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 1994;
Bregman & Houck 1997)
and NGC 4631
(Wang et al. 1995)
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] 158 µm 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.