Simulations of stars and gas in spiral galaxies

A new code, DUAL, is developed to model the 2D dynamics of stars and gas in spiral galaxies. DUAL combines grid-based hydrodynamics and N-body techniques. A triaxial model of the bulge of M31 is constructed from its surface brightness profile and shown, using hydrodynamic simulations, to reproduce observations of molecular gas kinemat- ics along the line of nodes of the disk. The bulge model rotates fast, with a pattern speed of 54 km s-1 kpc-1, and a ratio of bulge semi-major axis to corotation radius â = 1.2. The B band mass-to-light ratio is 6.5, the semi-major axis is 3.5 kpc, the bulge mass is 2.3 x 1010 Mâ and the angle between the projected minor axis of the bulge and the line of nodes of the disk is 15°. A thick gas disk (HWHM of 200 â 500 pc) is added to account for CO observations parallel to the disk line of nodes. The thickness is minimized if the gas velocity drops by 20% per kpc above the plane. By combining reasonable values of the disk mass-to-light ratio or dynamical friction arguments with the bulge model, M31 is shown to have a maximal disk with a halo mass fraction within 3.5 kpc of just a few percent. In contrast, the Cold Dark Matter theory predicts a fraction of 22% to 30%. The differences between optical and infrared morphologies of spiral galaxies are reproduced using hydrodynamic and N-body simulations of gas and stars. Gaseous spirals have tighter pitch angles than stellar spirals. This phenomenon is attributed to a phase shift between the stellar density and the potential. Gaseous images are more asymmetric, less smooth and more likely to have multiple arms. Morphological decoupling increases as the stellar arm-interarm contrast and Q parameter fall. The flocculence of a galaxy is quantified by decomposing the galaxy images into logarithmic spirals and denning a parameter closely related to the uniformity of the resulting 2D Fourier spectrum. I conclude that a significant amount of morphological decoupling in spiral galaxies is due to the difference in the dynamics of stars and gas, rather than dust, star formation or galaxy interactions.