There are two different kinematical components in the ellipticals with dust lanes. The first deals with the motions of the stars while the second deals with the gas associated with the dust. As listed in Table I, we possess information on the rotation and velocity dispersion for nine minor axis, one major axis, and two intermediate axis dust lane ellipticals. Data reduction is in progres for NGC 3302, NGC 4370, and NGC 5745 (Bertola et al. 1986).
The ionized gas has been detected in all but two minor axis dust lane
ellipticals
(Anon 0632-629 and Anon 0641-412). As a general result the rotation
axis of the gas
is perpendicular to that of the stars. This fact has deep implications
on the origin
of the gas and dust and gives strong support to the idea that they were
acquired from outside. A recent and detailed study
(Bland 1985)
of the ionized gas in NGC 5128 leads to the conclusion that the mass of
the gas is insignificant compared
with the mass of the stellar component. At this point it should be
mentioned that
sixteen ellipticals in our sample were detected by the IRAS
satellite. Dust mass estimates
(Zeilinger 1986)
from these data are in the range of 105 รท 106
M
, so
that the mass of the gas could be at least a couple of order of
magnitudes larger.
In this connection, an analysis on the possible self-gravitation of the
dust rings in ellipticals similar to that for polar rings in S0
(Sparke 1986)
is desirable.
Unfortunately no emission lines were detected in the only case of a major axis dust lane elliptical that has been studied (Anon 1029-459). If the gas is acquired from outside one would expect to find cases in which the gas is counter-rotating with respect to stars, as in NGC 7097 (Caldwell et al. 1986) and in NGC 4546 (Galletta 1986). From the data in Table I it appears immediately that no slow rotators relative to the isotropic oblate rotator are present among the dust lane ellipticals, although the brightest object reaches MB = -22.6. Whether this fact is due to the small sample at our disposal or is an intrinsic property of dust lane ellipticals, is a point to be checked with additional observations, in order to establish whether the nature of dust lane and normal ellipticals is similar or distinct.
It is worth noting that the rotational velocity of the major axis dust lane elliptical Anon 1029-459 is as high as 210 km sec-1, which is not reached in any other elliptical galaxy. This casts some doubt on whether ellipticals with dust lanes along the major axis, whose numbers have been so drastically reduced in our sample, really exist. This again is a point which needs future observations.
Considerable information on the intrinsic shape of the minor axis dust lane
ellipticals can be deduced from kinematical observations. For a minor
axis dust lane
elliptical the pure oblate configuration is excluded. In fact in this
case, if the gas
is captured from outside it tends to align with the major axis in the
inner regions
(Lake and Norman 1983),
causing a central discontinuity which is not observed. On
the other hand the pure prolate configuration is also excluded in those
cases with
high values of Vmax /
0 due to the
inability of the models to reproduce the case
(Richstone and Potter
1982).
Therefore it follows that the shape of a minor axis
dust lane elliptical is the triaxial one. The rotational velocity which
is observed
along the major axis of the galaxies is a combination of the streaming
velocity of
the stars and of the tumbling around an axis in the plane defined by the
minor and
intermediate axis. In order to give rise to a well defined dust lane,
the latter has
see an almost stationary potential. Therefore the conclusion is that the
tumbling
motion, if any, has to be very small with respect to streaming. Some
ideas as to
whether we are looking closer to the intermediate axis or to the minor
axis, can be
derived by the amount of streaming which is higher in the first case
than in the
second. Then by means of the tensor virial theorem, a lower limit of the
axial ratio
b / a or c / a can be deduced. If warps are present in the
dust lane, an upper limit to
these axial ratios can be set by assuming that the warps show the
direction of the
original infall, on the hypothesis that the warps are transient phenomena
(Caldwell 1984;
Bertola et al. 1985).
Recent photometric studies of dust lane ellipticals concern NGC 5266 (Caldwell 1984, Varnas et al. 1986) and about twenty other cases whose reduction is in progress (Bertola et al. 1986). All cases are characterized by the fact that the r1/4 law is more or less followed. It seems that the photometry does not help so much in discriminating S0's from major axis dust lane ellipticals.