Copyright © 1985 by Annual Reviews. All rights reserved
|
| Annu. Rev. Astron. Astrophys. 1985. 23:
147-168 Copyright © 1985 by Annual Reviews. All rights reserved |
In the previous section, we discussed cases where the outer ring is in the same plane as the main disk of the spiral or S0. However, a completely different situation occurs in polar ring galaxies. In these galaxies the material is located in a plane perpendicular to the main disk in a polar configuration.
Interestingly enough, one of the most peculiar galaxies in the Hubble Atlas (99), NGC 2685, is thought to be related. It is in a way a unique case, presenting several differences from the standard polar ring galaxies. Indeed, the gas surrounding the main body is in the form of a "helix" (99) rather than a polar ring. Stellar rotation data for NGC 2685 (107) show that the ratio of rotational velocities to central velocity dispersion is much higher than in an elliptical, while the velocity dispersion itself is low. Thus the stellar body should be a S0 seen edge on. This is confirmed by photometry (45). The gas motions in the helix indicate that its rotation axis is aligned with the major axis of the stellar body (29, 107, 117, 141). The colors of the helix are close to those of late-type spirals, and the numerous bright knots are probably regions of star formation. On deep photographs (e.g. 2) a faint outer ring is visible, rich in H I and rotating around the same rotation axis and in the same sense as the stars (117).
Several S0s with polar rings have been identified in the last few years, of which only one, ESO 474-G26, has an additional outer gaseous ring perpendicular to the first one (116). Schechter et al. (109) show on spectroscopic grounds that NGC 4650A is not a prolate galaxy with a gaseous ring encircling its minor axis, as was thought earlier (66), but rather that it is an S0 seen edge on with a ring (or rather, annulus) populated by polar orbits. Schweizer et al. (116) give an extensive and interesting discussion of S0s with polar rings. They present kinematic and photometric data on A0136-0801, showing it to be an S0 seen edge on, with a polar ring extending out to three times the radius of the main body and having a flat rotation curve. This contrasts with the situation in UGC 7576 and II Zw 73, where the H I measurements (110) indicate that the polar annuli exhibit little or no differential rotation, as would be expected for a thin ring seen in projection.
Polar rings offer a unique opportunity for probing galactic potentials far away from the center of the galaxy and in directions perpendicular to the disk. Thus, rotational velocities of the gas in the polar ring and stellar velocities in the main body of the galaxy were measured for A0136-0801 (116). Since the velocity dispersion in the outer parts of the disk is small, the stellar velocities are directly comparable to the gaseous ones. The near-agreement between the two sets of data argues that a presumed dark halo dominates the motions in the outer parts and is nearly spherical in shape. As the velocity mismatch is less than 10-15%, the halo equipotentials must have an axial ratio of the order of 0.9 or more (60, 116).
Schweizer et al. (116) list 8 sure cases, and another 14 candidates, of polar ring galaxies. The rings are invariably external to the disk, with the extent of the central hole being 1-2 disk radii. The absolute magnitudes of those galaxies for which redshifts are available range typically from -18 to -21 in B (H = 50 km s-1 Mpc-1). The geometrical centers of the polar ring and of the main body coincide very well, contrary to the ring galaxies discussed before. The plane of the ring makes an angle of roughly 90° with the main body, although deviations of up to 25° have been found (cf. 109, 110, 116, 153). The surface of the polar rings is not always planar. UGC 7576 has a bending of about 5° (151), while values of up to 10° can be inferred for other galaxies pictured in (116). Most of the bending is antisymmetric and regular, with a few exceptions like UGC 7576.
What kind of galaxies have polar rings? The handful of objects studied
so far look like S0s, and for three of them
(NGC 2685, NGC 4650A, and AO136-0801) there is both
spectroscopic and photometric evidence that the
galaxy is an S0. No sure cases of ellipticals or later-type disks have
yet been reported. Compared with other S0s, those with polar rings do
not seem in any other way exceptional. They have normal H I contents
(110)
and colors
(45,
83).
They are often found in binary systems or in
small, loose apparent groups, but with one possible exception, they
avoid clusters. In cluster environments the polar ring apparently can be
easily stripped or disrupted. There is a hint that S0s with polar rings
are smaller and less bright than average. Note that in the three cases
that have been properly studied in H I, the gas in the ring amounts to a
few times 109
M
,
i.e. high compared with a dwarf galaxy.
Are there many more S0s with polar rings waiting to be identified? Their peculiar morphology, with two planes of which one completely dominates the appearance if not seen edge on, makes identification difficult except in the favorable geometry exhibited by, for example, A0136-0801. Schweizer et al. (116) evaluate that 2% of all field S0s have polar rings. This estimate is thought to be good to better than an order of magnitude. Much care is necessary when identifying polar ring S0s, since in some cases confusion with other kinds of objects is possible. One such example has been NGC 4650A (66, 109); another one could be NGC 414A&B (143), claimed to be a ring galaxy with its companion practically superimposed, despite the fact that both galaxies in the pair are perfectly intact, the ring is accurately centered on one of them, and the radial velocity difference is 450 km s-1 (59).