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Session 68 - Invited.
Invited session, Thursday, June 13
Union Theater,
A small fraction of early-type galaxies have a highly unusual structure: a ring of gas, dust and stars lies roughly perpendicular to the apparent major axis of the central galaxy. Spectroscopy confirms the ring angular momentum to be almost normal to that of the central object; the galaxy has two distinct planes of rotation. This fact suggests that the polar material is a late acquisition: polar ring systems may represent incomplete galaxy mergers. The central galaxy is in general nearly gas-free, while the ring typically contains a few billion solar masses of neutral gas, so 21cm HI observations are an excellent way to study the distribution and dynamics of ring material. This talk will present new high-resolution HI maps and results of a recent HI survey, discussing implications for the formation and stability of polar rings.
The observed flat rotation curves of polar rings indicate appreciable amounts of dark matter in these galaxies. Since the ring orbits explore a plane nearly orthogonal to the rotation of the central galaxy, polar ring galaxies offer one of the few ways to probe the three-dimensional shape of the dark halo: unless the galaxy potential is spherical, polar orbits are oval rather than circular, and orbits inclined to the pole are subject to differential precession. Self-gravity of the ring material and viscous dissipation further modify the ring structure. Dynamical modelling will be presented for a number of polar ring systems, indicating that the dark halo is likely to be flattened in the same sense as the stellar body of the inner galaxy.
It is not known whether polar rings are in general lately-formed transient structures, or whether they are old and long-lived, dating from the early history of the Universe. Some of them are blue and show obvious signs of recent star formation, while others appear red, smooth, quiescent and vertically thin. This talk will discuss what can be learned from recent radio continuum, optical and near-IR imaging.
