Massive star-forming galaxies at high redshift require a supply of molecular gas from their gas reservoirs that is replenished by infall from the surrounding circumgalactic medium to sustain their immense star formation rates. Our knowledge of the extent and morphology of cold-gas reservoirs of early galaxies is still in its infancy, however. We present the results of stacking more than 80 hours of JVLA observations of CO(1–0) emission, which traces the cold molecular gas, in 19 z = 2.0−4.5 dusty star-forming galaxies from the AS2VLA survey. The visibility-plane stack reveals extended emission with a half-light radius of 3.8 ± 0.5 kpc, which is a factor of 2–3 more extended than the dust-obscured star formation and 1.4 ± 0.2× more extended than the stellar emission revealed by the JWST. Stacking the [C i](1–0) observations for 10 galaxies from our parent sample yielded a half-light radius ≤2.6 kpc, which is marginally smaller than CO(1–0). The CO(1–0) size is also comparable to that of the [C ii] haloes detected around high-redshift star-forming galaxies. This suggests that these arise from molecular gas. Photo-dissociation region modelling indicates that the extended CO(1–0) emission arises from clumpy dense clouds and not from smooth diffuse gas. Our results show that the bulk (up to 80%) of the molecular gas in these galaxies resides outside the star-forming region with only a small part directly contributing to the star formation.

We present JWST NIRCam imaging targeting 13 z ~ 3 infrared-luminous (L_IR ∼ 5 × 10¹²L_⊙) galaxies from the ALESS survey with uniquely deep, high-resolution (0 . ″ 08-0 . ″ 16) Atacama Large Millimeter/submillimeter Array 870 μm imaging. The 2.0-4.4 μm (observed frame) NIRCam imaging reveals the rest-frame near-infrared stellar emission in these submillimeter-selected galaxies at the same (sub)kiloparsec resolution as the 870 μm dust continuum. The newly revealed stellar morphologies show striking similarities with the dust continuum morphologies at 870 μm, with the centers and position angles agreeing for most sources, clearly illustrating that the spatial offsets reported previously between the 870 μm and Hubble Space Telescope morphologies were due to strong differential dust obscuration. The F444W sizes are 78% ± 21% larger than those measured at 870 μm, in contrast to recent results from hydrodynamical simulations that predict larger 870 μm sizes. We report evidence for significant dust obscuration in F444W for the highest-redshift sources, emphasizing the importance of longer-wavelength MIRI imaging. The majority of the sources show evidence that they are undergoing mergers/interactions, including tidal tails/plumes—some of which are also detected at 870 μm. We find a clear correlation between NIRCam colors and 870 μm surface brightness on ∼1 kpc scales, indicating that the galaxies are primarily red due to dust—not stellar age—and we show that the dust structure on ∼kpc scales is broadly similar to that in nearby galaxies. Finally, we find no strong stellar bars in the rest-frame near-infrared, suggesting the extended bar-like features seen at 870 μm are highly obscured and/or gas-dominated structures that are likely early precursors to significant bulge growth.

Unobscured quasars (QSOs) are predicted to be the final stage in the evolutionary sequence from gas-rich mergers to gas-depleted, quenched galaxies. Studies of this population, however, find a high incidence of far-infrared-luminous sources-suggesting significant dust-obscured star formation-but direct observations of the cold molecular gas fuelling this star formation are still necessary. We present a NOEMA study of CO(2-1) emission, tracing the cold molecular gas, in ten lensed z = 1-1.5 unobscured QSOs. We detected CO(2-1) in seven of our targets, four of which also show continuum emission (λ_rest = 1.3 mm). After subtracting the foreground galaxy contribution to the photometry, spectral energy distribution fitting yielded stellar masses of 10^9-11 M_⊙, with star formation rates of 25-160 M_⊙ yr^-1 for the host galaxies. These QSOs have lower L′_CO than star-forming galaxies with the same LIR, and show depletion times spanning a large range (50-900 Myr), but with a median of just 90(α_CO/4) Myr. We find molecular gas masses in the range ≤2-40 × 10⁹(α_CO/4) M_⊙, which suggest gas fractions above ~50% for most of the targets. Despite the presence of an unobscured QSO, the host galaxies are able to retain significant amounts of cold gas. However, with a median depletion time of ~90 Myr, the intense burst of star formation taking place in these targets will quickly deplete their molecular gas reservoirs in the absence of gas replenishment, resulting in a quiescent host galaxy. The non-detected QSOs are three of the four radio-loud QSOs in the sample, and their properties indicate that they are likely already transitioning into quiescence. Recent cosmological simulations tend to overestimate the depletion times expected for these z ~ 1 QSO-host galaxies, which is likely linked to their difficulty producing starbursts across the general high-redshift galaxy population.

We present deep ALMA Band 3 observations of the HCN, HCO⁺, and HNC(4-3) emission in SDP.81, a well-studied z = 3.042; strongly lensed galaxy. These lines trace the high-density gas, which remains almost entirely unexplored in z ≥ 1 galaxies. Additionally, these dense-gas tracers are potentially powerful diagnostics of the mechanical heating of the interstellar medium. While the HCN(4-3) and HNC(4-3) lines are not detected, the HCO⁺(4-3) emission is clearly detected and resolved. This is the third detection of this line in a high-redshift star-forming galaxy. We find an unusually high HCO⁺/HCN intensity ratio of ≥2.2. Based on the modelling of the photodissociation region, the most likely explanation for the elevated HCO⁺/HCN ratio is that SDP.81 has low mechanical heating, making up less than 10% of the total energy budget, along with a sub-solar metallicity of Z ≈ 0.5 Z_⊙. While such conditions might not be representative of the general population of high-redshift dusty galaxies, a lower-than-solar metallicity might significantly impact gas masses inferred from CO observations. In addition, we report the detection of CO(0-1) absorption from the foreground lensing galaxy and CO(1-0) emission from a massive companion to the lensing galaxy, approximately 50 kpc to the south-east.