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Session 82 - Spirals & Irregulars.
Display session, Friday, January 09
Exhibit Hall,
Long-slit spectra of the diffuse ionized gas in NGC 891 are presented, showing the vertical dependence of [N\,II]/H\alpha, [S\,II]/H\alpha, [O\,I]/H\alpha, and [O\,III]/H\beta through the brightest region of the DIG halo. The most surprising result, in complete contradiction to models in which the DIG is ionized by massive stars in the disk, is that [O\,III]/H\beta rises with height above the plane for z>1 kpc (even as [N\,II]/H\alpha, [S\,II]/H\alpha, and [O\,I]/H\alpha are rising, in line with expectations from such models). The run of [S\,II]/[N\,II] is also problematic, showing essentially no contrast with z. The [O\,III] emission probably arises from cooling gas at temperatures of \sim 10^5 K, which may originate in shocks, turbulent mixing layers, or some other source. Models in which the line emission comes from a mix of photo-ionized gas and shocks or turbulent mixing layers are considered in diagnostic diagrams, with the result that many aspects of the data can be explained by such composite models. Problems with the run of [S\,II]/[N\,II] still remain, however. Given the many uncertainties, the composite models can do little more than demonstrate the feasibility of these processes as secondary sources of energy input. A fairly robust result, however, is that the fraction of H\alpha emission arising from the second component probably increases with z. From values of [O\,I]/H\alpha, hydrogen is \gtrsim90% ionized (assuming T=10^4 K) up to z=1 kpc, the detection limit of the [O\,I] line.
