THE INTERSTELLAR MEDIUM IN DISTANT STAR-FORMING GALAXIES: TURBULENT PRESSURE, FRAGMENTATION, AND CLOUD SCALING RELATIONS IN A DENSE GAS DISK AT<i>z</i>= 2.3

Abstract

We have used the IRAM Plateau de Bure Interferometer and the Expanded Very\nLarge Array to obtain a high resolution map of the CO(6-5) and CO(1-0) emission\nin the lensed, star-forming galaxy SMMJ2135-0102 at z=2.32. The kinematics of\nthe gas are well described by a model of a rotationally-supported disk with an\ninclination-corrected rotation speed, v_rot = 320+/-25km/s, a ratio of\nrotational- to dispersion- support of v/sigma=3.5+/-0.2 and a dynamical mass of\n6.0+/-0.5x10^10Mo within a radius of 2.5kpc. The disk has a Toomre parameter,\nQ=0.50+/-0.15, suggesting the gas will rapidly fragment into massive clumps on\nscales of L_J ~ 400pc. We identify star-forming regions on these scales and\nshow that they are 10x denser than those in quiescent environments in local\ngalaxies, and significantly offset from the local molecular cloud scaling\nrelations (Larson's relations). The large offset compared to local molecular\ncloud linewidth-size scaling relations imply that supersonic turbulence should\nremain dominant on scales ~100x smaller than in the kinematically quiescent ISM\nof the Milky Way, while the molecular gas in SMMJ2135 is expected to be ~50x\ndenser than that in the Milky Way on all scales. This is most likely due to the\nhigh external hydrostatic pressure we measure for the interstellar medium\n(ISM), P_tot/kB ~ (2+/-1)x10^7K/cm3. In such highly turbulent ISM, the subsonic\nregions of gravitational collapse (and star-formation) will be characterised by\nmuch higher critical densities, n_crit&gt;=10^8/cm3, a factor ~1000x more than the\nquiescent ISM of the Milky Way.\n