Herschel extreme lensing line observations: Dynamics of two strongly lensed star-forming galaxies near redshift z = 2

We report on two regularly rotating galaxies at redshift z 2, using high-resolution spectra of the bright [C II] 158 μm emission line from the HIFI instrument on the Herschel Space Observatory. Both SDSS090122.37+181432.3 ("S0901") and SDSSJ120602.09+514229.5 ("the Clone") are strongly lensed and show the double-horned line profile that is typical of rotating gas disks. Using a parametric disk model to fit the emission line profiles, we find that S0901 has a rotation speed of vsin (i) 120 ± 7 km s^-1 and a gas velocity dispersion of σ g < 23 km s ^-1 (1σ). The best-fitting model for the Clone is a rotationally supported disk having vsin (i) 79 ± 11 km s^-1 and σ g ≲ 4 km s^-1 (1σ). However, the Clone is also consistent with a family of dispersion-dominated models having σ g = 92 ± 20 km s^-1. Our results showcase the potential of the [C II] line as a kinematic probe of high-redshift galaxy dynamics: [C II] is bright, accessible to heterodyne receivers with exquisite velocity resolution, and traces dense star-forming interstellar gas. Future [C II] line observations with ALMA would offer the further advantage of spatial resolution, allowing a clearer separation between rotation and velocity dispersion.