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.

We present a high-resolution study of the cold molecular gas as traced by CO(1-0) in the unlensed z ∼3.4 submillimeter galaxy SMM J13120+4242, using multiconfiguration observations with the Karl G. Jansky Very Large Array (JVLA). The gas reservoir, imaged on 0.″39 (∼3 kpc) scales, is resolved into two components separated by ∼11 kpc with a total extent of 16 ± 3 kpc. Despite the large spatial extent of the reservoir, the observations show a CO(1-0) FWHM linewidth of only 267 ± 64 km s-1. We derive a revised line luminosity of LCO(1-0)′ = (10 ± 3) × 1010 K km s-1 pc2 and a molecular gas mass of M gas = (13 ± 3)× 1010 (α CO/1) M ⊙. Despite the presence of a velocity gradient (consistent with previous resolved CO(6-5) imaging), the CO(1-0) imaging shows evidence for significant turbulent motions that are preventing the gas from fully settling into a disk. The system likely represents a merger in an advanced stage. Although the dynamical mass is highly uncertain, we use it to place an upper limit on the CO-to-H2 mass conversion factor α CO of 1.4. We revisit the SED fitting, finding that this galaxy lies on the very massive end of the main sequence at z = 3.4. Based on the low gas fraction, short gas depletion time, and evidence for a central AGN, we propose that SMM J13120 is in a rapid transitional phase between a merger-driven starburst and an unobscured quasar. The case of SMM J13120 highlights how mergers may drive important physical changes in galaxies without pushing them off the main sequence.