A Very Deep Spectrum of the Diffuse Ionized Gas in NGC 891

Abstract

A deep long-slit spectrum of the diffuse ionized gas (DIG) in the edge-on spiral galaxy NGC 891 has been obtained. The slit crosses the plane at a galactocentric radius of 5 kpc and covers DIG on both the east and west sides of the galaxy. The primary motivation was to attempt to detect the recombination line He I λ5876, 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; Heiles et al.), implying an ionizing spectrum much softer than had been inferred from the more readily observable forbidden lines, such as [N II] λ6583 and [S II] λ6717. 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/Hα ≈ 0.034, implying that helium is about 70% ionized and that the ionizing spectrum is significantly harder than in the Reynolds layer. The spectrum also allows very detailed mapping of the [N II] λ6583/Hα ratio as a function of height off the plane. This ratio reaches peak values of about 1.4 at heights of z = 2 kpc on the east side of the plane and z = 4 kpc on the west side. Beyond this height on the east side, the ratio clearly declines again, falling to about 1.0 at z = 4 kpc. Previous modeling indicates that such high values of [N II] λ6583/Hα require an ionizing spectrum significantly harder than that indicated by the He I/Hα ratio. Hence, despite the higher values of He I/Hα, the same dilemma exists as in the Reynolds layer case. These results suggest that we do not understand the heating and ionization of the diffuse gas well enough. A determination of the gas temperature would help, but the nondetection of the line [N II] λ5755 allows only upper limits to be set: 13,000 K on the east side and 10,000 K on the west side. Both the [N II] and Hα lines are detected beyond z = 5 kpc; thus, the DIG layer extends much further than indicated by previous narrowband 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. The very extended component was also seen in the image of Rand, Kulkarni, & Hester, but the inferred scale height is now larger. Apart from the known effects of dust absorption on the velocity centroids at low z, there is a smooth gradient in velocity centroids with increasing z above the dust lane in the sense that they become closer to the systemic velocity, changing by about 30 km s-1 from z = 1 kpc to z = 4.5 kpc. We postulate that the effect is due in part to a decreasing rotation speed and an outward radial migration of gas with z. An illustrative model suggests that the rotation speed at z = 4.5 kpc may be about 20 km s-1 slower than in the disk, although other effects could change this estimate. Such a change in rotation velocity is expected in galactic fountain models, but some simple experiments with stellar orbits show that the flow in a fountain may be more complicated than previously thought.