ALMA2040: Understanding the Early Assembly of Galaxies - The Formation of Star-Forming Clumps and their Role in Shaping Galaxy Evolution

The gist

ALMA2040 will connect cold gas, dust, and stellar structures across cosmic time to revolutionize our understanding of (i) the properties and evolution of GMCs and their connection to star-forming clumps, (ii) the spatial interplay between molecular gas, dust, and young stars, and (iii) the dynamics, survival, and formation of GMCs and stellar clumps.

Scientific context and open questions

Giant molecular clouds (GMCs) are central to understanding how galaxies form because they form from cooling and condensation of accreted gas provide the sites of star formation regulate by the gravitational state of galactic disks and by stellar feedback The properties of GMCs reflect galactic disk conditions, and their evolution is shaped by stellar feedback and host-galaxy dynamics. Hubble Space Telescope and James Webb Space Telescope observations reveal compact, star-forming structures from Cosmic Noon (z ≈ 2) to the Epoch of Reionization (z ≈ 6) but their link to the molecular gas reservoir is still unclear. The paper addresses unanswered questions regarding how GMC properties vary with redshift and environment, how molecular gas, dust, and young stars are arranged, or whether clumps form in unstable disks or disturbed systems.

ALMA2040: science case

ALMA2040 is uniquely suited to address these questions because its sub-mm capabilities will provide the molecular-gas and dust measurements that complement JWST and ELT observations of the stellar component. This facility will enable three complementary measurements in large, representative samples of star-forming, clumpy galaxies at z > 1: (1) resolve individual GMCs comparable to the ones in local galaxies and measure their molecular gas masses and physical conditions. (2) spatially resolve and map the dust continuum from galactic to GMC scales. (3) characterize molecular gas kinematics to connect cloud dynamics to the stability and evolution of their host galaxies.

Characterizing GMCs across redshift and galactic environments

Molecular hydrogen (H2), the fuel for star formation, cannot be directly observed at high redshift so CO is commonly used as its primary tracer. While some resolved observations of CO emission of individual GMCs have been carried out such measurements remain extremely scarce at z > 1. The first robust detections of individual GMCs in distant star-forming galaxies were obtained in the strongly lensed main-sequence systems Cosmic Snake and A521 at z ≈ 1. With ALMA observations, these studies resolved tens of clouds in CO with characteristic sizes of ∼ 30 − 200 pc and molecular gas surface densities of Σgas ≈ 103 − 104 M⊙ pc−2, about an order of magnitude above local values.

Resolving the dust distribution from galactic to GMC scales

Dust is an important component of galaxies and tightly connected to galaxy evolution because it affects the observed stellar emission by absorbing UV and optical radiation and re-emitting it in the far-infrared, it also plays a crucial role in cooling and therefore the formation of stars. While global dust properties of high-redshift galaxies start to be probed resolved dust measurements down to clump scales are still largely missing leaving open key questions such as what are the dust content and properties of high-z GMCs and what are the dust production mechanisms. ALMA2040 will resolve the dust continuum on the same physical scales targeted by JWST, from GMCs (≈ 10 − 100 pc in strongly lensed systems) to individual star-forming clumps in unlensed galaxies at Cosmic Noon and beyond. Critically, deriving dust temperature, dust mass, and optical depth at these scales cannot be achieved from a single continuum detection alone because dust temperature, emissivity index β, and mass are degenerate in single-band flux measurements.

Constraining molecular gas motions and GMC survival

Spatially resolved molecular lines from ALMA2040 will provide rotation curves, velocity dispersions, velocity gradients, and molecular surface densities distinguishing rotationally supported, dispersion-dominated (i.e., mergers), and dynamically disturbed systems. Combined with JWST or ELT stellar morphologies these measurements will test whether clumps preferentially form in gravitationally unstable disks, dynamically disturbed systems, or both, and how this balance changes with redshift and galaxy properties. The same data will determine whether individual GMCs are gravitationally bound, transient, or pressure-confined. ALMA2040 will extend such measurements to large samples over a broad range of redshift, stellar mass, starformation rate, and environment.

Technical Requirements

Pursuing the above science cases requires an increased sensitivity, higher spatial resolution, and wider band coverage at sub-mm frequencies which translates into a next-generation sub-mm interferometer with ≈ 10× increase spectral-line sensitivity to current ALMA (and ≈ 8× after WSU). ALMA2040 will probe high-z GMCs into the parameter space occupied by z ≈ 0 clouds reaching molecular masses of ∼ 106 M⊙ and radii of ∼ 40 pc. Using Larson relations such clouds are expected to have σV ∼ 3.5 km s−1 (assuming local relation) motivating a spectral resolution of ∆v <∼ 8 km s−1 (FWHM). A spatial resolution of ∼ 0.05′′ is likewise required to match the physical scales probed by JWST/NIRCam and, with lensing, to reach tens of pc.

Simulations with GHOST

Using the Generalised Hybrid Observation Support Tool (GHOST) we demonstrate how an ALMA2040 concept can be used to investigate GMCs and star-forming clumps in galaxies at high redshift. To this end we create two complementary galaxy models First we construct a model based on an observed clumpy galaxy at z ≈ 1 with a disk and ≈ 50 pc GMC-like clumps (Faisst et al. 2025) then “redshift” it to z = 5 using the corresponding cosmological angular-size and physical-size evolution (assuming Re ∝ (1 + z) −1, e.g., Ormerod et al. 2024). Second we construct a model with astropy.models resembling galaxies observed in large ALMA programs such as ALPINE (Le Fevre et al. 2019; B ethermin et al. 2020; Faisst et al. 2025), CRISTAL (Herrera-Camus et al. 2025), and REBELS (Bouwens et al. 2022) it contains a bulge and 15 clumps with different brightnesses.

Synergies with current and future facilities

By the 2040s, observations with JWST and large surveys with Euclid or the Nancy Grace Roman space telescope will have provided large, statistically representative samples of clumpy galaxies at z > 3. Similarly the ELT will deliver unprecedented spatial resolution for measuring cluster sizes and performing optical to near-infrared spectroscopy in galaxies with mild magnification and down to ≈ 20 pc scales in systems with µ ≈ 5. SKAO will add information on atomic gas from star-forming regions at high redshift at comparable scales. ALMA2040 will fit perfectly in this landscape providing the complementary census of molecular gas and dust making it possible to connect the stellar, atomic, and molecular phases across the scales from GMCs to galaxies and across the evolution from Cosmic Noon to the Epoch of Reionization.

1. No longer limited to strongly lensed systems for GMC detection: The improved system can "Detect GMCs in galaxies with modest (µ < ∼ 5) or no gravitational magnification," mitigating the bias toward rare, highly magnified systems.

2. Population study of cloud properties: The system can enable the statistical measurement of the intrinsic cloud-to-cloud and clump-to-clump scatter in various properties (mass, surface density, virial parameter, depletion time).

3. Redshift evolution tracking: The improved system can "extend resolved GMC studies to z > 3 using both CO and [CII], to test whether cloud properties, star-formation efficiencies, and depletion times evolve with redshift."

4. Physical condition mapping: The system can perform Measure the physical conditions of molecular gas through multi-J CO and [CI] observations by extending modeling techniques from local systems to individual clumps and GMCs.

5. Dust property characterization: The improved system can resolve dust temperature, dust mass, and optical depth at these scales by utilizing ALMA2040's simultaneous multi-band capability to sample both the Rayleigh-Jeans and near-peak regimes of the modified blackbody SED.

6. Dynamical state classification: The system can determine whether individual GMCs are gravitationally bound, transient, or pressure-confined by comparing resolved molecular gas kinematics with JWST stellar morphologies and ages.

Astrophysics seeks to explain how galaxies form and evolve through the interplay of stars, gas, and dust. Giant molecular clouds (GMCs), the main reservoirs and sites of star formation, link the cold interstellar medium to the birth of stars and stellar structures. Their properties reflect galactic disk conditions, and their evolution is shaped by stellar feedback and host-galaxy dynamics. Studying GMCs across cosmic time is therefore key to understanding how gas turns into stars and drives galaxy evolution. Hubble Space Telescope and James Webb Space Telescope observations reveal compact star-forming clumps in galaxies from Cosmic Noon to the Epoch of Reionization (z about6), but their link to the molecular gas reservoir is still unclear. We do not yet know how GMC properties vary with redshift and environment, how molecular gas, dust, and young stars are arranged, or whether clumps form in unstable disks or disturbed systems. Addressing these questions requires spatially resolved observations of the cold interstellar medium at the scales probed by optical/near-infrared facilities. ALMA2040, a transformational next-generation sub-mm facility, will provide the sensitivity and angular resolution needed to detect individual GMCs in representative lensed and unlensed galaxies at z>1. CO and [CII], complemented by dust continuum and molecular-gas kinematics, will measure the molecular gas reservoir, its physical conditions, and its dynamical state. Together with JWST and the ELT, ALMA2040 will connect cold gas, dust, and stellar structures across cosmic time. ALMA2040 will revolutionize our understanding of (i) the properties and evolution of GMCs and their connection to star-forming clumps, (ii) the spatial interplay between molecular gas, dust, and young stars, and (iii) the dynamics, survival, and formation of GMCs and stellar clumps.