Spectroscopy of the WIM and the Source of Ionization

The ionization problem for the WIM has two aspects. First, the energy requirements to keep the medium ionized are enormous, leading to OB stars as the only viable candidates. But do photo ionization models predict the correct spectrum for the WIM? Second, there is a transport problem, in that the mean free path for Lyman continuum photons in galactic disks is very small, typically less than a pc (this small number implied the existence of ``Strömgren spheres'' in the ISM in the first place). So how can the ISM be ionized at large distances from OB stars? Do Lyman continuum photons leak from HII regions, or are field OB stars responsible?

A characteristic of the Galactic WIM is its high [SII] over H emission line intensity ratio (e.g. Reynolds 1988). The WIM in other galaxies shows the same behavior (e.g. Walterbos & Braun 1992, 1994, Rand et al. 1990, Ferguson et al. 1996b, Hoopes et al 1996). Spectroscopic results by Greenawalt et al. (1997; see Figure 2 here) for M31 and imaging results by Ferguson et al. for NGC 55 (1996b) show [SII]/(H[NII]) to increase with decreasing H intensity. In M31, [NII]/H and [OIII]/H do not

  
Figure 2: Line ratios for the WIM in M31 (filled dots) compared to various ionization models. The crosses indicate bright HII regions in M31. The full drawn lines indicate loci of photo ionization models from Domgörgen & Mathis (1994), for various diluted radiation fields. The ``S'' points refer to mixing layers models (Slavin et al. 1993) for various temperatures and mixing speeds. The photo ionization models give satisfactory agreement and only a modest contribution from mixing layers appears to be allowed (from Greenawalt et al. 1997).

change. These results agree with photo ionization models of Domgörgen & Mathis (1994) who calculate various line ratios for diffuse gas exposed to a strongly diluted radiation field. Overall, the WIM in M31 shows similar, but not identical, spectral characteristics as the Reynolds layer. The differences (e.g. stronger [OIII]/H in M31) indicate an overall less diluted radiation field in M31 compared to the solar neighborhood, not surprising given that for M31 the relatively bright WIM in the spiral arms was observed.

Ferguson et al. (1996) imaged NGC 55 in [OII], detecting a smoothly increasing [OII]/(H +[NII]) ratio with decreasing H intensity of the WIM. The WIM in M31 may show the same behavior (Greenawalt et al. 1997). This trend appears not to be predicted in the photo ionization models of Domgörgen & Mathis (1994). Greenawalt et al. (1997) also looked at the predicted line ratios for mixing layer models (Slavin et al. 1993); it seems that a possible contribution from mixing layers to line emission from the WIM is less than 20%.

In distinguishing which spectral type of OB stars are playing a role in ionizing the WIM, knowledge of the ionization stage of helium is crucial, since only stars earlier than O8 can ionize helium in significant amounts. Early results for Galactic WIM at an average EM of 30 pc cm (Reynolds & Tufte 1995) indicated that most of the helium in the direction they studied had to be neutral. This caused significant problems for photo ionization models, since not enough ionizing radiation could be contributed by the late-spectral type stars implied to be responsible for the ionization of the WIM. More recently (see Reynolds, this volume), the He(5876Å) recombination line has been detected for Galactic WIM and in the halo gas of NGC891 (Rand 1997), but it appears that He is not fully ionized. Greenawalt et al. 1997) concluded that for relatively bright WIM in M31 (at EM above 50 pc cm), helium appears to be fully ionized. We could not derive information for WIM at lower intensity levels. It is clear that further measurements of the He recombination line are required in different environments.

© Copyright Astronomical Society of Australia 1997