Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996aas...188.4413r&link_type=abstract
American Astronomical Society, 188th AAS Meeting, #44.13; Bulletin of the American Astronomical Society, Vol. 28, p.894
Astronomy and Astrophysics
Astronomy
Scientific paper
We present new results on the Diffuse Ionized Gas (DIG) of NGC 891 from a deep long-slit spectrum. The primary motivation was an attempt to detect the HeI lambda 5876 recombination line, the strength of which relative to Hα provides a direct constraint on the hardness of the ionizing spectrum. In the DIG of the Milky Way, this line ratio has turned out to be surprisingly low (Reynolds & Tufte 1995, Heiles et al. 1996), implying an ionizing spectrum much softer than had been inferred from the more readily observable forbidden lines such as [NII]lambda 6583 and [SII]lambda 6717. The HeI line has been detected in NGC 891 to a height of about 1.5 kpc from the plane -- well into the diffuse gas layer. In the DIG, HeI/Hα ~ 0.035, implying that helium is about 70% ionized, and that the ionizing spectrum is significantly harder than in the Reynolds layer. The [NII]lambda 6583/Hα ratio smoothly rises with distance from the plane, reaching peak values of about 1.4. Previous modeling indicates that such high values of [NII]lambda 6583/Hα require an ionizing spectrum significantly harder than indicated by the HeI/Hα ratio. Hence, despite the higher values of HeI/Hα , the same dilemma exists as in the Reynolds layer. These results suggest that we do not understand the heating and ionization of the diffuse gas well enough. The non-detection of the [NII]lambda 5755 line implies upper limits on the gas temperature of 10,000--13,000 K. Both the [NII] and Hα lines are detected up to z~ 5.5 kpc; thus the DIG layer extends much further than indicated by previous narrow-band images. A model of the electron density distribution consisting of two components with scale heights of about 1 kpc and 5--6 kpc provides a good fit to the Hα emission profile. Apart from the known effects of dust absorption on the velocity profiles at low-z, there is a smooth gradient in velocity centroids with z in the sense that they become closer to the systemic velocity. The effect is probably due in part to a decreasing rotation speed with z and a decreasing degree of central concentration of the layer with z. The rotation speed at z=5 kpc may be about 15 km s(-1) slower than in the disk. This result is expected in galactic fountain models.
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