The interstellar gas flow in the inner disk of M31 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 M31 ’ 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 , \Upsilon _ { B } = 6.5 \pm 0.8 , and a ratio of co-rotation radius to bulge semi-major axis , \mathcal { R } = 1.2 \pm 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 .