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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 [CII] 158 micron emission line from the HIFI instrument on the Herschel Space Observatory. Both SDSS090122.37+181432.3 ("S0901") and SDSS J120602.09+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 v sin(i) = 120 +/- 7 km/s and gas velocity dispersion sigma < 23 km/s. The best fitting model for the Clone is a rotationally supported disk having v sin(i) = 79 +/- 11 km/s and sigma < 4km/s. However the Clone is also consistent with a family of dispersion-dominated models having sigma = 92 +/- 20 km/s. Our results showcase the potential of the [CII] line as a kinematic probe of high redshift galaxy dynamics: [CII] is bright; accessible to heterodyne receivers with exquisite velocity resolution; and traces dense star-forming interstellar gas. Future [CII] line observations with ALMA would offer the further advantage of spatial resolution, allowing a clearer separation between rotation and velocity dispersion.