DIG MORPHOLOGY

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] 158m emission (Madden et al. 1992), 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 kpc regions of disk for at least as long as the yr dynamical time (Norman & Ikeuchi 1989) to produce the chimneys. This connection between filaments and HII regions is also seen in NGC 2188 (Domgrgen, 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 () 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 () 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.

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.

© Copyright Astronomical Society of Australia 1997