DIG AND OTHER GASEOUS HALOS

The galaxies with the brightest extraplanar DIG layers also tend to have bright and extended radio continuum halos (Rand 1996). Examples are NGC 891 (e.g. Dahlem, Dettmar, & Hummel 1994), NGC 4631 (e.g. Golla & Hummel 1994), and NGC 5775 (Dettmar 1992; Duric, Irwin, & Bloemen 1996). At the other end of the scale, NGC 4244, with no indications of extraplanar DIG (Walterbos & Braun 1996), was not detected at all in radio continuum (Hummel, Sancisi, & Ekers 1984). Dettmar (1992) reports an association between filaments of radio and H emission in the halo of NGC 5775. One might expect an H-radio correlation if magnetic fields associated with the thermal gas serve to confine the cosmic-ray electrons as they try to escape the disk (N. Duric, pers. comm.). Observational connections between radio halo and star formation distributions in edge-ons has also been discussed by Dahlem, Lisenfeld, & Golla (1995). Most of the galaxies in Table 1 have not been observed with sufficient sensitivity and resolution to search for a radio halo.

Diffuse X-ray halo emission is also difficult to detect and has been searched for in only a few of these edge-ons with ROSAT. NGC 891 (Bregman & Pildis 1992; Bregman & Houck 1997) and NGC 4631 (Wang et al. 1994) are notable for having relatively bright X-ray halos occupying about the same radial range as the H halos. However, this trend is not perfect: patches of diffuse halo X-ray emission have been found in NGC 4565 (Vogler, Pietsch, & Kahabka 1996), which shows no extraplanar DIG.

NGC 891 has a very thick disk of 21-cm emission, and a careful study of the origin of this high-z gas has been carried out by Swaters (1994). The kinematic information allows inner-disk high-z gas, which may be accelerated by supernova activity, to be distinguished from outer-disk gas, which may be due to warps and flares. The result is that the most prominent inner-disk high-z emission is coincident with the brightest region of halo DIG and radio emission, sitting above the brightest [CII] 158m emission in the disk (all at about R=5-8 kpc on the north side). This HI is detected up to 2.5 kpc above the disk. There does not seem to be a good correlation between individual HI and H filaments, however, although the lower (20'') resolution of the HI may hamper the comparison. Other bright star forming edge-ons (NGC 3079, NGC 5775) show HI arcs, shells and filaments extending several kpc from the plane (Irwin & Seaquist 1990; Irwin 1994). Connections between these and H features are not yet clear.

The emergent scenario, then, which needs to be tested further, is that star formation activity, when sufficiently intense, leads to the elevation of gas from the thin, star-forming layer, through chimneys and superbubbles, leading to halo DIG, HI and X-ray emission. Meanwhile, diffusion and possibly localized regions of convection of cosmic rays produced in supernovae (e.g. Duric, Irwin, & Bloemen 1996) produce cosmic ray halos, traceable through radio continuum emission.

© Copyright Astronomical Society of Australia 1997