Arm Structure in Anemic Spiral Galaxies

Abstract

Anemic galaxies have less prominent star formation than normal galaxies of the same Hubble type. Previous studies showed they are deficient in total atomic hydrogen but not in molecular hydrogen. Here we compare the combined surface densities of HI and H 2 at mid-disk radii with the Kennicutt threshold for star formation. The anemic galaxies are below threshold, which explains their lack of prominent star formation, but they are not much different than other early type galaxies, which also tend to be below threshold. The spiral wave amplitudes of anemic and normal galaxies were also compared, using images in B and J passbands from the OSU Bright Spiral Galaxy Survey. Anemic galaxies have normal spiral wave properties too, with the same amplitudes and radial dependencies as other galaxies of the same arm class. Because of the lack of gas, spiral waves in early type galaxies and anemics do not have a continuous supply of stars with low velocity dispersions to maintain a marginally stable disk. As a result, they are either short-lived, evolving toward lenticulars and S0 types in only a few rotations at mid-disk, or they are driven by the asymmetries associated with gas removal in the cluster environment.