The ISM in distant star-forming galaxies: Turbulent pressure,
  fragmentation and cloud scaling relations in a dense gas disk at z=2.3
  We have used the 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 SMMJ2135-0102 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+/-25km/s, a ratio of
rotational- to dispersion- support of v/sigma=3.5+/-0.2 and a dynamical mass of
6.0+/-0.5x10^10Mo within a radius of 2.5kpc. The disk has a Toomre parameter,
Q=0.50+/-0.15, suggesting the gas will rapidly fragment into massive clumps on
scales of L_J ~ 400pc. We identify star-forming regions on these scales and
show that they are 10x denser 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 linewidth-size scaling relations imply that supersonic turbulence should
remain dominant on scales ~100x smaller than in the kinematically quiescent ISM
of the Milky Way, while the molecular gas in SMMJ2135 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 interstellar medium
(ISM), P_tot/kB ~ (2+/-1)x10^7K/cm3. In such highly turbulent ISM, the subsonic
regions of gravitational collapse (and star-formation) will be characterised by
much higher critical densities, n_crit>=10^8/cm3, a factor ~1000x more than the
quiescent ISM of the Milky Way.
