The star-forming molecular gas in high-redshift Submillimetre Galaxies

We present a model for the CO molecular line emission from high-redshift Submillimetre Galaxies (SMGs). By combining hydrodynamic simulations of gas-rich galaxy mergers with the polychromatic radiative transfer code, SUNRISE, and the 3D non-LTE molecular line radiative transfer code, TURTLEBEACH, we show that if SMGs are typically a transient phase of major mergers, then their observed compact CO spatial extents, broad linewidths and high excitation conditions (CO spectral energy distribution) are naturally explained. In this sense, SMGs can be understood as scaled-up analogues to local ultraluminous infrared galaxies (ULRIGs). We utilize these models to investigate the usage of CO as an indicator of physical conditions. We find that care must be taken when applying standard techniques. The usage of CO linewidths as a dynamical mass estimator from SMGs can possibly overestimate the true enclosed mass by a factor of ∼1.5–2. At the same time, assumptions of line ratios of unity from CO J = 3–2 (and higher lying lines) to CO (J = 1–0) will oftentimes lead to underestimates of the inferred gas mass. We provide tests for these models by outlining predictions for experiments which are imminently feasible with the current generation of bolometer arrays and radio-wave spectrometers