Annu. Rev. Astron. Astrophys. 1985. 23: 147-168 Copyright © 1985 by Annual Reviews. All rights reserved

4.3. Gas Around S0s

Some hope of finding more polar ring galaxies is provided by the detection and subsequent mapping of neutral hydrogen in S0 galaxies. The detection statistics (9, 145) seem to be beset with confusion problems. The confusion of dwarfs in the same beam in the single-dish spectra can be resolved by subsequent mapping (e.g. 118, 122). Several noise peaks at the expected place have apparently been taken as signal, in view of the failure of confirmation by other observers, both for ellipticals and S0s (e.g. 61). Yet more serious problems remain.

Closer inspection of better plate material revealed that many galaxies classified S0 on poor plates are in fact early-type spirals. Of the list of 13 southern S0s detected by van Woerden et al. (144), only 3 are classified S0 in the Revised Shapley-Ames Catalog [RSA (101)], one of which has a low-surface-brightness disk containing the hydrogen (NGC 5084; compare 15 with 146). The remainder are early-type spirals, mostly barred, and frequently having outer rings (as discussed in Section 3). In fact, Hawarden et al. (51) present several such cases, some of them having a ring shape. The subject of galaxies with low-surface-brightness disks having high rotational velocities indicative of giant galaxies is a fascinating one, and one on which more papers will be forthcoming (e.g. A. Bosma & K. C. Freeman, in preparation). We note in this connection that on deep-sky-limited exposures of several galaxies, an outer ring (or pseudoring) appears, apparently closely related to the outer rings already discussed in Section 3. Old puzzles such as Hoag's object (56, 84) and NGC 6028 (157), which are seemingly related, are still unclear.

The mapping results now available for the H I distributions in S0s (62, 63, 118, 122, 146, 147) confirm the impression of heterogeneity: Confusion with dwarfs remains a problem, and many galaxies labeled S0s are early-type ordinary or barred spirals. In particular for the latter, outer H I rings are found coincident with outer optical rings, though in some cases the H I extends beyond these. Several other galaxies, however, are of a different nature. NGC 4203 (see also 19) may have a gas disk that looks like a pseudoring structure, and NGC 1023 is surrounded by a large amount of H I that is concentrated partly in dwarfs, but that is also in extended structures vaguely reminiscent of rings (50, 98).

The most interesting cases are those in which the gas seems to be in out-of-plane orbits with respect to the main body. A case in point is NGC 3998, which has been treated as either an S0 (146) or an elliptical galaxy (10, 61), but most recently as an S0 with a polar ring (11), for which the H I ring is nearly at right angles with the optical image (63). NGC 2787 and NGC 4262 are SBOs with clear H I rings that are not aligned with the main body. Furthermore, the ratio of ring size to bar size is larger than the value of 2.2 that is found so often in normal barred galaxies with outer rings. Thus the H I rings around these galaxies may lie in a near-polar orbit. For NGC 4262 the outermost isophote in (150) makes an angle of 54° with that of the gas ring, if both are assumed to be circular.

Thus there could be several S0s with H I polar rings, with a frequency of 2-3 out of 200 galaxies searched for H I; of these, we think roughly half can stand up to close scrutiny and still be called S0s. The total number of S0s with polar rings seems therefore not to exceed a few percent.