NGC 891, having an optical appearance similar to the Milky Way, and being nearby (D = 9.5 Mpc) and almost perfectly edge on (Swaters 1994), was the first target in the search for extraplanar DIG layers (Rand, Kulkarni, & Hester 1990; Dettmar 1990). There, a bright, widespread DIG halo was found, showing truly diffuse emission as well as several quasi-vertical filaments of height 2-3 kpc. The halo could be traced up to z = 3.5 kpc, but recent deep spectroscopy (Rand 1997; see below) shows that it extends to at least z = 5.5 kpc. Furthermore, the spectra confirm the earlier result that there are two vertical components to the emission, with electron density scale heights (assuming that the gas temperature and filling factor are constant with z) of 1 kpc and 5-6 kpc. The emission is concentrated to within about R = 8 kpc of the center of the galaxy, with the brightest region at about R = 5 - 8 kpc on the north side and a rather sharp cutoff beyond this region. Disk star formation, as traced by [CII] 158µm emission (Madden et al. 1993), shows a radial profile with very similar asymmetries (Rand 1994).
Almost without exception, the
H
filaments have their footprint
in bright HII regions in the disk
(Rand 1994).
Some are quite narrow and may be walls of chimneys
(Norman & Ikeuchi 1989),
others are broader and may be "ionization cones" : regions where
relatively exposed O and B stars have been able to ionize large
volumes of diffuse gas above and below the midplane
(Miller & Cox 1993,
Dove & Shull 1994).
In the case of the former, the connection
with HII regions suggests that star formation persists in these ~ 1 kpc
regions of disk for at least as long as the ~ 107 yr
dynamical time
(Norman & Ikeuchi 1989)
to produce the chimneys.
This connection between filaments and HII regions is also seen in
NGC 2188
(Domgörgen, Dahlem,
& Dettmar 1996)
and NGC 55
(Ferguson, Wyse, &
Gallagher 1996;
Wyse, this volume).
Since these observations, over a dozen edge-ons have been observed
with similar sensitivity, with the result that bright, extended DIG
layers as in NGC 891 are the exception rather than the rule
(Table 1).
Other galaxies with bright layers include NGC 4631 (although only
above the central few kpc;
Rand, Kulkarni & Hester
1992)
NGC 5775
(Dettmar 1992),
and NGC 3079
(Veilleux, Cecil, &
Bland-Hawthorn 1995).
However, most galaxies seem to show only one or a few isolated
patches or filaments of emission above the HII region layer, while
some show no extraplanar DIG at all
(Rand 1996).
The prominence of
the extraplanar DIG emission is related to the star formation activity
in the disk. Insofar as far infrared luminosity (LFIR)
is a
tracer of star formation activity (debate continues on what fraction
of the emission comes from dust heated by the ISRF; however, one
cannot use H emission as a
star formation tracer because of
the severe extinction problem), then the galaxies with high surface
densities of star formation (LFIR /
D252) generally have the
brightest and most prominent extraplanar DIG layers
(Rand 1996).
This general connection is strengthened by the afore-mentioned correlation
of halo DIG and disk [CII] emission profiles in NGC 891 and the
filament-HII region associations, and it is probably more accurate to
say that active star-forming regions tend to have associated
extraplanar DIG.
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Current directions in this area include attempts at better characterization of the background emission to allow heavier smoothing, thus making possible searches for emission at much fainter levels. Examples are the "shift and stare" technique (Donahue, Aldering, & Stocke 1995) and "charge shuffling" (Bland-Hawthorn, this volume). In this way, one can hope to determine the extent of halos at low column densities and their possible relevance as QSO absorption line systems.