EMISSION LINE RATIOS AS DIAGNOSTICS OF IONIZATION AND HEATING

In the Reynolds layer, photo-ionization by massive stars in the disk is considered the most likely primary ionization mechanism (Reynolds, this volume). The available energy easily satisfies that required to maintain the layer, and photo-ionization models [e.g. Mathis 1986; Sokolowski 1994 (partially reproduced in Bland-Hawthorn, Freeman, & Quinn 1997)] have been the most successful in explaining the behavior of line ratios (enhanced [SII]/H, [NII]/H, very weak [OI]/H, [OIII]/H relative to HII regions are due to dilution of the stellar radiation as it propagates away from the Galactic plane). However, there are observations that these models cannot explain - one of which is the weakness of HeI/H (Reynolds & Tufte 1995; Reynolds, this volume), a ratio with a fairly straightforward interpretation in terms of the hardness of the ionizing radiation [see also Heiles et al. (1996) for an even more extreme result from radio recombination lines]. The HeI emission is much weaker than expected from models given the high [NII]/H and [SII]/H values [see Domgrgen & Mathis (1994)]. The forbidden lines are highly temperature sensitive and their interpretation is complicated by issues of abundances, gas heating, and depletion of important coolants such as Fe, Si, and Ca (Sokolowski 1994). But regardless of the forbidden lines, the low HeI/H is problematic in itself and may indicate that we do not understand stellar atmospheres in the extreme UV well enough (see e.g. Cassinelli et al. 1995), and/or which spectral types of massive stars are most important for the ionization (Reynolds & Tufte 1995; Heiles et al. 1996).

It is also not clear to what degree other sources of ionization contribute: shocks (which may play a role in irregulars; Martin 1996), turbulent mixing layers (Slavin, Shull, & Begelman 1993), cooling fountain gas (Shapiro & Benjamin 1993) and decaying neutrinos (e.g. Sciama 1995).

In NGC 891, the run of [NII]/H vs. z at R=5 kpc on the north side has been measured by Rand (1997). This shows a smooth increase with z, from 0.4 in the midplane to 1.4 at z=2 kpc (east side of midplane) and z=4 kpc (west side). Previous, less sensitive spectroscopy by Dettmar & Schulz (1992) and Keppel et al. (1991) also showed an increase with z for [NII]/H as well as [SII]/H. The same trend in [SII]/H was also seen in narrow-band images (Rand, Kulkarni, & Hester 1990). The spatial behavior is as expected in photo-ionization models (the ratios rise with distance from the disk ionizing sources as the radiation field is diluted) but values as high as 1.4 are difficult to explain (Sokolowski 1994). Note that in M31 (Walterbos, this volume) and the Milky Way, [NII]/H is roughly constant at only 0.3-0.5, and well modeled by Domgrgen & Mathis (1994). It is not clear whether the lower value is due to a less dilute radiation field, or whether features of some models of Sokolowski designed to raise the value, such as a very high upper IMF cutoff, do not apply in these cases.

HeI is detected in NGC 891, but, as in the Reynolds layer, the weakness of HeI/H implies a much softer spectrum than indicated by [NII]/H, and also softer than expected for a population of ionizing stars (Rand 1996). A measurement of the gas temperature would help in understanding the forbidden line strengths, but only upper limits exist from the non-detection of the [NII] line: 13,000 K (east side) and 10,000 K (west side).

Veilleux et al. (1995) have formed an [NII]/H map of NGC 3079, where again there is a general increase with distance from HII regions, reaching similar values as in NGC 891.

Golla, Dettmar, & Domgrgen (1996) present long-slit spectra of NGC 4631, and again find an increase of [NII]/H and [SII]/H height from the plane. However, the rate of increase of the two ratios with z is about the same (both running from a minimum of about 0.13 in the plane to a maximum of about 0.5 at z=1 kpc in their slit position C), whereas their models predict a stronger trend in [SII]/H than in [NII]/H (because of a more dramatic change in the predominant ionization state of S between HII regions and diffuse gas). The solution of this discrepancy is not clear, but again it points towards a departure from existing models.

Finally, Ferguson, Wyse, & Gallagher (1996) present H+[NII], [SII] and [OII] images of NGC 55, and find that both [SII]/H+[NII] and [OII]/H+[NII] increase with height from the plane (like N and S, O becomes increasingly singly ionized with greater distance from the HII regions). The latter ratio shows a very strong contrast, from about 0.1 in the midplane to as high as 2 in the halo. Again, such high values are hard to explain.

Clearly, more work needs to be done to constrain the sources of ionization and heating. One approach is to model a combination of photo-ionized gas with shock-ionized gas using various shock speeds, and turbulent mixing layers of various temperatures, as has been done for the much brighter diffuse emission of irregulars by Martin (1996). While such composite models may be able to explain the observations, one still desires a physical scenario which joins these several processes together. For instance, in a model where the diffuse halos are fed by superbubble and chimney activity, what sort of shocks are predicted to run through the halo gas as a result, what sort of mixing interfaces should exist, and how might line ratios change with environment?

As for extra sources of non-ionization heating, possibilities include photoelectric heating from dust grains (Reynolds & Cox 1992) and the dissipation of turbulence (Minter & Balser 1997). The latter authors include a heating rate from turbulence derived from scintillation observations in a photo-ionization model of the DIG and can thereby explain simultaneously [SII]/H, [NII]/H, and [HeI]/H at low-z in the Reynolds layer if the composite stellar temperature is low enough.

© Copyright Astronomical Society of Australia 1997