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Session 44 - The Local Diffuse ISM.
Display session, Tuesday, June 11
Great Hall,
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 He\,I \lambda 5876 recombination line, the strength of which relative to H\alpha 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 amp; 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 [N\,II]\lambda 6583 and [S\,II]\lambda 6717. The He\,I 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, He\,I/H\alpha \approx 0.035, implying that helium is about 70% ionized, and that the ionizing spectrum is significantly harder than in the Reynolds layer. The [N\,II]\lambda 6583/H\alpha ratio smoothly rises with distance from the plane, reaching peak values of about 1.4. Previous modeling indicates that such high values of [N\,II]\lambda 6583/H\alpha require an ionizing spectrum significantly harder than indicated by the He\,I/H\alpha ratio. Hence, despite the higher values of He\,I/H\alpha, 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 [N\,II]\lambda 5755 line implies upper limits on the gas temperature of 10,000--13,000 K. Both the [N\,II] and H\alpha lines are detected up to z\approx 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 H\alpha 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.
