We analyze the kinematics as a function of stellar age for Andromeda ( M31 ) mass analogs from the Illustris and Illustris TNG cosmological simulations .
We divide the star particles into four age groups : < 1 Gyr , 1 - 5 Gyr , 5 - 10 Gyr , and > 10 Gyr , and we 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 in the subgroup that corresponds to having had a 4:1 merger in a given time range compared to analogs that have not experienced a 4:1 merger in the same time frame .
These differences are the most significant for recent 4:1 merger events ( < 4 Gyr ago ) and are therefore most visible in the 1-5 Gyr stellar age group where we find a difference in AD as large as \sim 55 km s ^ { -1 } .
This subset of analogs that have had a 4:1 merger within the last 4 Gyr are also very consistent with AD measurements of stars in M31 ’ s disk , providing evidence that M31 may in fact have recently merged with a galaxy nearly 1/4 of its mass .
Further work using high resolution zoom-in simulations is required to explore the contribution of internal heating to AD .