We measure the dust and gas content of the three submillimeter galaxies (SMGs) in the GN20 proto-cluster at z = 4.05 using new IRAM Plateau de Bure interferometer (PdBI) CO(4–3) and 1.2–3.3 mm continuum observations. All these three SMGs are heavily dust obscured, with UV-based star formation rate (SFR) estimates significantly smaller than the ones derived from the bolometric infrared (IR), consistent with the spatial offsets revealed by HST and CO imaging. Based also on evaluations of the specific SFR, CO-to-H2 conversion factor and gas depletion timescale, we classify all the three galaxies as starbursts (SBs), although with a lower confidence for GN20.2b that might be a later stage merging event. We place our measurements in the context of the evolutionary properties of main sequence (MS) and SB galaxies. ULIRGs have 3–5 times larger L' CO/Mdust and M dust/M ⋆ ratios than z = 0 MS galaxies, but by z ~ 2 the difference appears to be blurred, probably due to differential metallicity evolution. SB galaxies appear to slowly evolve in their L' CO/Mdust and M dust/M ⋆ ratios all the way to z> 6 (consistent with rapid enrichment of SB events), while MS galaxies rapidly increase in M dust/M ⋆ from z = 0 to 2 (due to gas fraction increase, compensated by a decrease of metallicities). While no IR/submm continuum detection is available for indisputably normal massive galaxies at z> 2.5, we show that if metallicity indeed decrease rapidly for these systems at z> 3 as claimed in the literature, we should expect a strong decrease of their M dust/M ⋆ , consistent with recent PdBI and ALMA upper limits. We conclude that the M dust/M ⋆ ratio could be a powerful tool for distinguishing starbursts from normal galaxies at z> 4.

Using NOEMA and ALMA 3mm line scans, we measured spectroscopic redshifts of six new dusty galaxies at 3.5 < z < 4.2 by solidly detecting [CI](1-0) and CO transitions. The sample was selected from the COSMOS and GOODS-North super-deblended catalogs with far-infrared (FIR) photometric redshifts z_phot > 6 based on template IR spectral energy distribution (SED) from known submillimeter galaxies at z = 4–6. Dust SED analyses explain the z_phot overestimate from seemingly cold dust temperatures (T_d) and steep Rayleigh-Jeans (RJ) slopes, providing additional examples of cold dusty galaxies impacted by the cosmic microwave background (CMB). We therefore studied the general properties of the enlarged sample of 10 “cold” dusty galaxies over 3.5 < z < 6. We conclude that these galaxies are deceivingly cold at the surface but are actually warm in their starbursting cores. Several lines of evidence support this scenario: (1) The high infrared surface density Σ_IR and cold T_d from optically thin models appear to violate the Stefan-Boltzmann law; (2) the gas masses derived from optically thin dust masses are inconsistent with estimates from dynamics and CI luminosities; (3) the implied high star formation efficiencies would conflict with cold T_d; and (4) high FIR optical depth is implied even using the lower, optically thick dust masses. This work confirms the existence of a substantial population of deceivingly cold, compact dusty starburst galaxies at z ≳ 4, together with the severe impact of the CMB on their RJ observables, paving the way for the diagnostics of optically thick dust in the early Universe. Conventional gas mass estimates based on RJ dust continuum luminosities implicitly assume an optically thin case, which leads to overestimation of gas masses by a factor of 2–3 on average in compact dusty star-forming galaxies.

We present an identification of dust-attenuated star-forming galactic-disk substructures in a typical star-forming galaxy (SFG), UDF2, at z = 2.696. To date, substructures containing significant buildup of stellar mass and actively forming stars have yet to be found in typical (i.e., main-sequence) SFGs at z > 2. This is due to the strong dust attenuation common in massive galaxies at the epoch and the scarcity of high-resolution, high-sensitivity extinction-independent imaging. To search for disk substructures, we subtracted the central stellar-mass disk from the JWST/NIRCam rest-frame 1.2 μm image (0.″13 resolution) and subtracted, in the visibility plane, the central starburst disk from Atacama Large Millimeter/submillimeter Array (ALMA) rest-frame 240 μm observations (0.″03 resolution). The residual images revealed substructures at rest-frame 1.2 μm colocated with those found at rest-frame 240 μm, ≃2 kpc away from the galactic center. The largest substructure contains ≃20% of the total stellar mass and ≃5% of the total star formation rate of the galaxy. While UDF2 exhibits a kinematically ordered velocity field of molecular gas consistent with a secularly evolving disk, more sensitive observations are required to characterize the nature and the origin of this substructure (spiral arms, minor merger, or other types of disk instabilities). UDF2 resides in an overdense region (N ≥ 4 massive galaxies within 70 kpc projected distance at z = 2.690–2.697) and the substructures may be associated with interaction-induced instabilities. Importantly, a statistical sample of such substructures identified with JWST and ALMA could play a key role in bridging the gap between the bulge-forming starburst and the rest of the galaxy.

We present and maps of six nearby star-forming galaxies, NGC 253, NGC 1068, IC 342, M82, M83, and NGC 6946, obtained with the James Clerk Maxwell Telescope as part of the MALATANG survey. All galaxies were mapped in the central 2′ × 2′ region at 14″ (FWHM) resolution (corresponding to linear scales of ∼0.2–1.0 kpc). TheL_IR–L′_denserelation, where the dense gas is traced by the and the emission, measured in our sample of spatially resolved galaxies is found to follow the linear correlation established globally in galaxies within the scatter. We find that the luminosity ratio,L_IR/L′_dense, shows systematic variations withL_IRwithin individual spatially resolved galaxies, whereas the galaxy-integrated ratios vary little. A rising trend is also found betweenL_IR/L′_denseratio and the warm-dust temperature gauged by the 70μm/100μm flux ratio. We find that the luminosity ratios of IR/HCN (4–3) and IR/HCO⁺(4–3), which can be taken as a proxy for the star formation efficiency (SFE) in the dense molecular gas (SFE_dense), appear to be nearly independent of the dense gas fraction (f_dense) for our sample of galaxies. The SFE of the total molecular gas (SFE_mol) is found to increase substantially withf_densewhen combining our data with those on local (ultra)luminous infrared galaxies and high-zquasars. The mean line ratio measured for the six targeted galaxies is 0.9 ± 0.6. No significant correlation is found for the ratio with the star formation rate as traced byL_IR, nor with the warm-dust temperature, for the different populations of galaxies.

We present CO (J = 1−0) and/or CO (J = 2−1) spectroscopy for 31 galaxies selected from the ongoing Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, obtained with multiple telescopes. This sample is combined with CO observations from the literature to study the correlation of the CO luminosities ( ) with the mid-infrared luminosities at 12 ( ) and 22 μm ( ), as well as the dependence of the residuals on a variety of galaxy properties. The correlation with is tighter and more linear, but galaxies with relatively low stellar masses ( M _⊙) and blue colors ( and/or NUV − r ≲ 3) fall significantly below the mean – relation. We propose a new estimator of the CO (1−0) luminosity (and thus the total molecular gas mass M _mol) that is a linear combination of three parameters: , , and g − r. We show that, with a scatter of only 0.18 dex in log , this estimator provides unbiased estimates for galaxies of different properties and types. An immediate application of this estimator to a compiled sample of galaxies with only CO (J = 2−1) observations yields a distribution of the CO (J = 2−1) to CO (J = 1−0) luminosity ratios (R21) that agrees well with the distribution of real observations, in terms of both the median and the shape. Application of our estimator to the current MaNGA sample reveals a gas-poor population of galaxies that are predominantly early-type and show no correlation between molecular gas-to-stellar mass ratio and star formation rate, in contrast to gas-rich galaxies. We also provide alternative estimators with similar scatters, based on r- and/or z-band luminosities instead of . These estimators serve as cheap and convenient M _mol proxies to be potentially applied to large samples of galaxies, thus allowing statistical studies of gas-related processes of galaxies.

We present well-resolved near-IR and submillimeter analysis of the three highly star-forming massive (> 10¹¹ M_⊙) galaxies within the core of the RO-1001 galaxy group at z = 2.91. Each of them displays kpc scale compact starbursting cores with properties consistent with forming galaxy bulges, embedded at the center of extended, massive stellar disks. Surprisingly, the stellar disks are unambiguously both quiescent and severely lopsided. Therefore, “outside-in” quenching is ongoing in the three group galaxies. We propose an overall scenario in which the strong mass lopsidedness in the disks (ranging from factors of 1.6 to > 3) likely generated under the effects of accreted gas and clumps, is responsible for their star-formation suppression, while funnelling gas into the nuclei and thus creating the central starbursts. The lopsided side of the disks marks the location of impact of accretion streams, with additional matter components (dust and stars) detected in their close proximity directly tracing the inflow direction. The interaction with the accreted clumps, which can be regarded as minor mergers, leads the major axes of the three galaxies to be closely aligned with the outer Lyman-α-emitting feeding filaments. These results provide the first piece of observational evidence of the impact of cold accretion streams on the formation and evolution of the galaxies they feed. In the current phase, this is taking the form of the rapid buildup of bulges under the effects of accretion, while still preserving massive quiescent and lopsided stellar disks at least until encountering a violent major merger.