The energetics of turbulent molecular gas and star formation

The role interstellar turbulence plays in regulating star formation is a much debated research topic. In this paper, I take an observational view point in presenting observations of H₂ line emission from extragalactic sources. I highlight key results from these observations. (1) H₂ line emission is a main cooling channel of molecular gas. It is a tracer of mechanical energy dissipation complementing mass tracers in describing the dynamical state of molecular gas in galaxies. (2) Spectroscopy of warm H₂ observations with the Spitzer Space Telescope and the SINFONI spectro-imager at ESO provide evidence of shock excited H₂ line emission in galaxies that exemplify the main agents of galaxy evolution. (3) The dissipation of mechanical energy involves a turbulent energy cascade and the cycling of interstellar matter across ISM phases, including the formation of H₂ gas from warm atomic gas. (4) In Stephan's Quintet and the radio galaxy 3C326, two sources with a high H₂ luminosity to mass ratio (i.e. a high dissipation rate per unit mass), turbulence is observed to quench star formation. In the Antennae merger, star formation is observed to proceed where the turbulent kinetic energy is being dissipated.