![]() | Annu. Rev. Astron. Astrophys. 1991. 29:
239-274 Copyright © 1991 by Annual Reviews. All rights reserved |
3.2 Rotation
Kinematic observations of elliptical galaxies have provided several surprises.
First, it was found that luminous ellipticals rotate slowly
(32,
169).
A modern observation of NGC 1600 gives
a rotational velocity of 1.9 ± 2.3 km s-1 along the
major axis, resulting in
a v / < 0.013
(182).
Although in one particular
case projection effects might play a role, the luminous ellipticals as a class
have a mean v /
0.2. Binney showed that this is significantly
lower than expected for oblate isotropic rotators
(35). Hence the
shapes of bright elliptical galaxies are supported by anisotropies in the
velocity distribution
(34,
36).
A useful diagnostic indicator
is (v /
)*,
which is the measured v /
divided by (v /
)iso,
the value expected for oblate rotationally flattened galaxies. A good
approximation is (v /
)iso = [
/ (1 -
)]1/2, where
is the ellipticity of the
galaxy (189). The
parameter (v /
)* is almost independent of inclination for
oblate models.
Bright ellipticals have a mean (v /
)*
0.4.
The second surprise was the rapid rotation of spiral bulges, with
v / > 0.5
(117,
171,
194,
231,
370).
Most observations have been done for
(nearly) edge-on bulges in early-type spirals. Contamination of bulge light
by light from the disk is a major problem, and many studies have tried to
avoid this problem by observing at several position angles and/or offset slit
positions. The total number of well-observed bulges is still relatively
small. Detailed modeling, taking into account the influence of the disk, has
shown that bulges rotate about as fast as expected for oblate isotropic
rotators (117,
177,
178,
185).
The third surprise was the observation that ellipticals with
-18 > MB > -20.m5 have (v / )*
0.9, showing that
intrinsically faint ellipticals rotate about as rapidly as bulges, most of
which have comparable luminosities
(68).
There is a relation
between luminosity and v /
, in the sense that more luminous ellipticals
rotate slower. However, the scatter about the relation is large, and its
origin is not well-understood. Wyse and Jones noted that v /
correlates
with surface brightness, in the sense that galaxies with low surface
brightness have lower v /
(376).
They speculated that the high
surface brightness galaxies had dissipated more energy during their formation,
and have thus a higher rotation. This is not a complete explanation, as the
specific angular momentum J / M is also lower for the bright galaxies
(27).
Subsequently, it was found that galaxies with ``disky'' isophotes have
systematically high v /
(23,
25,
58). Galaxies with
``boxy'' isophotes have a large spread in v /
. A possible explanation is
that the galaxies with disky isophotes contain relatively luminous disks, as
indicated by a recent statistical analysis of the available data
(295).
The disks may dominate the observed kinematics, because the
measurement techniques are more sensitive to the low velocity dispersion
system in a multi-component galaxy (e.g.,
122,
231,
370). Other
parameters, like X-ray and radio-emission are
also correlated with the isophotal shape
(28,
193). This
suggests that boxy ellipticals and disky
ellipticals are intrinsically different, and may have had a different
formation history. Detailed analysis of isophote shapes and kinematic
properties may help estimate the light contribution of the disks (e.g.,
26,
295,
312).
Very recently, several groups have found slow rotation in dwarf ellipticals
(27,
60,
155).
Bender and Nieto
(27)
observed five faint ellipticals with
MB > -18m. All of
these ellipticals have v / < 0.3, and four have (v /
)* < 0.5. The
lower surface brightness galaxies generally are rotating slower. This result
has added to the confusion in explaining the rotation of ellipticals. It is
possible that the low mass and low surface brightness systems have formed in a
different way. Supernova driven winds may have been more important during their
formation (e.g.,
77).
These winds may help to decrease the
v /
of a galaxy. Other types of explanations have been considered also
(27).
![]() |
Figure 2. The rotational parameters of
ellipticals correlated with
dynamical and structural parameters. In (a), the rotation
velocity v is
plotted versus the mean velocity dispersion < |
Recent results on the rotation of globular cluster systems and planetary
nebulae in the outer regions of elliptical galaxies indicate that their
kinematics differ from that of the luminous central regions. Data on globular
cluster systems in the two Virgo ellipticals M87 and NGC 4472 suggest that their
v /
0.3
(256), comparable
to the v /
of the
globular cluster system in our galaxy and Andromeda, and consistent with the
flattening of the luminous part of the galaxies. The number of observed
globulars in these two cases is still small, and hence these results are very
preliminary. The planetary nebula system in NGC 5128 (Cen A) is reported to be
rotating with a velocity of 100 km s-1 at
4re, as compared to 40 km s-1 for
the luminous, inner, part of the galaxy
(119,
372). This may be
interpreted as a gradient in the
rotational velocity, but it is also possible that the formation process has
caused a systematic difference between the rotational properties of the
different components.
We have collected the available high quality data from references
(23,
27,
67,
68,
125,
182).
Photometric parameters and distances were taken from
(114),
or, if not available, from the
references mentioned. The data are shown in
Figure 2. All of these plots show
a correlation and large scatter. In Figure 2e,
the specific angular momentum
is plotted against absolute magnitude. The solid line is the relation expected
for galaxies with v /
= 1. There is a significant spread below this line for
luminous galaxies.
At present, no satisfactory explanation can be given for these results. The
systematic differences between low-mass and high-mass galaxies cannot be
explained in a scenario of purely dissipationless formation
(68).
Simulations of dissipationless hierarchical formation predict a large scatter
for v / , but no large
systematic trend with mass
(18,
378).
The lower specific angular
momentum of bright galaxies
cannot be explained completely by differences in the dissipated energy during
their formation.
The planetary nebula and globular cluster systems provide very interesting independent results. The fact that they appear to rotate faster than the luminous part of the galaxy they are associated with argues strongly against formation in a monolithic dissipative collapse. In such a picture the central parts are predicted to rotate faster than the outer parts. Merger simulations have shown that the inner parts of galaxies can lose their angular momentum quickly to the outer parts (15, 17). It is possible that the planetary nebula and/or globular cluster systems have the same kinematics as the halo; this exciting possibility deserves great attention.