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 < z_spec < 3.8 between 9.0 < $\mathrm{log}({M}_{* }/{M}_{\odot })$ < 11.0. We find that galaxies with $\mathrm{log}({M}_{* }/{M}_{\odot })$ > 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 ( $\mathrm{log}({M}_{* }/{M}_{\odot })$ > 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 < 3.5. We speculate that the rising contribution of ex situ stellar mass to the total stellar mass growth of massive galaxies is driving the higher integrated velocity dispersion and rising SFHs of massive galaxies at z ∼ 2.0 compared to galaxies of similar stellar masses at z > 3.