The notion that certain galaxies expel dust from their main gas layer into the lower halo (z-height > 1 kpc) has only been recognized quite recently. For example, the starburst-driven expulsion of ionized gas from the centre of far-infrared (FIR) bright galaxies, like M 82, is now believed to entrain denser, neutral material forming a "vertical'' network of gas and dust streamers at the working surface of the expansion (Hughes et al. 1994; Alton et al. 1999a; Phillips 1993; Ichikawa et al. 1994). For non-starburst or quiescent galaxies, the occurence of such structures, or indeed any kind of protuberance from the notional dust lane, has rarely been remarked upon in the literature and, if so, usually en passant. Sofue et al. (1994) carried out a dedicated study of the dust lane in nearby NGC 253 and catagorize an ensemble of filaments and loops surmounting the traditional dust layer (scale-size 1 kpc). This galaxy, however, is known to be experiencing a starburst phase similar to that of M 82 (e.g. McCarthy et al. 1987) and, furthermore, its inclined aspect ( ) makes the actual height of such features above the stellar midplane uncertain.
Keppel et al. (1991) noted an abundance of "dust chimneys'' leaking out to heights of 1-2 kpc above the main absorption layer in NGC 891 (so-called "fishbone effect''). This nearby, perfectly edge-on disk has recently come under closer scrutiny by Howk & Savage (1997; HS97) who used optical images of sub-arcsecond resolution to isolate and describe the extraplanar material. In another approach, Alton et al. (1998a, 1999b) mapped the submillimeter (submm) thermal emission from the dark lane in NGC 891 in order to try and infer properties for the extraplanar dust structures (see also Israel et al. 1999). Whilst the submm observations remain difficult to interpret, the conclusion from the optical study was that high-latitude (z-height > 0.4 kpc) dust features have a B-band opacity of 0.5-1 and comprise % of the gas associated with the disk (assuming a ratio of gas-to-dust similar to the solar neighborhood).
Given that NGC 891 is known to have a reasonably active halo, with both neutral and ionized gas extending up to several kpc from the midplane (Dettmar 1990; Swaters et al. 1997; Rand et al 1990), it is not obvious whether dust chimneys are commonplace amongst disk galaxies. Indeed, we have shown (Rand 1996; hereafter R96) that NGC 891 possesses one of the brightest ionized gas extraplanar layers amongst nearby edge-on galaxies suggesting that the transfer of energy and mass between disk and halo is very pronounced in this case. Using the WIYN imager, Howk & Savage (1999; hereafter HS99) recently extended their optical observations to 7 edge-on systems, of which 5 were found to contain extraplanar dust. At the same time that the HS99 study was still in progress, we procured B-band images of the 10 galaxies which we had already observed in the H emission line (R96). The objective for both groups is to establish how commonplace dust chimneys are amongst spiral galaxies and to clarify how extraplanar dust might relate to recent star-formation in the disk below. The notion that dust chimneys may be connected with young stars stems from the observation, in starburst galaxies, that massive stars and supernovae collectively lift dust out of the main disk (an effect known as the "superwind''; Chevalier & Clegg 1985; Heckman et al. 1990).
There is significant overlap between the galaxies presented here and those investigated by HS99. Our sample is essentially fixed by those disks we had already imaged in H and, furthermore, there are perhaps only a dozen or so edge-on spirals which are sufficiently close to allow detailed study of extraplanar structures. Although HS99 preside over higher resolution optical images than ourselves we employ a radiative transfer model to separate more reliably the standard dust layer from putative extraplanar features (Sect. 3). This is important because if the object of interest is not perfectly edge-on like NGC 891, then signficant confusion arises between structure within the disk and features which truly extend out of the disk (indeed, for this reason, we reject from the final analysis some of the galaxies which HS99 label as manifesting extraplanar dust). After using unsharp-masking techniques to locate unequivocal extraplanar features we relate the occurence of such structures to H emission emanating from both the corresponding halo region and the disk below (Sect. 4.3). For a number of reasons, tracing extraplanar dust by extinction alone is likely to give a lower estimate to the amount of material existing outside the main dust/gas layer. Consequently, we analyse the vertical distribution of submm emission detected in NGC 891 (at the transparent wavelengths of 450 and m) in order to constrain the maximum amount of extraplanar material that may be present (Sect. 5.1).
An important corollary of this work is the rate of mass loss from quiescent disks (Sect. 6.4) and whether this is likely to be a rich source of gas and dust for the intergalactic medium (IGM). Numerically, quiescent spirals constitute one of the primary galaxy types both in the field and in rich clusters (Dressler 1980). Potentially, therefore, any seepage from the main stellar disk may contaminate the medium between galaxies with both metals and light-absorbing particulates (Wiebe et al. 1999). The latter, when dispersed over a large volume, are particularly effective at both blocking and reddening light passing through the IGM (Heisler & Ostriker 1988; Masci & Webster 1999; Alton et al. 1999b). This effect would explain the preferential optical-reddening of radio quasars viewed through the IGM of nearby clusters (Webster et al. 1995; Fall & Pei 1993; Malhotra 1997) and could eventually be implicated in the relatively large scale-size ( kpc) inferred for QSO absorption-line systems (Lanzetta et al. 1987). At the very least, even if dust chimneys do not possess sufficient kinetic energy to liberate grains completely from the host disk, they are still likely to play an important role in the circulation of metal-rich, dusty gas around the ISM. This mixing dynamo would constitute a key process in the chemical evolution and star-formation history of galactic disks (Edmunds 1996).
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