ALMA2040: The High-Redshift Circumgalactic Medium. Imaging the fueling and feedback of galaxies in the early universe

The gist: The authors present three key scientific cases for an ALMA2040 telescope to image extended [CII]-traced CGM halos around Milky-Way-progenitor galaxies at z = 4–8, resolved cold, clumpy outflows on ∼100 pc scales tied to individual star-forming clumps, and multi-species/-transition emission and absorption diagnostics that jointly constrain the density, chemistry, and radiation fields permeating the cool CGM and its multiphase gas flows.

Scientific Context

Galaxies form as part of a wider interconnected web mediated by an extended gaseous atmosphere called the Circum-Galactic Medium (CGM) The CGM is fed and shaped by gas inflows and mergers from the intergalactic medium and by feedback-driven outflows and radiation from the central galaxy This dynamic region is imprinted by the past, present and future evolution of its central galaxy and environment The cool atomic and molecular phases are of particular interest as they dominate the mass budget of outflows and provide a reservoir of direct star forming fuel for the galaxy to accrete In the early universe where accretion rates, mergers, and feedback processes peak, the cycle of gas through the CGM is most vigorous Sub-mm observations have detected cold gas in extended halos and gas flows around high redshift galaxies but are thus far limited to the most massive, extreme or gravitationally lensed sources, using just a handful of the strongest emission and absorption lines. A significant increase (5 − 10×) in spectral line sensitivity compared with the current ALMA is needed to push physical scales, densities and properties to a new regime.

Imaging of the Cool CGM around Normal Star-Forming Galaxies at z ≃ 4 − 8

Emission-line mapping is a powerful probe for directly imaging the cool CGM, complementing the sight-line view from absorption where the declining C IV/C II ratio towards the reionisation era already points to a progressively more neutral, cooler CGM Deep ALMA observations show that [C II] 158 µm emission, the brightest tracer of cool/cold gas, is systematically more extended than the rest-UV and dust continuum in star-forming galaxies at z = 4–7 This emission reaches 10–15 kpc scales in stacked samples (Fujimoto et al. 2019; Ginolfi et al. 2020b) and, in the deepest individual follow-ups, taking the form of halos, plumes, and tidal-like extensions. These halos show that enriched cool gas is already in place around galaxies within the first ∼0.8–1.5 Gyr, but their origin remains open: star-formation-driven outflows, accreting streams, stripped satellite material, or a combination of these (Pizzati et al. 2020, 2023).

Synergies with current and future facilities

ALMA2040’s 10× gain in line-sensitivity will match the boost from lensing provided here, such that maps of comparable quality become routine for unlensed normal-starforming galaxies at z ≃ 4–8, free of the distortion and differential magnification that complicate lensed observations. Halo mapping then moves from stacks to individual, representative galaxies, and two measurements become standard: (i) Cool-gas budgets: surface-brightness profiles and coolgas masses of the CGM out to > ∼ 10–15 kpc, with individual maps for log(M⋆/M⊙) > ∼ 9.5 galaxies in a few hours per source and the log(M⋆/M⊙) ∼ 9 regime reached in modest sub-stacks. (ii) CGM kinematics: resolved velocity and line-width maps that help to distinguish outflowing shells, extended rotating reservoirs, and accreting gas.

Resolving cold outflows in the early universe

While the gas in outflows spans many orders of magnitude in both temperature and density, it is the cold, dense, and neutral atomic / molecular phase that is thought to dominate the total mass budget (e.g. Fluetsch et al. 2019). High-resolution simulations predict that the properties of outflows depend strongly on the small-scale local properties of the regions that launch them, such as the local SFR surface density ΣSFR (e.g. Kim et al. 2020; Rathjen et al. 2023). Cold outflows are much more difficult to detect than warmer gas phases observed in the restoptical, and consequently the connection between the cold outflowing gas and the local conditions of the regions that launch winds is poorly understood. The recent CRISTAL ALMA Large Program (Herrera-Camus et al. 2025) illustrates the challenges of identifying cold outflows and connecting them to the kpc-scale regions that drive them with the current capabilities of ALMA.

Physical and chemical Properties of the Cool CGM and its Gas Flows

Every spectral line or chemical species probes a unique distribution of gas properties and by themselves reveal either a restricted and/or overly general view of the total gas reservoir. The bright [CII] 158 µm emission line, which has been instrumental in revealing extended CGM reservoirs and outflowing gas around high-redshift galaxies (Sec. 2.1 and 2.2), broadly emits across molecular, atomic, and ionised gas phases. Determining the relative contribution of these phases requires additional lines and species. Absorption lines of light hydride molecules (one heavy element atom attached to one or more hydrogen atoms), observed against a galaxy’s own dust continuum, have provided a complimentary view of the neutral CGM at high redshift. CH+ for example traces the dissipation of mechanical energy in diffuse, partially molecular gas, allowing their line properties to infer the sizes of turbulent high-redshift gas halos (Falgarone et al. 2017).

Technical Requirements

Transformative advances in our understanding of the high-redshift cold CGM and gas flows will require a next-generation interferometer with 10× higher line sensitivity compared to existing ALMA across the prime sub/millimetre atmospheric windows. This science case simultaneously requires excellent surface-brightness sensitivity to diffuse emission on > ∼ 10 kpc scales and ∼100 pc resolution for compact outflow structures. Increased line sensitivity is the primary requirement to detect the cold CGM and gas flows in the typical low-mass galaxies that evolve to become Milky Way-like by z = 0. Detailed inventories of outflows and the CGM benefit from an ability to detect multiple spectral lines simultaneously.

Synergies with current and future facilities

The Square Kilometre Array (SKA) and Next Generation Very Large Array (ngVLA) will provide complementary views of the cold phase of the CGM. The ngVLA will detect outflows in the ground-state transitions of many molecular species in typical main-sequence galaxies, including molecular transitions that trace gas at densities far higher than the ions and molecules accessible to ALMA2040 (CO, HCN, HNC, CS, etc.). The SKA, meanwhile, can directly observe the neutral atomic hydrogen 21 cm transition in absorption in galaxies that host even modest levels of radio AGN activity.

Future Facilities

At warmer ∼104-106 K temperatures, the completion of the next generation of ground-based Extremely Large Telescopes (ELTs) in the early/mid-2030s and the 2040s-era launch of the Habitable Worlds Observatory (HWO) will advance this field in two key ways. Firstly, the extremely high angular resolution of these facilities will result in maps of galaxies that detect and resolve any and all star-forming clumps, if present, on scales down to ≈200 pc.

1. Enhanced Cold CGM Mapping: The improved system can perform direct imaging of extended [CII]-traced CGM halos around Milky-Way-progenitor galaxies at z = 4–8, allowing for the mapping of "surface-brightness profiles and coolgas masses of the CGM out to >∼ 10–15 kpc."

2. Kinematic Decomposition: The system can execute per-object decomposition on halo scales to separate gas components, moving beyond biased recovery by isolating "the rotating component (e.g., Di Teodoro & Fraternali 2015; Davis et al. 2013; Roman-Oliveira et al. 2023)."

3. Outflow Characterization: The improved system can resolve small-scale structure of cold outflows down to ≈100 pc scales, enabling the detection of substructures predicted by simulations, such as much larger 100–500 pc-scale dense knots that survive direct acceleration.

4. Multiphase Diagnostics: The system can perform multi-species/-transition emission and absorption diagnostics to constrain the gas environment, enabling it to determine the relative contribution of these phases by analyzing lines like [CII] 158 µm, CO, and [CI].

5. Physical Property Inference: The AI system can infer crucial physical conditions by analyzing chemical tracers; for instance, it can use absorption lines of light hydride molecules (one heavy element atom attached to one or more hydrogen atoms) to constrain properties like the CR ionisation rate.

Galaxies do not form in isolation, but rather as part of a wider interconnected web mediated at their interface by an extended gaseous atmosphere - the Circum-Galactic Medium (CGM). This dynamic and multiphase region is fed and shaped by gas inflows and mergers from the intergalactic medium and by feedback-driven outflows and radiation from the central galaxy. The CGM is imprinted by the past, present and future evolution of its central galaxy and environment. The cool atomic and molecular phases are of particular interest as they dominate the mass budget of outflows and provide a reservoir of direct star forming fuel for the galaxy to accrete. In the early universe where accretion rates, mergers, and feedback processes peak, the cycle of gas through the CGM is most vigorous. Sub-mm observations have detected cold gas in extended halos and gas flows around high redshift galaxies but are thus far limited to the most massive, extreme or gravitationally lensed sources, using just a handful of the strongest emission and absorption lines. A significant increase (5-10 times) in spectral line sensitivity compared with the current ALMA is needed to push physical scales, densities and properties to a new regime. We present three key scientific cases for an ALMA2040 telescope: (i) direct imaging of extended [CII]-traced CGM halos around Milky-Way-progenitor galaxies at z=4-8; (ii) resolved detection of cold, clumpy outflows on about 100 pc scales tied to individual star-forming clumps; and (iii) multi-species/-transition emission and absorption diagnostics that jointly constrain the density, chemistry, and radiation fields permeating the cool CGM and its multiphase gas flows.