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 approximate to 2, using high-resolution spectra of the bright [C II] 158 mu m 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 nu sin(i) approximate to 120 +/- 7 kms(-1) and a gas velocity dispersion of sigma(g) < 23 km s-1 (1 sigma). The best-fitting model for the Clone is a rotationally supported disk having nu sin(i) approximate to 79 +/- 11 km s(-1) and sigma(g) less than or similar to 4 kms-1 (1 sigma). However, the Clone is also consistent with a family of dispersion-dominated models having sigma(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.