2011ApJ...742...11S

We have used the Institut de Radioastronomie Millimétrique (IRAM) Plateau de Bure Interferometer and the Expanded Very Large Array to obtain a high-resolution map of the CO(6-5) and CO(1-0) emission in the lensed, star-forming galaxy SMM J2135-0102 at z = 2.32. The kinematics of the gas are well described by a model of a rotationally supported disk with an inclination-corrected rotation speed, v_rot= 320±25 km/s, a ratio of rotational-to-dispersion support of v/σ = 3.5±0.2, and a dynamical mass of (6.0±0.5)x10¹⁰ M_☉ within a radius of 2.5 kpc. The disk has a Toomre parameter, Q = 0.50±0.15, suggesting that the gas will rapidly fragment into massive clumps on scales of L_J∼ 400 pc. We identify star-forming regions on these scales and show that they are ∼10xdenser than those in quiescent environments in local galaxies, and significantly offset from the local molecular cloud scaling relations (Larson's relations). The large offset compared to local molecular cloud line-width-size scaling relations implies that supersonic turbulence should remain dominant on scales ∼100x smaller than in the kinematically quiescent interstellar medium (ISM) of the Milky Way, while the molecular gas in SMM J2135 is expected to be ∼50x denser than that in the Milky Way on all scales. This is most likely due to the high external hydrostatic pressure we measure for the ISM, P_tot/k_B∼ (2±1)x10⁷ K/cm3. In such highly turbulent ISM, the subsonic regions of gravitational collapse (and star formation) will be characterized by much higher critical densities, n_crit> = 10⁸/cm3, a factor ≳ 1000x more than the quiescent ISM of the Milky Way.