9.1 Polar Ring Galaxies

The method that has provided the most clear cut evidence for flattened halos uses polar ring galaxies (PRGs). A catalogue of such objects was compiled by Whitmore et al. (1990). The main drawback of such objects is that they are rather rare, so few have been studied in detail. Nevertheless, given the difficulties with other techniques, they are a welcome tool for probing dark halos.

Schweizer, Whitmore and Rubin (1983) and Whitmore, McElroy and Schweizer (1987) pioneered early attempts to use the kinematics of polar rings to infer something about the halo shape. Subsequently, Sackett and Sparke (1990) developed a method which uses all the observational data in deriving a realistic gravitational potential. They applied their technique to the PRG NGC 4650A. This galaxy also has a planar disk, so that there is velocity information in two planes. Consequently, the overall potential, and thus the shape of the halo, is reasonably well constrained. Sackett and Sparke (1990) fit the two rotation curves with a model consisting of a bulge, disk, axisymmetric dark halo and massive polar ring. They concluded that the halo of NGC 4650A probably had a flattening of between E3 and E7, although a spherical halo could not be ruled out. (Recall that the designation En is defined so that n = 10(1 - b/a), where a and b represent the major and minor axis, respectively.) More recent work (Sackett DMW) appears to exclude a spherical halo and points to an E6 halo surrounding NGC 4650A (see Figure 3). However, it should be stressed that NGC 4650A has the flattest halo of the three galaxies originally studied by Whitmore et al. (1987), so that further studies of more of these objects would be valuable.

Figure 3. Model fits to the equitorial disk (upper panel) and polar ring (lower panel) of the polar ring galaxy NGC 4650A. The upper panel shows the predicted velocties for dark halos of different flattenings. The E6 halo is the preferred solution. Figure courtesy of P. Sackett.

Sackett (1991) has pointed out that the full projected velocity field of the ring around 4650A and similar galaxies may be a particularly powerful diagnostic. The non-circularity of polar orbits leads to a misalignment between kinematic and morphological axes (cf. Teuben 1991 and references therein). This kinematical twisting acts as a probe of the underlying potential. The principal difficulty may lie in separating such twists from physical warps, but studying kinematic twists has the potential to yield shape information.

At least one galaxy halo appears to be appreciably flattened. Unfortunately, PRGs are unusual objects and their halos may be atypical. For instance, they have clearly suffered some kind of merger, and this could have given rise to a flattened halo. Alternatively, it may require a flattened halo to define a plane dynamically to ensure that the ring that forms is polar at all. In this case the scarcity of PRGs may argue for the majority of halos to be spherical. It is therefore necessary to compliment studies of PRG galaxies with work on the shapes of halos around other types of galaxy.