The molecular gas in the interstellar medium (ISM) of star-forming galaxy populations exhibits diverse physical properties. We investigate the CO excitation of 12 dusty luminous star-forming galaxies at 2-4 by combining observations of the CO from to. The spectral line energy distribution (SLED) has a similar shape to NGC 253, M82, and local ultra-luminous infrared galaxies, with much stronger excitation than the Milky Way inner disc. By combining with resolved dust continuum sizes from high-resolution 870 m ALMA observations and dust mass measurements determined from multiwavelength spectral energy distribution fitting, we measure the relationship between the CO SLED and probable physical drivers of excitation: star-formation efficiency, the average intensity of the radiation field, and the star-formation rate surface density. The primary driver of high-CO excitation in star-forming galaxies is star-formation rate surface density. We use the ratio of the CO(3-2) and CO(6-5) line fluxes to infer the CO excitation in each source and find that the average ratios for our sample are elevated compared to observations of low-redshift, less actively star-forming galaxies and agree well with predictions from numerical models that relate the ISM excitation to the star-formation rate surface density. The significant scatter in the line ratios of a factor within our sample likely reflects intrinsic variations in the ISM properties that may be caused by other effects on the excitation of the molecular gas, such as cosmic ray ionization rates and mechanical heating through turbulence dissipation.

We present the initial results of an ongoing survey with the Karl G. Jansky Very Large Array targeting the CO(J = 1-0) transition in a sample of 30 submillimeter-selected, dusty star-forming galaxies (SFGs) at z = 2-5 with existing mid-J CO detections from the Atacama Large Millimeter/submillimeter Array and NOrthern Extended Millimeter Array, of which 17 have been fully observed. We detect CO(1-0) emission in 11 targets, along with three tentative (∼1.5σ-2σ) detections; three galaxies are undetected. Our results yield total molecular gas masses of 6-23 × 10¹⁰ (α _CO/1) M _⊙, with gas mass fractions, f _gas = M _mol/(M _*+M _mol), of 0.1-0.8 and a median depletion time of (140 ± 70) Myr. We find median CO excitation ratios of r ₃₁ = 0.75 ± 0.39 and r ₄₁ = 0.63 ± 0.44, with significant scatter. We find no significant correlation between the excitation ratio and a number of key parameters such as redshift, CO(1-0) line width, or Σ_SFR. We only find a tentative positive correlation between r ₄₁ and the star-forming efficiency, but we are limited by our small sample size. Finally, we compare our results to predictions from the SHARK semi-analytical model, finding a good agreement between the molecular gas masses, depletion times, and gas fractions of our sources and their SHARK counterparts. Our results highlight the heterogeneous nature of the most massive SFGs at high redshift, and the importance of CO(1-0) observations to robustly constrain their total molecular gas content and interstellar medium properties.