UC Berkeley

ABSTRACT We analyse the kinematics as a function of stellar age for Andromeda (M31) mass analogues from the IllustrisTNG cosmological simulation. We divide the star particles into four age groups: <1, 1–5, 5–10, and >10 Gyr, and compare the kinematics of these groups to that of the neutral gas cells. We calculate rotation curves for the stellar and gaseous components of each analogue from 2 to 20 kpc from the centre of mass. We find that the lag, or asymmetric drift (AD), between the gas rotation curve and the stellar rotation curve on average increases with stellar age. This finding is consistent with observational measurements of AD in the disc of the Andromeda galaxy. When the M31 analogues are separated into groups based on merger history, we find that there is a difference in the AD of the analogues that have had a 4:1 merger the last 4, 8, or 12 Gyr compared to analogues that have not experienced a 4:1 merger in the same time frame. The subset of analogues that have had a 4:1 merger within the last 4 Gyr are also similar to AD measurements of stars in M31’s disc, providing evidence that M31 may in fact have recently merged with a galaxy nearly one-fourth of its mass. Further work using high-resolution zoom-in simulations is required to explore the contribution of internal heating to AD.