Substructure around M31: Evolution and effects

We investigate the evolution of a population of 100 dark matter satellites orbiting in the gravitational potential of a realistic model of M31. We find that after 10 Gyr, seven subhalos are completely disrupted by the tidal field of the host galaxy. The remaining satellites suffer heavy mass loss and, overall, 75% of the mass initially in the subhalo system is tidally stripped. Not surprisingly, satellites with pericentric radius less than 30 kpc suffer the greatest stripping and leave a complex structure of tails and streams of debris around the host galaxy. Assuming that the most bound particles in each subhalo are kinematic tracers of stars, we find that the halo stellar population resulting from the tidal debris follows an r(-3.5) density profile at large radii. We construct B-band photometricmaps of stars coming from disrupted satellites and find conspicuous features similar in both morphology and brightness to the observed giant stream around Andromeda. While an assumed star formation efficiency of 5% in the simulated satellite galaxies results in good agreement with the observed V-band surface brightness of M31's stellar halo, an efficiency of 1%-2% accounts for the brightness of the giant stream. During the first 5 Gyr, the bombardment of the satellites heats and thickens the disk by a small amount. At about 5 Gyr, satellite interactions induce the formation of a strong bar, which, in turn, leads to a significant increase in the velocity dispersion of the disk.