Kinematic Downsizing at z ∼ 2

Abstract

We present results from a survey of the internal kinematics of 49 star-forming galaxies at z∼ 2 in the CANDELS fields with the Keck/MOSFIRE spectrograph, Survey in the near-Infrared of Galaxies with Multiple position Angles (SIGMA). Kinematics (rotation velocity V _rot and gas velocity dispersion {σ }_g) are measured from nebular emission lines which trace the hot ionized gas surrounding star-forming regions. We find that by z∼ 2, massive star-forming galaxies ({log} {M}_* /{M}_⊙ ≳ 10.2) have assembled primitive disks: their kinematics are dominated by rotation, they are consistent with a marginally stable disk model, and they form a Tully-Fisher relation. These massive galaxies have values of {V}_{rot}/{σ }_g that are factors of 2-5 lower than local well-ordered galaxies at similar masses. Such results are consistent with findings by other studies. We find that low-mass galaxies ({log} {M}_* /{M}_⊙ ≲ 10.2) at this epoch are still in the early stages of disk assembly: their kinematics are often dominated by gas velocity dispersion and they fall from the Tully-Fisher relation to significantly low values of V _rot. This “kinematic downsizing” implies that the process(es) responsible for disrupting disks at z∼ 2 have a stronger effect and/or are more active in low-mass systems. In conclusion, we find that the period of rapid stellar mass growth at z∼ 2 is coincident with the nascent assembly of low-mass disks and the assembly and settling of high-mass disks.