Hydrodynamic simulations of the triaxial bulge of M31

TL;DR

This study models the gas flow in M31's inner disk using hydrodynamic simulations, revealing a fast-rotating triaxial bulge with implications for dark matter distribution.

Contribution

It introduces a new hydrodynamics code to model gas flow in M31, deriving the bulge's properties from surface brightness and comparing with observations.

Findings

The bulge is a fast rotator with a specific mass-to-light ratio.

The co-rotation radius is about 1.2 times the bulge semi-major axis.

Dark halo density must be low, contradicting CDM predictions.

Abstract

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, Y = 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.