Asymmetric Drift of Andromeda Analogs in the IllustrisTNG Simulation
  We analyze the kinematics as a function of stellar age for Andromeda (M31)
mass analogs from the IllustrisTNG cosmological simulation. We divide the star
particles into four age groups: less than 1 Gyr, 1 to 5 Gyr, 5 to 10 Gyr, and
greater 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 analog from 2 kpc to 20 kpc from the center 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 disk of the Andromeda
galaxy. When the M31 analogs are separated into groups based on merger history,
we find that there is a difference in the AD of the analogs that have had a 4:1
merger the last 4 Gyr, 8 Gyr, or 12 Gyr compared to analogs that have not
experienced a 4:1 merger in the same time frame. The subset of analogs that
have had a 4:1 merger within the last 4 Gyr are also similar to AD measurements
of stars in the disk of M31, 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.
