The PRC is divided into four categories, based on the certainty of the
identification. This identification is difficult because a polar-ring
galaxy is only obvious if it is seen in a certain orientation on the
sky. A graphic illustration of this problem is shown in
Figure 1, in
which a flat disk and an orthogonal ring, whose inner radius is the
same size as the outer radius of the disk, are shown from a variety of
viewing orientations. The angle
is the amount the ring has been
rotated, while the angle
is the amount the
disk was subsequently
rotated. Finally, the model is silhouetted to depict a two-dimensional
observation on the sky. Only when both ring and disk are seen
relatively edge-on is the identification obvious. If the projection
angle on the sky is not optimal, a polar-ring galaxy can masquerade as
an Sa galaxy (e.g., upper left of Fig. 1) or a
barred "theta" galaxy
(e.g., lower left of Fig. 1). These cases of
mistaken identity could
generally be distinguished by detailed kinematic and photometric
observations. This figure also indicates that for every polar-ring
galaxy we identify, there are one or two others that have been missed
because of their orientation on the sky. The criteria used to
categorize the galaxies in the PRC are described below.
|
Figure 1. The appearance of a polar-ring galaxy as seen from a variety of viewing orientations. Note that less than half of the viewing orientations allow easy identification of the system as a polar-ring galaxy. |