It is certainly something of a surprise to find no obvious correlation between dust expelled
from the main absorption layer and either HII regions in the disk or enhanced DIG
brightness. Even for NGC 4013, which constitutes a possible exception to this statement,
only a general association is evident, with chimneys tending to occur over regions of the
disk where the DIG is most luminous. There is certainly no detailed correspondance apparent
with, for example, individual dust filaments forming a boundary layer to H
"bubbles'' expanding out of the disk. Observationally, there are severe obstacles in looking
for a correlation between H
emission and extraplanar dust. As far as the dust is
concerned we are most sensitive to dust features on the near-side of the disk (given that we
have selected on the basis of optical extinction). At the same time, H
emission
emanating from anywhere up to z-heights of 2 kpc is susceptible to extinction from these
same features. Indeed, for a typical chimney optical depth of
,
6563 Å emission-line radiation emitted by the DIG will suffer up to 0.6 mag of extinction depending
on whether the dust feature is situated in front of, or behind, the ionized gas. This value
excludes attenuation by the more general dust lane although we expect this effect to be
generally small (for example, with an edge-on, midplane optical depth AB=10,
0.1 mag at 1 kpc or 4 scale-heights above the disk). Such
considerations might argue that the recognition of extraplanar dust and the detectablility
of DIG emission are mutually-exclusive. The problem is distinctly exacerbated by our
necessary selection of edge-on disks. Choosing less inclined galaxies would, admittedly,
mitigate the effects of extinction, but at the same time, we could never be sure that the
structures we discover are extraplanar.
In the following sections, we try to assess how well our unsharp-masking technique reveals the true distribution of extraplanar grains. We do this in two different ways. Firstly, we use images of the (optically-thin) submillimeter emission recently detected from NGC 891. Secondly, we examine B-band images of the edge-on spiral NGC 55, which at a distance of only 1.6 Mpc offers superior spatial detail over galaxies comprising the R96 sample.
Alton et al. (1998a) obtained deep 450 and m images of NGC 891 with a spatial
resolution of 10'' and 16'' respectively (see also Israel et al. 1998). These
submillimeter maps, obtained with the SCUBA array on the James Clerk Maxwell Telescope,
trace primarily thermal emission from cold dust residing in the edge-on disk (temperature
17 K). Notably, however, fairly structured submm emission is detected up z=2 kpc in
both these images (Fig. 13). Given a nominal FWHM beamwidth of 15'' (700 pc) and
9'' (400 pc) at 850 and
m, respectively, it is reasonable to ask whether this
apparent high-z emission can identified with the extinction chimneys identified in our
B-band image. Indeed, an initial inspection of the SCUBA data (Alton et al. 1999b) showed
that the extended submm emission is much broader than the instrumental PSF, suggesting that
it might well correspond to high-latitude dust. To make a more careful comparison between
the extraplanar emission and the vertical extinction features, we use the results of the
radiative transfer fit given in Table 3. Adopting the parameters derived for the
distribution of optical depth, we produce an image of the standard dust layer which, after
smoothing with the appropriate PSF, can be compared with the submm maps. Our objective here
is to identify an excess in the submm maps which might be ascribed to the extraplanar
extinction features. We utilize calibration images of Uranus, taken at regular intervals
during the SCUBA observing schedule, to produce a signal-weighted map of the PSF at both 450
and 850
m. Before convolving the model galaxy with the measured PSF we ensure that the
chop direction is aligned for both the object and the beam images. This is important because
the chop for NGC 891 was along the minor axis and the side lobes of the beam are known to be
more pronounced along the chop direction. Having smoothed, in turn, the simulated disk with
the respective 450 and
m PSF, profiles in z-height were created, for both
wavelengths, at various locations along the major axis.
It was found that the widths of the simulated profiles were considerably larger than
the observed data. After several trials, we established that a modified exponential
z-height of 0.22 kpc gave a far more consistent fit between model and observation (at both
450 and m and for all locations along the disk). This value is somewhat smaller
than the original scale-height of 0.31 kpc but it seems likely that B-band radiation
transfer modelling of NGC 891 will over-estimate the height of the general dust layer
by incorporating high-z extinction features into the photometric fit (they are
particularly conspicuous in a blue filter). Figure 14 shows the simulated profiles,
with a 0.22 kpc scale-height, alongside measured profiles of regions of the disk exhibiting
high-z structure in the submm image. Our conclusion from this figure is that the detected
submm emission even at 30-60'' (1.5-3.0 kpc) is attributable to the main dust layer. The
wings of the PSF appear to amplify the tail of the exponential distribution so that spurious
high-z "features'' are produced which extend up to 3 beamwidths from the midplane. To
clarify the situation we also ascertained the fraction of emission occuring at z>1 kpc in
both the 450 and
m SCUBA images. This quantity can be expected to vary at different
locations along the disk if it is not attributable to the wings of the PSF. Moreover,
the high-z flux should correlate at both 450 and
m if it has a physical, rather
than an instrumental, origin. Figure 15 demonstrates quite clearly that a constant
fraction of 450 and 850
m flux is recorded at z>1 kpc severely undermining our
precursory impression that the submm filaments might represent high-latitude dust. The
detectability of high-z extinction features in thermal emission is, in fact, doubtful if
one considers that a chimney of intrinsic
will be smoothed out parallel to
the major axis by a 16'' beam in the
m image. The
that is then
effectively measured is
or 0.5 mJy/beam for cold (17 K) dust (Alton et al.
1999b). The anticipated emission level, therefore, is an order of magnitude smaller than
that recorded at z=1 kpc from the (convolved) main dust layer (Fig. 14).
Although it is disappointing not to be able to use submm images to infer the true
distribution of high-z dust, we are still in a position to place an upper limit on the
total amount of material in the extraplanar layer. Refering once again to Fig. 14,
the observed submm emission seldom deviates more than
from the simulated profiles.
We therefore attribute, as an upper limit,
of the flux to grains located outside
the standard exponential layer (5 mJy/16'' beam and 16 mJy/10'' beam at 850 and
m
respectively). Integrating over a z-height of 1-2 kpc we find that <5% of both the
450 and
m flux can originate from high-latitude dust. Thus, with the proviso that
high-z grains are not much colder than 17 K, the SCUBA observations indicate that less
than 5% of galactic dust exists outside the notional absorption layer. For lower grain
temperatures this upper limit must be relaxed somewhat - dust temperatures of 10 K would
allow up to 9% of galactic dust to reside outside the disk. Considered together, however,
our optical and submm images for NGC 891 tend to constrain the fraction of dust outside the
main extinction lane to
%. If dust chimneys possess solar-type gas-to-dust ratios,
our results imply that only a few percent of the neutral gas present in spiral galaxies
reside above the main disk.
At a distance of 1.6 Mpc (Puche et al. 1991), NGC 55 is ten times closer than the galaxies
comprising the Rand (1996) sample (Table 1). Under normal conditions of optical
seeing, this object provides a remarkable opportunity to resolve regions down to a physical
size of 13 pc - a region much smaller than the anticipated width of dust chimneys (30 pc).
NGC 55 is a late-type galaxy classified as SB(s)m (de Vaucouleurs et al. 1991). With a
D25 size of 18 kpc, it is physically smaller than most of the spirals in our sample
(cf. NGC 891 with a D25 corresponding to 37 kpc) and its overall optical appearance is
more irregular than a typical
disk. In emission-line radiation, NGC 55 possesses
bright, knotted structure in the disk and a spectacular network of curvi-linear filaments
extending up to 2.6 kpc away from the midplane (Ferguson et al. 1996; Bomans & Grebel
1993). The DIG is most conspicuous within the inner 3-4 kpc of the galaxy where the
extraplanar H
structures correspond well to fragmented shells of gas presumably
swept up by superbubbles expanding from recent enhanced star-formation in the central disk.
To compare with the H
image taken by Ferguson et al. (1996), we obtained archival
CCD data of NGC 55 in a blue filter. Since this object is located in the Sculptor Group
(Dec =
)
the only image that could be retrieved was a 120 second exposure of
the central
region, which had been taken on the 4-m Anglo-Australian
Telescope. These archival frames were reduced in the standard manner and an unsharp-mask
B-band image created using the same method adopted for the R96 sample. Figure 16
compares the extinction evident in the unsharp-mask image with the pronounced network of
H
filaments located at the centre of the galaxy. We have superimposed on the disk
a box to denote the apparent centroid of the most prominent H
shells. The dust
does not extend to such large distances from the midplane as the diffuse emission-line gas
(only up to
z=700-1000 kpc in fact). However, there is a suggestion that, perhaps, at
either side of the active centre, the dust clouds have been prised away from the midplane by
the superbubble's impact on the main gas layer. If this is the case, the situation would be
reminiscient of M 82, where the central starburst cavity levers ambient dust and gas away
from a rather chaotic-looking absorption layer (Ichikawa et al. 1994).
We should point out that the dust lane in NGC 55 assumes an equally non-uniform, distorted
shape in the outlying parts of the disk as it does close to the central star-forming
activity. This can be inferred from the extensive R-band image of Ferguson et al. which
was used in the subtraction of the continuum from their H
filter. Indeed, the
maximum z-height at which dust manifests itself through R-band absorption varies little
along the major axis and is typically no more than
kpc. Parallel to the disk, the
horizontal width of the "vertical'' absorption structures is generally
40 pc which is
consistent with the width of dust chimneys in NGC 891, as measured by the Hubble Space
Telescope (HS97). The fact that vertically-extending dust structures do not appear to be
concentrated where the DIG is brightest might be considered indicative that disruption in
the disk and halo occur on different timescales. A chaotic dust lane may be the remnant of a
more widespread epoch of star-formation whilst the brightest H
filaments above the
central disk represent a more recent episode of stellar creation. We elaborate these ideas
in the next section.
Although we obtain significantly more spatial detail in using NGC 55, our reliance on unsharp-masking to identify concentrations of high-latitude dust means that we are always sensitive to the relative geometry between stars and dust. The proximity of NGC 55, however, makes it an ideal candidate for future submm/mm imaging where sampling of the optically-thin thermal emission from high-z grains can be carried out.
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