Tracing small-scale structure formation through cosmic time with strong gravitational lensing: A case for a next-generation sub-millimetre/millimetre interferometer

arXiv:2610.20710 · astro-ph.IM, astro-ph.CO, astro-ph.GA · Submitted 2026-10-14 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: I'm Vera.

Jocelyn: And I'm Jocelyn, here with Subrahmanyan.

Vera: We're looking at the paper: "Tracing small-scale structure formation through cosmic time with strong gravitational lensing".

Jocelyn: The gist: A 255-element sub-mm/mm interferometer could probe this critical mass regime, detecting gravitational perturbations in strongly lensed dusty star-forming galaxies (DSFGs) from objects down to at least 105 M⊙.

Vera: First, who's behind it and why it matters.

Paper summary: Vera: To summarize this paper, "Tracing small-scale structure formation through cosmic time with strong gravitational lensing: A case for a next-generation sub-millimetre/millimetre interferometer," the main idea is that we can use strong gravitational lensing to detect dark matter haloes and wandering black holes in the mass range under one hundred seven solar masses <ref:2610.20710#pg2,haloes and wandering black holes>.

Jocelyn: The paper claims that these objects leave a key signature in the gravitational perturbation of strongly lensed dusty star-forming galaxies, and this method is powerful because it allows us to probe dark matter haloes and wandering black holes that might otherwise be invisible to current facilities.

Subrahmanyan: They are looking at the context of the standard cosmological paradigm where galaxies grow hierarchically through mergers in haloes of non-baryonic dark matter, and they ask how seeds for massive black holes formed and grew into the SMBHs we see today.

Vera: The paper is focused on this unexplored mass range under one hundred seven solar masses because objects at cosmological distances are too dark or too faint for existing tools to characterize in large numbers <ref:2610.20710#pg2,dark or too faint for existing>.

Jocelyn: They argue that by probing these low-mass haloes, we can discriminate between dark matter models, like cold dark matter versus models with self-interacting dark matter, because SIDM generates more diverse density profiles than CDM.

Subrahmanyan: They also focus on wandering black holes and primordial seeds; they suggest that a census of wandering black holes below one hundred six solar masses across different redshifts could discriminate between various seeding channels, like star cluster seeds versus direct collapse seeds <ref:2610.20710#pg2>.

Vera: The paper shows how strong gravitational lensing is a uniquely sensitive probe for mass, revealing low-mass haloes through perturbations to the surface brightness of lensed images, which is important because it bypasses the need for an electromagnetic signature from these dark objects.

Jocelyn: And they discuss the technical requirements for a next-generation sub-mm/mm interferometer like ALMA2040, highlighting the need for high angular resolution and surface brightness sensitivity to actually detect these faint perturbers.

Subrahmanyan: The authors point out that to break the degeneracy between dark matter physics and baryonic physics, it is essential to probe dark matter haloes below one hundred seven solar masses which are expected to be devoid of stars and gas <ref:2610.20710#pg2>.

Conclusion: Vera: So, looking at the title "Tracing small-scale structure formation through cosmic time with strong gravitational lensing: A case for a next-generation sub-millimetre/millimetre interferometer," it really encapsulates the scope of this work.

Jocelyn: It’s about using lensing as a tool to track how small structures formed over cosmic time, and why that requires us to build instruments like ALMA2040 to see what's happening at these low mass scales.

Subrahmanyan: The implication is that by successfully detecting these low-mass haloes and wandering black holes, we could gain crucial insights into the nature of dark matter seeding and how those structures evolved during galaxy assembly.

Vera: It suggests that the future of understanding dark matter might rely heavily on these gravitational probes rather than just looking for direct electromagnetic signals from these small objects.

Jocelyn: And it shows that combining this with other upcoming facilities, like Euclid and the Vera C. Rubin Observatory, could give us a much fuller picture of cosmic structure and black hole growth across different epochs.

Hannah R. Stacey, John P. McKean, Mamta Pandey-Pommier, Carla Cornil-Baéiotto, Simona Vegetti, Aristeidis Amvrosiadis, Franc¸oise Combes, Giulia Despali, Andreas L. Faisst, Giovanni Ferrami, Davide Massari, Dieu D. Nguyen, James W. Nightingale, Antti Rantala, Èristiana Spingola

European Southern Observatory (ESO) · Kapteyn Astronomical Institute, University of Groningen · South African Radio Astronomy Observatory (SARAO) · Department of Physics, University of Pretoria · Pole Scientific, Catholic University of Lyon · Instituto de Física y Astronomía, Universidad de ValparaƑso · Max-Planck-Institut fur Astrophysik · Department of Physics, University of Oxford · Observatoire de Paris, LUX · Dipartimento di Fisica e Astronomia Augusto Righi, Universita di Bologna · INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna · INFN – Sezione di Bologna · IPAC, California Institute of Technology · Department of Astronomy, University of Michigan · Newcastle University · Institute of Astronomy, University of Cambridge · È Kavli Institute for Cosmology (KICC)

astro-ph.IM, astro-ph.CO, astro-ph.GA

Submitted: 2026-10-14

Updated: 2026-10-14

Comments: Part of the ALMA2040 White Paper Series

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 83/100

The gist: The gist: A 255-element sub-mm/mm interferometer could probe this critical mass regime, detecting gravitational perturbations in strongly lensed dusty star-forming galaxies (DSFGs) from objects down

Key concepts

Low-mass dark matter haloes: These are dark matter structures predicted by Cold Dark Matter models, ranging from very small scales down to Jupiter-mass. They are crucial for understanding how dark matter is distributed on subgalactic scales and can be distinguished between different dark matter theories like CDM and SIDM.
Strong gravitational lensing: This phenomenon occurs when massive objects, like galaxies or clusters, bend the path of light from background sources. This bending distorts the images of those sources, creating magnified arcs that allow astronomers to measure the mass distribution of both visible and dark matter.
Sub-mm/mm interferometer: This is a proposed next-generation radio telescope with 255 antennas designed for high angular resolution and surface brightness sensitivity. It is specifically suited to detect the subtle gravitational distortions caused by very low-mass perturbers in lensed galaxies.

Terminology

Gravitational Lensing — 1× in this paper · explained in 2 episodes

Summary

The gist: A 255-element sub-mm/mm interferometer could probe this critical mass regime, detecting gravitational perturbations in strongly lensed dusty star-forming galaxies (DSFGs) from objects down to at least 105 M⊙.

Scientific context and open questions

Low-mass haloes and the nature of dark matter

Cold dark matter (CDM) predicts a large number of low-mass haloes with a well-defined mass function and mass density profile (down to Jupiter-mass scale; Wang et al. 2020). Alternative models like warm dark matter (WDM), ultra-light dark matter (ULDM), and self-interacting dark matter (SIDM) affect how dark matter is distributed on subgalactic scales. A key difference between these models is how dark matter is distributed on subgalactic scales. SIDM generates more diverse density profiles than CDM (Vogelsberger et al. 2012; Despali et al. 2019). Potential seed candidates include Population III remnants, heavy seeds from direct collapse, and medium-weight seeds from dense star clusters. Cosmological simulations predict that some of these black holes never settle to galaxy centres: minor mergers and satellite stripping deposit them in wide orbits, and they wander in the halo (Ricarte et al. 2021). Estimates at the high end predict massive galaxies (M200 ∼ 1013 M⊙) may host 100s of wanderers > 105 M⊙ (Ricarte et al. 2021; Di Matteo et al. 2023). A census of < 106 M⊙ wandering black holes across redshifts could discriminate between different seeding channels (especially between star cluster and direct collapse seeds) and probe sinking efficiency, while > 108 M⊙ recoiling black holes probe late-stage pre-merger dynamics.

Gravitational detection of low-mass objects

Strong gravitational lensing by galaxies and galaxy clusters provides a uniquely sensitive probe of mass, whether dark or baryonic. It has therefore become a powerful diagnostic of the nature of dark matter, particularly because it can reveal low-mass haloes (both sub-haloes and line-of-sight structures) via perturbations to the surface brightness of lensed images (see Vegetti et al. 2024 for a review). Current high-resolution, near-infrared imaging (e.g. JWST/NIRCam) of galaxy- and clusterscale strong lenses is sensitive to low-mass haloes in the mass range 108 to 1010 M⊙ (for median CDM mass concentrations; e.g. Despali et al. 2022; Nightingale et al. 2024). Powell et al. (2025) presented the current state-of-the-art in this field, using milliarcsecond interferometric imaging at cm-wavelengths (McKean et al. 2025) to detect a perturber with a well-defined mass of just 106.05±0.02 M⊙ within a projected radius of 80 pc (Fig. 2). A combination of milliarcsecond angular resolution and high surface brightness sensitivity is required to probe objects in this largely unexplored mass regime.

Technical requirements for a next-generation sub-mm interferometer

Multiple factors play into the ability to detect and characterise low-mass perturbers via lensing. Angular resolution and surface brightness sensitivity are both important. Contamination from lens light at optical and near-infrared wavelengths (Nightingale et al. 2024), and the structure and magnification of the lensed sources (Koopmans 2005; Despali et al. 2022; Tajalli & Vegetti 2026) are also important factors. Useful statistics are essential; therefore, a significant sky volume depends on the number of lens systems, the size of lensed arcs, and the distance to the lensed source. A next-generation sub-mm/mm interferometer that could succeed ALMA (hereafter, ALMA2040) offers unique advantages and complements other forthcoming facilities.

Synergies with current and future facilities

Euclid’s own strong-lensing analysis may report some low-mass halo detections, but at masses comparable to the existing near-infrared work described in §1.3 (> 108.8 M⊙; O’Riordan 2026). Meanwhile, the Vera C. Rubin Observatory recently began its 10-year Legacy Survey of Space and Time (LSST), which will probe the dynamic Universe by continuously monitoring 18000 deg2 of sky (Ivezic et al. 2019) and discover a similar number of galaxy-scale strong lenses to Euclid (Shajib et al. 2025). The SKA is expected to discover around 105 new gravitationally lensed radio sources (McKean et al. 2015; McCarty & Connor 2025; Pandey-Pommier et al. 2026). The ELT will provide an angular resolution comparable to ALMA2040 (Padovani & Cirasuolo 2023). LISA will detect gravitational waves from merging massive black hole binaries across cosmic history (Amaro-Seoane et al. 2023). Together, these synergies provide an observational picture of dark matter structures and black hole growth that can only be delivered with a next-generation submm/mm interferometer.

Simulated data

We created realistic simulated observations of a gravitational lens with a 255-antenna interferometer to demonstrate improved angular resolution and dynamic range relative to ALMA. We show images of the mock data created using CLEAN for visualisation in Fig. 4. Both low-mass objects create ≥ 15σ significance distortions in surface brightness relative to the perturber-free simulated data.

Improvements for AI systems

Improved Dark Matter Model Discrimination: The improved AI system can discriminate between dark matter models by finding and characterising low-mass haloes because these objects differ dramatically in abundance and density profiles between dark matter models (Fig. 1).
Enhanced Black Hole Census: The system can provide a census of wandering black holes by constraining different seeding channels and sinking efficiency, as the study aims to discriminate between different seeding channels (especially between star cluster and direct collapse seeds) and probe sinking efficiency.
Lensing Perturber Classification: The AI can classify perturber detections by comparing simulated observations to physical models, specifically distinguishing between a point mass (describing a wandering black hole), a King profile (describing a globular cluster), and a single NFW profile (a dark-matter-dominated halo).
Cross-Wavelength Target Selection: The system can increase the statistical sample of low-mass perturbers by performing cross-matching between Euclid/JWST surveys and sub-mm/mm surveys, enabling better target selection for ALMA2040 observations.

Abstract

The nature of dark matter and the growth of supermassive black holes (SMBHs) remain major open questions in modern astrophysics. Both leave a key signature in an overlapping, largely unexplored mass range: <10 7 M dark haloes and wandering black holes. These may be undetectable directly, but detectable through gravitational perturbation of strongly lensed images, and partly or entirely distinguishable from one another through lensing data alone. We demonstrate through realistic simulations that a 255-element sub-mm/mm interferometer could probe this critical mass regime, detecting gravitational perturbations in strongly lensed dusty star-forming galaxies (DSFGs) from objects down to at least 10 5 M. Combined with lens-galaxy characterisation from the European Extremely Large Telescope (ELT), which strengthens the physical interpretation of detected perturbers, and the unprecedented lens samples expected from Euclid and the Square Kilometre Array (SKA), this facility could deliver a census of low-mass dark matter haloes and wandering massive black holes across cosmic time.