STRUCTURE OF POLAR RING GALAXIES - SHOCK-WAVES IN THE GAS OF POLAR RINGS

Polar ring galaxies are peculiar SO galaxies with gaseous rings rotating perpendicularly to the SO disks. The gas in the ring rushes into the potential well of the SO disk with rotational speeds of 130-250 km s-1, exceeding the sound velocity of the ionized gas. Hence, if the stellar disk of an SO galaxy is dense enough, i.e., if the potential well is deep enough to severely disturb the motions of the gas, shock waves might form, like the galactic shock waves in spiral or barred galaxies. We examined the mass models of the SO disks of three well-observed polar ring galaxies, A0136-0801, MCG 5-29-86, and NGC 4650A, and found that their SO disks are dense enough to produce shock waves in the region of R/r0 less-than-or-equal-to 4.5, where R and ro are the radius of the polar ring and the exponential scalelength of the SO disk, respectively. We propose that shock waves are a possible mechanism for the formation of the central holes in polar rings.