MOSEL Survey: Tracking the Growth of Massive Galaxies at 2 < z < 4 Using Kinematics and the IllustrisTNG Simulation

We use K-band spectroscopic data from the Multi-Object Spectroscopic Emission Line survey to analyze the kinematic properties of galaxies at z > 3. Our sample consists of 34 galaxies at 3.0 10.2 at z > 3 have 56 ± 21 km s−1 lower integrated velocity dispersion compared to galaxies at z 2 of similar stellar mass. Massive galaxies at z > 3 have either a flat or declining star formation history (SFH), whereas similar stellar mass galaxies at z ~ 2.0 exhibit a slight peak in the past 500 Myr. Comparing with the IllustrisTNG cosmological simulation, we find that (i) the dynamical mass of massive galaxies in simulations (log(M*/M⊙) > 10.0) increases by ~0.1 dex at a fixed stellar mass between z = 2.0–3.0, and (ii) dynamical mass growth is coupled with a rapid rise in the ex situ stellar mass fraction (stars accreted from other galaxies) for massive galaxies at z 3.