Unveiling star formation at sub-solar metallicity with ALMA2040

The gist: ALMA2040 will identify similarities and differences between solar- and sub-solar-metallicity star-forming regions, answering how different environmental conditions influence the physical and chemical pathways leading to star formation.

Scientific Context

Sub-solar metallicity gas clouds or stars have an amount of elements heavier than hydrogen and helium lower than solar (i.e. Z < Z⊙) <ref:2610.213315, as heavy elements and dust grains act as the primary coolants in interstellar gas clouds. Therefore, sub-solar metallicity clouds should have higher gas temperatures <ref:2610.21331#pg6>. If core masses are determined by Jeans fragmentation, bound cores on the verge of gravitational collapse would require higher densities and/or pressure confinement in these low-metallicity, and thus warmer, clouds, thereby reducing the efficiency of forming lowmass (M ≃ 1M⊙) cores and, ultimately, low-mass stars <ref:2610.21331#pg6>. This effect should be particularly strong in clusters hosting young massive stars, which have been observed to affect the brown dwarf regime of the Initial Mass Function (IMF) in solar-metallicity environments <ref:2610.21331#pg6>. On the side of planet formation, protoplanetary disks around sub-solar metallicity stars should contain fewer heavy solid materials, and hence be associated with a reduced efficiency in the formation of large solid planetary cores <ref:2610.21331#pg6>. In addition, the reduced dust content could accelerate disk dispersal and shorten lifetimes compared to solar-metallicity regions <ref:2610.21331#pg6>. Therefore, studying regions of different metallicity is fundamental to understand the impact of environment on the process of star formation <ref:2610.21331#pg6>.

The Milky Way is a perfect laboratory to test these predicted differences, as it is a barred spiral with a radius of ∼27 kpc (Lopez-Corredoira et al. 2018) with a metallicity gradient between ´∼ 0.1 − 2 Z⊙ from the outermost edge to its centre <ref:2610.21331#pg6>. The outer Galaxy (hereafter, OG), usually defined as the portion of the Galactic disk beyond ∼ 11 kpc, is characterized not only by sub-solar metallicities, but also by several other properties that significantly differ from those of the inner Galaxy <ref:2610.21331#pg6>. In summary, compared with the local Galaxy the OG exhibits: (i) metallicity reduced by factors ∼ 2 − 10 depending on the Galactocentric distance, RGC (e.g., Luck & Lambert 2011); (ii) lower dust abundance, and hence gas-to-dust ratios above the ∼ 100 value inferred locally (e.g., Giannetti et al. 2017); (iii) lower gas surface densities (e.g., Heyer & Dame 2015; Braine et al. 2023) and ambient pressure (e.g., Wolfire et al. 2003; Schruba et al. 2019); (iv) lower star-formation rate (e.g. Roman-Duval et al. 2010). These properties resemble those found in nearby low-metallicity galaxies like the Magellanic Clouds (e.g., Russell & Dopita 1992; Choudhury et al. 2018) and, to some extent, galaxies at earlier cosmic epochs <ref:2610.21331#pg6>. Among the nearest laboratories to study in detail the star formation process at sub-solar metallicities, the Large and Small Magellanic Clouds (LMC and SMC, respectively) occupy a prominent place <ref:2610.21331#pg6>. The LMC possesses profound differences compared with the OG in the Milky Way <ref:2610.21331#pg6>. In addition to having a sub-solar metallicity (∼ 0.5 − 0.3 Z⊙, Roman-Duval et al. 2014), the intensity of the UV radiation field is higher (10–100 times, but with large variations) when compared to the Galactic values <ref:2610.21331#pg6>. The cosmic-ray density is also a factor of 4 lower than that measured in the solar neighborhood (e.g., Sewiło et al. 2022; Shimonishi et al. 2026). Similarly, the SMC has a metallicity of ∼0.25 − 0.1 Z⊙, the lowest metallicity of any gas-rich galaxy in close proximity (Bolatto et al. 2007) <ref:2610.21331#pg6>. Even if ten times further away than the OG, both the LMC and SMC are sufficiently close to enable detailed studies of individual star-forming regions <ref:2610.21331#pg6>.

The Outstanding Questions

Star formation models generally consider a common set of physical processes —the “ingredients” driving the basic physics of star formation— including self-gravity, thermal support, turbulence, magnetic fields, stellar feedback, and others (Fig. 1) <ref:2610.21331#pg6>. However, their relative importance may differ between models as these processes depend on the local environment <ref:2610.21331#pg6>. In particular, as changes in Z reflect in different coolling and ionisation rates, among others (Fig. 1), the relative importance of such “ingredients” for star formation may vary in low-Z environments with respect to local ones <ref:2610.21331#pg6>. Therefore, the fundamental question is: how do environmental conditions influence the physical and chemical pathways leading to star formation? <ref:2610.21331#pg6>. This overarching question raises several specific issues: (1) Are the properties of prestellar cores universal? (2) Does cloud fragmentation, and the resulting stellar IMF, depend critically on metallicity? (3) How does dust evolution depend on metallicity, and what is the impact in the survival of protoplanetary disks? (4) Is the reduced amount of metals and dust shielding at low metallicity critical for the formation and survival of molecules? <ref:2610.21331#pg6>.

Starless and Prestellar Cores at Sub-Solar Metallicities

Starless cores in clouds of our solar neighborhood are cold overdensities within clouds, with average densities ≥ 104 cm−3, that is about ten times larger than the surrounding medium, and with temperatures of about 10 K <ref:2610.21331#pg6>. Prestellar cores are starless cores characterized by higher central densities (≥ 105 cm−3, Aikawa et al. 2026) which, owing to the high density contrasts between their nuclei and outer edges, are dynamically unstable and tend to gravitationally collapse and form stars <ref:2610.21331#pg6>. In prestellar cores, turbulence is subsonic (Goodman et al. 1998) and lines are typically much narrower than 1 km s−1 <ref:2610.21331#pg6>. The combination of low temperatures and high densities leads to catastrophic molecular freeze-out in the nucleus of prestellar cores, as (non-thermal) desorption rate of CO can no longer counteract the adsorption rate, which increases with gas density (Caselli 4 <ref:2610.21331#pg6>. CO freeze-out boosts deuterium fractionation, enormously increasing the abundance of deuterated species such ad H2D+, N2D+, DCO+, NH2D, and others (e.g., Ceccarelli et al. 2014) <ref:2610.21331#pg6>. A prototypical prestellar core is L1544, at a distance of ∼ 140 pc (Fig. 2, Chacon-Tanarro et al. 2017) <ref:2610.21331#pg6>. A critical difference between starless cores and a prestellar core like L1544 is that the main cooling mechanism in starless cores is CO line emission, while in prestellar cores (at least in the CO-depleted nucleus) is dust thermal emission <ref:2610.21331#pg6>. At subsolar metallicity, both the dust and the CO abundance are expected to decrease, but they do not decline proportionally, or do significant differences emerge? <ref:2610.21331#pg6>. Moreover, is the typical layered chemical structure characterised by high abundance of deuterated species and CO depletion in the nucleus modified in low-metallicity warmer and less dense environments? <ref:2610.21331#pg6>. So far, physical and chemical properties of prestellar cores have been derived almost totally towards nearby (d ≤ 1 kpc) star-forming regions (e.g., Andre et al. 2014; Caselli et al. 2025) <ref:2610.

1. textbfImprove core mass function modeling for low-metallicity environments by incorporating metallicity effects on fragmentation cascades and turbulence models to determine how cloud fragmentation, and the resulting stellar IMF, depend critically on metallicity. This improved system can predict if the effect of metallicity on cloud fragmentation and the assembly of the CMF remains beyond our current understanding by simulating how changes in Z reflect in different coolling and ionisation rates.

2. textbfEnhance protoplanetary disk evolution simulations to account for reduced heavy element availability by integrating constraints on dust opacity, gas content, and chemical composition based on sub-solar metallicity observations. The system can determine whether the lower metallicity primarily reduces disk masses and dust reservoirs, changes the gas-to-dust ratio, or fundamentally modifies the physical and chemical pathways of disk evolution.

3. textbfDevelop a high-resolution observational data processing pipeline capable of achieving a 9× smaller PSF area through 3× longer baselines to resolve physical scales relevant to star formation across different metallicity regimes. This system can specifically target the resolution needed to resolve linear scales of 10 000 au (core envelope scale), 1000 au (core nucleus scale), and 100 au (protoplanetary disk scale) at distances ≥ 15 kpc.

4. textbfCreate a multi-transition molecular line analysis module that allows for simultaneous observations of numerous chemical tracers by utilizing the ≥ 512 GHz instantaneous bandwidth. This improved system can perform robust relative comparisons between lines and accurately determine excitation temperatures and column densities across different physical environments.

A fundamental question in astrophysics is whether the processes governing star formation are universal or depend on the physical conditions of their local galactic environment. The Milky Way and the Magellanic clouds offer ideal laboratories to explore this question, as the gas conditions in them span up to orders of magnitude in metallicity, gas surface density, ambient pressure, dust abundance, radiation field, and cosmic-ray flux. In particular, both the outer Milky Way and the Magellanic clouds provide the closest low-metallicity environments with conditions similar to those of metal-poor galaxies, while remaining sufficiently close to soon allow spatial resolution of individual star-forming cores and disks. Recent observations with ALMA and other facilities have begun to unveil the physical and chemical properties of star-forming regions in low-metallicity environments. Yet, their distances from the Sun still require transformational gains in current ALMA capabilities to address fundamental open questions: (1) Are the properties of prestellar cores universal? (2) Does cloud fragmentation, and the resulting stellar IMF, depend critically on metallicity? (3) How does dust evolution depend on metallicity, and what is the impact in the survival of protoplanetary disks? (4) Is the reduced amount of metals and dust shielding at low metallicity critical for the formation and survival of molecules? ALMA2040 will identify similarities and differences between solar- and sub-solar-metallicity star-forming regions, and answer the question: how do different environmental conditions influence the physical and chemical pathways leading to star formation?