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

Abstract

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 −1 , a ratio of rotational-to-dispersion support of v /σ = 3.5 ± 0.2, and a dynamical mass of (6.0 ± 0.5) × 10 10 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 ∼10 × 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 line-width–size scaling relations implies that supersonic turbulence should remain dominant on scales ∼100 × smaller than in the kinematically quiescent interstellar medium (ISM) of the Milky Way, while the molecular gas in SMM J2135 is expected to be ∼50 × 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) × 10 7 K cm −3 . In such highly turbulent ISM, the subsonic regions of gravitational collapse (and star formation) will be characterized by much higher critical densities, n crit > = 10 8 cm −3 , a factor ≳1000 × more than the quiescent ISM of the Milky Way.