Kinematic downsizing at z ∼ 2

We present results from a survey of the internal kinematics of 49 star-forming galaxies at $z\sim 2$ 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 ${\sigma }_g$) are measured from nebular emission lines which trace the hot ionized gas surrounding star-forming regions. We find that by $z\sim 2$, massive star-forming galaxies ($log{M}_{\ast }/M_{\odot }\gtrsim 10.2$) 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 $V_{rot}/{\sigma }_g$ 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 ($log{M}_{\ast }/M_{\odot }\lesssim 10.2$) 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 $z\sim 2$ 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 $z\sim 2$ is coincident with the nascent assembly of low-mass disks and the assembly and settling of high-mass disks.