A survey of molecular gas in luminous sub-millimetre galaxies

We present the results from a survey of ¹²CO emission in 40 luminous sub-millimetre galaxies (SMGs), with 850-μm fluxes of S_{850 μm} = 4-20 mJy, conducted with the Plateau de Bure Interferometer. We detect ¹²CO emission in 32 SMGs at z ∼ 1.2-4.1, including 16 SMGs not previously published. Using multiple ¹²CO line (J_up = 2-7) observations, we derive a median spectral line energy distribution for luminous SMGs. We report the discovery of a fundamental relationship between ¹²CO FWHM and ¹²CO line luminosity in high-redshift starbursts, which we interpret as a natural consequence of the baryon-dominated dynamics within the regions probed by our observations. We use far-infrared luminosities to assess the star formation efficiency in our SMGs, finding that the slope of the L'_CO-L_FIR relation is close to linear. We derive molecular gas masses, finding a mean gas mass of (5.3 ± 1.0) × 10¹⁰ M_⊙. Combining these with dynamical masses, we determine the redshift evolution of the gas content of SMGs, finding that they do not appear to be significantly more gas rich than less vigorously star-forming galaxies at high redshifts. Finally, we collate X-ray observations, and study the interdependence of gas and dynamical properties of SMGs with their AGN activity and supermassive black hole masses (M_BH), finding that SMGs lie significantly below the local M_BH-σ relation.