Arm Structure in Anemic Spiral Galaxies

Anemic galaxies have less prominent star formation than normal galaxies ofthe same Hubble type. Previous studies showed they are deficient in totalatomic hydrogen but not in molecular hydrogen. Here we compare the combinedsurface densities of HI and H2 at mid-disk radii with the Kennicutt thresholdfor star formation. The anemic galaxies are below threshold, which explainstheir lack of prominent star formation, but they are not much different thanother early type galaxies, which also tend to be below threshold. The spiralwave amplitudes of anemic and normal galaxies were also compared, using imagesin B and J passbands from the OSU Bright Spiral Galaxy Survey. Anemic galaxieshave normal spiral wave properties too, with the same amplitudes and radialdependencies as other galaxies of the same arm class. Because of the lack ofgas, spiral waves in early type galaxies and anemics do not have a continuoussupply of stars with low velocity dispersions to maintain a marginally stabledisk. As a result, they are either short-lived, evolving toward lenticulars andS0 types in only a few rotations at mid-disk, or they are driven by theasymmetries associated with gas removal in the cluster environment.Comment: 15 pages, 3 figures, accepted by A