The appearence of the dust lane, regardless of its orientation with respect to the stellar body, can be described in the following way:
the dust lane lies exactly along an axis of the stellar body and presumably represents a disk seen exactly edge-on. A good example is NGC 6702.
a slight curvature is present, suggesting a disk seen almost edge-on. NGC 5485 is an example.
in a few cases and notably in NGC 1947, a set of parallel dust lanes is present on one side of the galaxy. The lanes could be the result of a system of coplanar rings seen at an angle.
a full ring is visible; from its flattening, and on the hypothesis of intrinsic circularity, one can deduce the tilt angle. The best example is NGC 5266 and possible cases are Anon 0151-498 and Anon 0418-583.
the prototype is NGC 5128 where the dust lane, aligned along the minor axis, bends toward the major axis in the outer regions. Additional cases are NGC 5363, NGC 5266, Anon 0151-498, and Anon 1029-459.
In Fig 1 six representative cases are illustrated with high resolution photographs obtained at the prime focus of the 3.9m AAT.
Figure 1. Dust lane ellipticals photographed with the 3.9m AAT.
A straightforward question arises from the above morphological description: is the dust distributed to form a disk or is it confined to a ring? While in a few cases the ring is clearly visible, in the case of NGC 5128 there is evidence that the gas associated with the dust forms a disk both in ionized (Bland 1985) and neutral (van Gorkom 1986) hydrogen.
Concerning the extent of the dust lane with respect to the galaxy, a variety of situations are encountered. There are galaxies where the dust lane is confined to the innermost regions of the galaxy (e.g. NGC 4374), and galaxies like NGC 5128 where the dust lane can be followed up to the limit of detection of the luminous stellar material. In Table I we have indicated for each galaxy whether the dust extends to the inner, intermediate, and outer regions. One sees that inner and outer dust lanes are present in approximately equal number. Elliptical galaxies with inner dust lanes are easily found with modern detectors and certainly in the future they will outnumber those with extended dust lanes.
Before proceeding to analyze the properties of ellipticals with dust lanes it is necessary to define the sample according to precise morphological criteria. In order to do this the catalog of dusty elliptical galaxies and its updated version (Ebneter and Balick 1985, Ebneter 1986) have been used as basic references, carefully selecting from them the ellipticals with well defined dust lanes. Patchy dust structures were excluded, even those regularly distributed along one axis, as in the case of NGC 1316 (Fornax A).
The first part of Table I lists the elliptical galaxies with dust lanes along the minor axis. Due to the special character of this morphology, the identification of these objects is rather unambiguous. In fact we do not know of any disk galaxy, the alternative to an elliptical, where the bulge is elongated perpendicularly to the plane defined by the disk. However, one should be very careful on this point, since there is a tendency in the usual classification schemes to call S0 any galaxy where a stellar body is crossed by a dust lane, regardless of orientation. Thirty objects are listed in Table I as minor axis dust lane ellipticals. In Ebneter's catalog there are 34 objects listed as minor axis dust lane ellipticals, and 23 of these are in common with those of Table I. Also in Table I there are 6 objects which are classified in different way by Ebneter, while 9 cases given by the latter are discarded.
It is worth mentioning why NGC 3108 has been omitted from Table I; in addition to the dust lane crossing the stellar body along the major axis, there is a trace of a luminous ring associated with the dust. This galaxy is probably the second instance of an elliptical galaxy with an outer luminous ring, the other being AM 2020-5050, recently discovered by Whitmore at al. (1987).
|ident.||Bmag||Vr||MB||b/a||Vmax||0||Vmax / 0||(V/)*||ext||Ref.|
The second part of the Table I lists nine elliptical galaxies with a dust lane along the major axis. They are all in the catalog by Ebneter (1986), who, however, lists 37 objects belonging to this type. The reason for the drastic reduction lies in the fact that an edge-on S0 galaxy can be easily confused with an elliptical galaxy with a dust lane along the major axis. If the disk-to-bulge ratio in an S0 galaxy reaches a value such that the luminous disk is at the limit of detection, then it is very difficult to make a distinction. The size of the disk with respect to the bulge plays also a role in the misclassification of major axis dust lane ellipticals. There are cases in which the presence of a disk is missed if the exposure is too low; and vice-versa inner disks in S0 are cancelled out by a deep exposure. Given such difficulties we tried to select only those cases in which the presence of an even tenuous luminous disk can be ruled out. In connection with the relationship between S0 and major axis dust lane ellipticals it is interesting to remark that the morphology of Anon 1029-459 (Fig.1), where the dust lane along the major axis is strongly warped on both sides, closely resembles that of the S0 NGC 5866 (Sandage 1961) with the only difference that the latter possesses an additional luminous disk. Is the origin of the dust lane in these two galaxies the same?
The last part of Table I lists the elliptical galaxies with a dust lane along an axis intermediate between the major and minor axis. Seven cases were selected out of the 18 given by Ebneter (1986). This configuration can be explained by supposing that we are looking at triaxial galaxies at an angle to the planes where the gas and dust are permitted to settle, contrary to what happens in the cases of major and minor axis dust lanes, where the line of sight lies approximately in these planes. An alternative explanation is that we are looking at gas and dust not yet settled.
It is worth mentioning a couple of elliptical galaxies whose morphology is related to that of dust lane ellipticals, viz. NGC 5077 (Bertola et al. 1986) and NGC 7097 (Caldwell et al. 1986). These are characterized by a narrow distribution of ionized gas along the minor and major axis respectively. No dust has been detected. Very probably the same mechanism that has produced the dust lane ellipticals has given rise to such peculiar objects.
NGC 1052, which is listed in Table I as a minor axis dust lane elliptical according to images by Sparks et al. (1985), is characterized by an HI distribution (van Gorkom et al. 1986) much more extended than the dust, but aligned also with the minor axis.
Spindle galaxies with the gaseous ring along the minor axis of the stellar body seem not to be related to minor axis dust lane ellipticals for the following reasons: in several cases the stellar body is clearly not an elliptical galaxy but an S0 with a well defined disk and bulge. In addition there is a tendency for the warps of the gaseous ring (e.g. NGC 4650A) to align with the minor axis of the galaxy in the outer regions, contrary to the behaviour of the minor axis ellipticals. This suggests a different dynamical origin for the two phenomena.