Galaxies are gravitationally bound systems of stars, dust, and gas. They vary is size and shape and may
contain up to a billion billon stars. Earth is part of the Milky Way galaxy and it was not until 1923 that
Edwin Hubble established that there exists galaxies outside of the Milky Way. Hubble observed that most
galaxies were either elliptical (E) or spiral (S or SB) in shape with a few that were intermediate between the two
shapes. Today we call these transition galaxies lenticular (S0) galaxies. There were a few galaxies that
Hubble could not classify based on their shape and he called these galaxies
irregular galaxies (Ir or Irr).
In order to make sense of why most galaxies exhibited elliptical or spiral morphology, Hubble arranged
the morphological classes into a sequence now known as the Hubble Sequence. This is shown below in
Figure 1. He believed this sequence to be an evolutionary sequence for galaxies where ellipticals would
evolve into spirals. Because of this, he called elliptical galaxies "early-type" and spiral galaxies
"late-type" galaxies. This terminology is still prevalent even though astronomers now know that the
Hubble Sequence is NOT an evolutionary sequence for galaxies.
The elliptical galaxies are denoted by the capital letter E and a number. The number is one minus the apparent axial ratio of the
ellipsoid multiplied by 10. An elliptical galaxy with a number of 0 would have a circular profile whereas an elliptical with a number
of 7 would have a very elongated profiles. Galaxies with number higher than 7 have not been observed.
Spiral galaxies are denoted by the S or SB (depending on whether or not there is a bar at the center) and a lower case letter from
a to d. The lower case letter reflects how open the spiral arms of the galaxy are with a being used for spirals
with tight spiral arms and d for those with very open spiral arms.
According to Hubble's interpretation of the Hubble Sequence, early-type galaxies begin their lives very spheroidal in shape. They become elongated
until a disk structure develops as seen in the S0 galaxies. From here, spiral arms emerge and expand from the disk structure giving rise
to the late-type galaxies.
Polar ring galaxies are NOT on the Hubble Sequence. They are composite galaxies that consists of a gas-poor, early-type (usually S0) central
host galaxy surrounded by a polar ring rich in cool gas. The plane of the polar ring is nearly perpendicular to the semi-major axis of the central host.
Consequently, the rotation vectors of the two components are nearly orthogonal. The polar ring typically exhibits strong H-alpha emission that is characteristic
of recent star formation and thus resembles a late-type, gas-rich disk. On the surface, a polar ring galaxy appears to resemble a spiral galaxy with a
large bulge in the center. However, for Hubble Sequence spiral galaxies, the plane of rotation for the disk and bulge lie in the same plane. This is the
reason polar ring galaxies are more appropriately classified as irregulars. Polar ring galaxies are very rare. They constitute less than 0.5% of all S0
The most intriguing question concerning polar ring galaxies is their formation. It is known that highly inclined rings are unstable and should smear
out within a few orbital periods. Yet, the polar rings appear to be long-lived structures and astronomers have attempted to answer the question of how
such a system may have formed and what is the extent of their life.
Two main formation scenarios have been proposed. The first explains the formation of polar ring galaxies as the accretion of a gas-rich dwarf galaxy
or some other gas-rich object by an early-type galaxy (Quinn 1991; Hernquist & Weil 1993). The second scenario proposed by Bekki (1998) suggests that
polar ring galaxies result from a "polar" merger of two disk galaxies with different masses. These two scenarios are tested through simulation for validity.
It is important for us to know as much as possible about the structure of polar ring galaxies in order to determine whether or not these and other formation
scenarios can accurately reproduce the observed morphology of polar ring galaxies.