Gas in NGC 891: Constraints on Ionization Models

Dec 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995aas...187.4811r&link_type=abstract

American Astronomical Society, 187th AAS Meeting, #48.11; Bulletin of the American Astronomical Society, Vol. 27, p.1354

Astronomy and Astrophysics

Astronomy

1

Scientific paper

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 & 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 Hα 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 HeI lambda 5876 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. Hα and [NII]lambda 6583 are seen up to > 5 kpc above the plane, much higher than in previous observations.

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