The Warm Ionized Medium in Disk Galaxies

Emission line imaging with CCDs on even modest size telescopes is an excellent method for studying the WIM in galaxies, provided care is taken in flat fielding and subtraction of continuum light. Imaging with Fabry-Perot systems is another fruitful observational approach, as discussed by Bland-Hawthorn (this volume). Most imaging studies have focused on H, sometimes including [NII](6548+6583Å), and on [SII](6716+6731Å) emission lines (e.g. Walterbos & Braun 1992, 1994, Hoopes et al. 1996, Ferguson et al. 1996a,b). The H intensity one observes is directly proportional to the Emission Measure (EM), the integral of the electron density squared along the line of sight. For ionized gas at 10,000K, The WIM is too diffuse to obtain density information from the ratios of forbidden lines such as the [SII] doublet. Thus we have no direct probe of the electron density, hence column density of ionized gas, in observations of external galaxies. For the Galactic WIM, this information does exist through observations of pulsar dispersion measurements (see e.g. Kulkarni & Heiles 1988 for a review).

  
Figure 1: A 5-hour exposure in H[NII] of the nearby spiral M33 obtained with the Burrell Schmidt telescope at Kitt Peak. The image shows the central 4.2 by 4.2 kpc. The brightest HII region in M33, NGC 604 is located just left and above the middle. The grey scale saturates at an EM of 500 pc cm. Continuum light has been subtracted. The WIM seems to cover almost the entire area of the disk not occupied by traditional bright HII regions (from Greenawalt 1997).

We show an example of a deep H image for the nearby spiral M33 in Figure 1. The WIM is brightest in regions with a high surface density of bright HII regions, although sometimes WIM patches or filaments are located far away from traditional HII regions (see also Hunter et al. 1990, 1992). The WIM covers a large fraction of the disk, here as much as 100%. Its morphology is a combination of diffuse emission and curved filaments, perhaps consistent with a ``dented sheet''. Typical Emission Measures contributed by the WIM reach up to as much as 50 pc cm in spiral arms, down to as low as a few pc cm at the faintest levels so far detected. For comparison, in the solar neighborhood the Reynolds layer has an emission measure of about 5 pc cm perpendicular through the disk.

H imaging allows straightforward determination of a crucial quantity: the fractional H luminosity contributed by the WIM in a galaxy. A detailed method on how to separate the WIM contribution from the total H luminosity has been described by Hoopes et al. (1996). Results are now available for M31 (Walterbos & Braun 1994), NGC 253, NGC 300 (Hoopes et al. 1996), NGC 247, NGC 7793 (Ferguson et al. 1996a), NGC 55 (Hoopes et al. 1996, Ferguson et al. 1996b, M81, M51, and M33 (Greenawalt 1997). A surprising result emerges: the contribution from DIG is 40 10 %, irrespective of the star formation rate in all these galaxies. Such a result might be expected if the H emission we observe from the WIM were in fact scattered light from bright HII regions in galaxies. However, the distinct spectral signature (see next section) and the distinct morphology of the WIM that is discernible in nearby galaxies (e.g. Walterbos & Braun 1994) make this unlikely. If the extinction is systematically less in the WIM compared to that in HII regions, this number may be somewhat less. This has only been addressed for M31 (Walterbos & Braun 1994, Greenawalt et al. 1997), where the corrected WIM fraction probably remains at least 20 to 30%. The high number for this fraction is relevant in that it forces us to accept that OB stars have to be the dominant ionization mechanism. In addition, the fact that this fraction is constant among galaxies argues against a strong influence for an external ionizing source. Instead, the constant fraction must be reflecting some fundamental property of the ISM and the distribution of ionizing sources in galaxies. Either the medium is similarly porous in galaxies with widely different star formation rates per unit area, or the ratio of field OB stars to total number of OB stars is similar in all galaxies.

© Copyright Astronomical Society of Australia 1997