Simulations of dual morphology in spiral galaxies

Gas and stars in spiral galaxies are modelled with a combination of hydrodynamic and N-body techniques. The simulations reveal morphological differences mirroring the dual morphologies seen in B and Kʹ band observations of many spiral galaxies: gaseous images have tighter pitch angles, are more asymmetric, more flocculent and more likely to have multiple arms. Morphological decoupling increases as the stellar arm-interarm contrast and the Q parameter fall. The flocculence of a galaxy is quantified by decomposing the images into logarithmic spirals and defining a parameter closely related to the uniformity of the resulting 2D Fourier spectrum. Thus, a significant amount of morphological decoupling in spiral galaxies is shown to be due to the difference in the dynamics of stars and gas, rather than dust, star formation or galaxy interactions.