Abstract. The interstellar gas flow in the inner disk of M 31 is modelled using a new, two dimensional, grid based, hydrodynamics code. The potential of the stellar bulge is derived from its surface brightness profile. The bulge is assumed to be triaxial and rotating in the same plane as the disk in order to explain the twisted nature of M 31's central isophotes and the non circular gas velocities in the inner disk. Results are compared with CO observations and the bulge is found to be a fast rotator with a B-band mass-to-light ratio, ΥB = 6.5 ± 0.8, and a ratio of co-rotation radius to bulge semi-major axis, R = 1.2 ± 0.1, implying that any dark halo must have a low density core in contradiction to the predictions of CDM. These conclusions would be strengthened by further observations confirming the model's off axis CO velocity predictions.

A detailed hydrodynamical model of the gas flow in the triaxial gravitational potential of the bulge of the Andromeda galaxy (M31) has recently been proposed by Berman (2001) astro-ph/0103209, and shown to provide excellent agreement with the CO emission line velocities observed along its major axis. In the present paper, we confirm the validity of that model by showing that it can also reproduce the CO velocities observed off the major axis - a much more robust test. The CO observations, however, tend to span a wider range of velocities than a direct application of the original model of Berman would suggest. This situation can be improved significantly if the molecular disk is made thicker, a requirement already encountered in dynamical simulations of other spiral galaxies, and typically attributed to a broadening of the molecular layer in galactic fountain-like processes. In the central regions of M31, however, it is unclear whether there actually is a thick molecular disk, or whether broadening the molecular layer is merely an artificial theoretical means of accounting for some disk warping. Other effects not included in the model, such as hydraulic jumps, might also contribute to a widening of the velocities.Comment: 6 pages, 3 figures, accepted for publication in MNRA

Abstract. 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 K 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.

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