Broken and unbroken: the Milky Way and M31 stellar haloes

We use the Bullock & Johnston suite of simulations to study the density profiles of L*-type galaxy stellar haloes. Observations of the Milky Way and M31 stellar haloes show contrasting results: the Milky Way has a `broken' profile, where the density falls off more rapidly beyond ~ 25 kpc, while M31 has a smooth profile out to 100 kpc with no obvious break. Simulated stellar haloes, built solely by the accretion of dwarf galaxies, also exhibit this behavior: some haloes have breaks, while others don't. The presence or absence of a break in the stellar halo profile can be related to the accretion history of the galaxy. We find that a break radius is strongly related to the build up of stars at apocentres. We relate these findings to observations, and find that the `break' in the Milky Way density profile is likely associated with a relatively early (~ 6-9 Gyr ago) and massive accretion event. In contrast, the absence of a break in the M31 stellar halo profile suggests that its accreted satellites have a wide range of apocentres. Hence, it is likely that M31 has had a much more prolonged accretion history than the Milky Way.