The Interstellar Medium in Distant Star-forming Galaxies: Turbulent Pressure, Fragmentation, and Cloud Scaling Relations in a Dense Gas Disk at z = 2.3
Résumé
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) × 1010 M &sun; 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) × 107 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 > = 108 cm-3, a factor gsim1000× more than the quiescent ISM of the Milky Way.
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