A next-generation sub-millimeter interferometer will be uniquely capable of revealing the cold interstellar medium, dust reservoirs, and obscured star formation in the first galaxies. Providing robust redshifts, gas kinematics, feedback and attenuation-free estimates of the metallicity will require a five-fold increase in ALMA’s collecting area and extended baselines to deliver ELT-like spatial resolutions on thousands of normal (MUV > −20) early galaxies.
What drives stellar and black-hole growth through cosmic time? Mapping the physics and chemistry of gas and dust in distant galaxies
The interplay between the baryonic matter in galaxies (gas, dust, stars) has a fundamental role in our understanding of galaxy formation and evolution through cosmic time. The (sub)mm regime provides a unique window to measure the physical conditions and chemistry in the cold and dusty gas reservoirs, that fuel the formation of new stars and the growth of massive black holes in distant galaxies, complementing multi-wavelength facilities (ELT, SKA). Large sub(mm) interferometers (ALMA, NOEMA) have made a revolutionary impact mapping the gas and dust in galaxies through cosmic time. However, significant progress is hampered by limited spectral-line sensitivity. This limits the field to small-number statistics and completely restricts access to all but the brightest gas tracers in typical galaxies in the distant universe, and thereby the physics and chemistry of the gas and dust, even after the ALMA wide-band sensitivity upgrade (WSU). A next-generation ALMA (sub)mm interferometer with an order-of-magnitude increase in spectral line sensitivity and surveying capabilities is critical to make a transformative leap in the 2040s.
Probing cold gas and dust in quiescent galaxies across cosmic time: What quenches massive galaxies in the early Universe?
The discovery of massive quiescent galaxies at z > 3 by JWST has revealed that star formation can cease rapidly in the early Universe, yet the physical mechanisms responsible for quenching remain unknown. A key missing element is the systematic measurement of the cold gas and dust content in statistically representative quiescent populations, as understanding those is the gateway to distinguishing quenching mechanisms and evolutionary pathways. Such observations are currently not feasible because state-of-the-art sub-millimeter facilities (e.g. ALMA) cannot routinely detect the faint molecular gas and dust reservoirs of quiescent galaxies, nor pinpoint outflowing molecular gas. A new-generation facility capable of providing transformative access to the cold interstellar medium of quiescent galaxies across redshift is therefore essential. When combined with the rich legacy of JWST, ELT, and SKAO surveys, ALMA2040 will deliver a comprehensive, multi-phase view of feedback and quenching, establishing when, how, and why galaxies shut down their star formation across cosmic time.
Tracing the emergence of galaxy discs and dark matter halos in the early Universe
The earliest phases of galaxy assembly (z > 3, 1 - 2 Gyr after the Big Bang) mark the emergence of the first galactic discs, the progenitors of the Milky Way and local spirals, and the initial coupling between baryons and dark matter (DM) halos. Addressing when and how discs settled, how galaxies evolved into present-day spirals, and what the structural properties of DM halos were at these epochs requires spatially resolved kinematics of the cold gas in typical high-z galaxies out to large radii. Only a next-generation mm/sub-mm interferometer (ALMA2040) can open this window, enabling the first statistical measurements of galaxy dynamics and DM halo properties in the early Universe.
High-z AGN: The role of massive black holes in galaxy evolution
Decades of intensive campaigns and theoretical efforts have firmly established the central role that massive black holes (BHs; MBH = 106−10 M⊙) play in galaxy evolution. In the coming decades, we need to secure precise dynamical measurements of BH masses by mapping the gas motion within their gravitational sphere of influence (< 100 pc). We need to characterise the relation between the BHs and their hosts and how they change/evolve with cosmic time. Finally, we need to assess how the outflows from the central BHs impact the growth of the hosts. A next-generation sub-mm/mm interferometer would open up a novel window to address all of these challenges.
AGN Feeding and Feedback: Resolving the Torus
Active Galactic Nuclei (AGN) represent a key phase in galaxy evolution, as they regulate and influence star formation within their host galaxies. It is therefore crucial to understand how the central supermassive black holes (SMBH) are fueled by material from the galaxy disk, and how the AGN feedback impacts the surrounding gas. Most information so far comes from very nearby active galaxies (z < 0.008), providing the best spatial resolution and revealing randomly oriented dusty molecular tori that fuel SMBHs. Increased sensitivity and spatial resolution would enable progress in two fundamental areas that are beyond the capabilities of both existing and planned facilities: (i) probing the dynamics of the material feeding the AGN down to the accretion disk in nearby galaxies; and (ii) spatially resolving the tori and circum-nuclear regions of more energetic and cosmologically significant AGN, such as local quasars.
Resolving the Baryon Cycle over Cosmic Time: What regulates star formation in galaxies across cosmic time?
Galaxies are dynamic ecosystems where gas is continuously cycled between different phases and transformed through star formation. As gas cools down in the interstellar medium (ISM), it forms molecular clouds where new stars are born. These stars then return energy and metals to the ISM through radiation, winds, and supernovae. This baryon cycle is further shaped by gas accretion from the circumgalactic medium (CGM) and the expulsion of enriched material via outflows. Although the baryon cycle evolves with redshift z, the physical processes linking its evolution to galaxy growth remain poorly understood, raising key questions about gas accretion, star formation, galactic structure, and the interplay between these processes. These questions can be addressed with spatially resolved sub-mm observations of typical star-forming galaxies (SFGs) at key cosmological epochs (z ≳ 1). This requires a next-generation sub-mm interferometer (ALMA2040) beyond the capabilities of current and planned observatories.
Tracking the flow of interstellar gas across scales: from galaxies to star clusters
While the baryon cycle of the interstellar medium – and its ability to form stars – plays a key role in galaxy evolution, unraveling the physical processes driving its evolution remains an unsolvable challenge with present-day observing facilities, which are unable to map entire galactic ecosystems down to the scales of individual star-forming clumps. Here, we outline the technical requirements that would enable, for the first time, tracking the flows of matter and energy across the vast range of interacting and evolving environments in galaxies. Only through connecting the small (pc) scale physics of star formation to large (tens-of-kpc) galactic scales can we answer the long-standing question of what regulates star formation across cosmic time.
What drives star formation across local galaxies? Resolving the underlying physics behind star-forming scaling relations
A central open question in galaxy evolution is the physical origin of star-forming scaling relations. Galaxies with similar molecular gas masses show differences in star-formation efficiencies, indicating underlying, poorly constrained variations in the physical state of the molecular ISM. A key science objective for the 2040s, enabled by high-resolution and deep (sub)mm observations, will be linking molecular gas physical conditions to the drivers of star-forming scaling relations, replacing purely empirical descriptions with physically grounded ones.
Molecular atlas of galaxies: High-resolution wideband molecular line inventories of nearby galaxies
Star formation and associated feedback processes play a key role in galaxy evolution. Astro-chemistry is a powerful tool for characterising the physical conditions of dense star-forming gas and understanding the reciprocal relation between stellar feedback and the interstellar medium (ISM). The advent of large spectral surveys of a few individual galaxies have demonstrated the potential of molecular gas inventories to reveal the physical conditions of star-forming gas in great detail. A next-generation (sub)mm facility in the 2040s would extend this science to the entire local galaxy population, providing for the first time a comprehensive view of the physical and chemical conditions of the cold molecular ISM in the local universe.
Chemical complexity from molecular clouds to star-forming cores: Understanding its emergence, evolution, local and galactic variations
The study of the molecular feedstock of star-forming regions is pivotal for understanding the physical and chemical conditions that govern the formation of stars and planetary systems from their natal clouds, and its potential to provide ingredients for the emergence of life. Only a new generation (sub)millimeter interferometer with a 10× enhancement, both in spectral line sensitivity and wide-field imaging capabilities, will allow us to capture the true extent of chemical diversity (complex prebiotic molecules and rare isotopologues) towards representative samples of sources, and to follow its emergence, evolution and variations locally and at Galactic scales.
Star Formation across the Galaxy: Resolving fragmentation, accretion and feedback throughout the entire Milky Way
Star formation is a multi-scale highly dynamic process, regulated by the combination of gravity, turbulence, feedback and magnetic fields, which connects multiple spatial scales: from large molecular clouds (a dozen pc in size) down to protostellar disks (a few ten au in size). Resolving the processes of fragmentation, accretion and feedback that result in the formation of stellar systems throughout the entire range of environments in the Milky Way is key to constructing a comprehensive understanding of the formation process of stars of all masses, which populate not only our Galaxy but the wider Universe.
Planet formation on Solar System scales: How do gas giants form and shape their planetary system architectures?
Giant planets are often regarded as primordial protoplanetary “architects”. Their formation and interactions with planetesimals and other planets during and after the protoplanetary disc phase are key to lay down the blueprint of the planetary systems we observe today. Catching nascent gas giants is our best opportunity to unravel these crucial steps in the planet formation process and shed light on the origin of our own solar system. Over the last decade, ALMA has revolutionised our understanding of planet formation, yet detecting signatures of a Jupiter-like planet remains out of its reach. We argue that a next-generation sub(mm) interferometer with improved sensitivity (×25 in the continuum and ×10 for line emission) and ×3 higher angular resolution compared to ALMA will allow detecting (sub)mm emission around a forming Jupiter-like planet orbiting a Sun-like star within 150 pc and kinematically characterising its interactions with its natal disc. This new facility will provide unique and unprecedented insights into how gas giants carve gaps, accrete material, form satellites, and affect their planetary system architectures.
Probing the chemistry of planet-forming material on solar system scales: What is the elemental and isotopic composition of terrestrial planet-building material?
The initial chemical composition of planets and primitive bodies in our Solar System and exoplanets around other stars is ultimately set by their nascent protoplanetary disks. The observations of key C-, O- and N-bearing molecules in inner (exo)planet-forming zones ( <10 au) remain extremely challenging with current facilities. To construct a complete picture of planet formation, a more powerful observing facility is needed. It must provide an increase in line sensitivity ( > a factor of 10) at a spatial resolution of < 5 au at 150 pc to map the composition on terrestrial planet-forming scales (within the water snowline) and detect molecules that are the feedstock for potentially habitable worlds.
Planet forming disk populations and their connection to the environment: How do planets form in the most common star-forming conditions in the galaxy?
Existing sub-mm observatories such as ALMA have provided statistics on the planet-forming mass reservoirs within circumstellar disks around stars in stellar clusters out to ∼ 400 pc. However, these discs are not where most planets form. Most stars, and hence planets, form in more massive clusters, which are only found at larger distances. Environmental conditions that affect the disks such as UV radiation field, ambient gaseous mass reservoir, and stellar density, change significantly as a function of cluster mass, implying that our current knowledge of the disk population from current facilities is severely biased. Characterising planet formation in stellar clusters towards the peak of the cluster mass function requires determining the distribution of planet forming disk masses out to ∼3kpc. This would be achievable with a facility with a factor 20 increase in continuum sensitivity compared to current facilities.
The complex dynamics of the early stages of star and planet formation
Young planet-forming disks are dynamic, magnetized systems embedded in star-forming environments that shape their properties. We need to reveal the physical and chemical processes that govern mass delivery, angular momentum removal, and the enrichment of disks with complex molecules in various environments. A next-generation (sub)mm facility in the 2040s would transform our understanding of how planetary systems are born.
Planetary system architectures in the final stages of planet formation: How do late-stage processes shape planet assembly and volatile delivery?
The processes in the final stages of planet formation are crucial for a full picture of planet as- sembly and composition. After the younger protoplanetary disks dissipate, second-generation dust and gas produced by collisions of planetesimals or planets are found within debris discs/planetesimal belts, analogues of the Asteroid and Kuiper Belt. Resolved images of these belts allow us to trace the dynamical history of these systems, including giant impacts and planet-belt interactions, in the crucial 10 to few 100 Myr-old era of terrestrial planet formation. High-resolution mm and optical/near-IR imagers have revolutionised our view of the brightest planetesimal belts, providing first glimpses of diverse structures, many of which show plau- sible planet imprints. Yet these results are strongly biased towards the brightest belts around early-type stars, and even for these, crucial details remain unresolvable due to their typically low surface brightness. A future (sub-) mm facility with a 25x greater continuum sensitivity, would transform our understanding of late-stage planet formation, enabling the first statisti- cally meaningful surveys of faint outer planetesimal belts across all spectral types as well as imaging of planetary collisions in inner regions of planetary systems.
Evolved Stars: Dust formation/mass loss
Evolved stars drive the chemical enrichment of the Universe, acting as primary producers of elements and cosmic dust. Resolving their role requires empirical constraints on the interplay between pulsation, convection, mass-loss, and the formation of dust at challenging small scales in their outflows. Current observational studies are limited to a small sample of nearby stars, leaving fundamental questions unsolved. The next generation of instrumentation for ESO Expanding Horizons is essential to achieve the prescription of mass loss, dust formation and enrichment that is required to properly understand the life cycle of baryonic matter.
Evolved stars, multiplicity and planets: Shaping and Being Shaped by Stellar Evolution
Stellar evolution rarely occurs in isolation. From planetary systems to binary and higher-order multiples, companions shape the fate of stars and their feedback into the interstellar medium. Understanding how planets and stellar companions survive, interact with, or are destroyed by stellar evolution is essential both for predicting the long-term future of planetary systems like our own and for understanding how stars contribute to galactic chemical evolution.
From climate to chemistry: understanding atmospheric variability, dynamics, and volatile and organics origins across worlds
A next-generation (sub)millimetre interferometer (ALMA2040) will open a new observational regime for studying atmospheres, volatiles, and organic chemistry across Solar System bodies and beyond. Many processes governing atmospheric dynamics, climate variability, and chemical evolution remain inaccessible due to limitations in sensitivity, resolution, spectral coverage, and imaging speed. ALMA2040 will enable investigations not achievable today, including snapshot global measurements of planetary wind fields, vertically resolved circulation studies, and detections of faint molecular tracers and isotopologues in cold or rarefied environments. These capabilities are essential for probing long-term climate variability on terrestrial planets, stratospheric dynamics on giant planets and Uranus, complex chemistry and circulation on Titan, auroral processes, and volatile inventories of comets and small bodies. By accessing a new parameter space in spatial, spectral, and temporal domains, ALMA2040 will establish robust Solar System benchmarks for atmospheric dynamics, chemistry, and isotopic evolution, providing a foundation for interpreting exoplanet atmospheres within a comparative framework.
The energetic atmosphere of the Sun: How do small-scale dynamic processes heat and structure the solar atmosphere?
Understanding the mechanisms that heat the Sun’s chromosphere and corona to temperatures far exceeding those of the solar photosphere below remains one of the central challenges in solar physics. Small-scale, rapidly evolving processes – including magnetoacoustic shocks, turbulence, Alfven waves, reconnection-driven jets, and nano-flares – are thought to contribute significantly to the heating budget. However, direct observational constraints on their spatial, temporal, and thermal signatures remain hard to obtain. While the Atacama Large Millimeter/submillimeter Array (ALMA) already provides unique diagnostic capabilities, offering temperature diagnostics and high-cadence imaging, the current capabilities are still limiting the study of such dynamic, small-scale phenomena. Improved spatial resolution, enhanced uv-coverage, and simultaneous multi-band observations, as proposed for ALMA2040, would enable transformative progress in capturing dynamic processes across multiple heights and allow for the first time the time-dependent three-dimensional tomography of the chromosphere.
Shaping the future of Global Interferometric: Imaging Strong Gravity and Magnetic Fields
The observational validation of General Relativity (GR) has been propelled in recent years by recent breakthroughs in Very Long Baseline Interferometry (VLBI) augmented by ALMA. We explore ALMA2040 opportunities to transform these studies through greatly improved sensitivity and a multi-frequency approach. The focus will be on placing most stringent constraints on GR and alternative theories in the strong-gravity regime, and on understanding the formation and launching of relativistic jets.
Strong gravitational lensing with a next-generation sub-millimetre/millimetre array: Astrophysical tests of the nature of dark matter in the 2040s
A fundamental open question in physics is the nature and properties of dark matter. Different dark matter models predict different structure formation on small scales. Strong gravitational lensing is a proven effective tool to test models of dark matter by measuring the prevalence and properties of low-mass haloes. We argue that a next-generation sub-mm array would enable powerful constraints and overcome the hindrances of other planned facilities.
The Magnetized Universe: From planet-forming disks to the high-redshift universe
The interstellar medium (ISM) is magnetized at all scales and phases in our Galaxy and other galaxies across the Universe. Spectro-polarimetric millimeter observations are essential to probe the role of the interstellar magnetic fields in the dynamics of the cold universe, and how this affects the star formation process, and the planet formation in nearby star forming regions. Such observations are also essential for understanding magnetic fields in active stars and the shaping and evolution of the circumstellar environments of evolved stars.
Pulsars & Fast transients: Understanding fundamental physics using mm-wavelength observations of neutron stars
Bursty coherent radio emission from pulsars, magnetars and fast radio transients is originated in some of the most extreme environments in the universe. However, our understanding of the exact physical mechanism behind the scenes is still vague. ALMA2040 will allow for systematical exploration of the mm-wavelength spectral window for pulsars and fast radio transients, offering unprecedented opportunity to discover new observational phenomena and boost the understanding of emission physics.
Understanding cosmic explosions
Millimeter-wavelength observations provide a uniquely sensitive window into the physics of cosmic explosions, from supernovae (SNe) and tidal disruption events (TDEs) to nova eruptions, stellar mergers, and electromagnetic counterparts to gravitational-wave (GW) sources. Over the past decade, discoveries across the transient sky have revealed that many of the most informative phases of shock interaction, particle acceleration, and jet formation unfold at frequencies above the traditional cm-wave domain, where synchrotron self-absorption weakens and thermal, nonthermal, and dust components can be disentangled. Upcoming wide-field mm surveys will detect a new population of short-lived transients but will be limited by coarse localization. A next-generation mm-facility, with dramatically improved sensitivity, bandwidth, fast-response modes, and extended baselines, is essential for extracting the physical insights encoded in these events. This white paper outlines the scientific context, the key open questions, and the technical capabilities required to unlock the full potential of millimeter time-domain astrophysics in the 2040s.
Mm-jets in the multi-messenger era: The launching, collimation, acceleration, feedback and evolution of astrophysical jets across all scales
Relativistic jets exist across the full cosmic mass scale—from young stellar objects (YSOs) and X-ray binaries (XRBs) to active galactic nuclei (AGN), gamma-ray bursts (GRBs), and tidal disruption events (TDEs)—yet the physical processes governing their formation, evolution, and impact remain only partially understood. Millimeter and sub-millimeter observations uniquely probe the region where jets become optically thin and where particle acceleration and collimation can be directly observed. This white paper outlines the transformational science enabled by a next-generation mm/sub-mm interferometer providing an order-of-magnitude sensitivity gain over current facilities, rapid-response capability, simultaneous frequency coverage over one frequency decade, high-fidelity polarization, and full integration with global very-long-baseline interferometry (VLBI) networks. Such a facility will capture jet physics in real time in XRBs, reveal the earliest afterglow phases of GRBs, measure jet launching and feedback in protostars, image event-horizon-scale jet bases in AGN, and follow the birth and evolution of jets in TDEs. Together, these advances will provide a coherent, multi-scale understanding of how jets ignite, accelerate, and interact with their environments—and, crucially, how these processes vary with the nature of the central engine—establishing millimeter astronomy as a cornerstone of time-domain and multi-messenger astrophysics in the 2040s and beyond.