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The free-streaming length of dark matter from JWST observations of 28 strong gravitational lenses

This study utilizes JWST observations of 28 strong gravitational lens systems to constrain the free-streaming length of dark matter, confirming Cold Dark Matter predictions by ruling out deviations on scales above $10^{7.2} M_{\odot}$ and measuring a projected subhalo mass consistent with the standard cosmological model.

Original authors: D. Gilman, A. M. Nierenberg, T. Treu, K. N. Abazajian, T. Anguita, V. N. Bennert, A. J. Benson, S. Birrer, S. G. Djorgovski, X. Du, C. Gannon, S. F. Hoenig, R. E. Keeley, A. Kusenko, H. R. Larsson, M.

Published 2026-06-05

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CC BY 4.0

Original authors: D. Gilman, A. M. Nierenberg, T. Treu, K. N. Abazajian, T. Anguita, V. N. Bennert, A. J. Benson, S. Birrer, S. G. Djorgovski, X. Du, C. Gannon, S. F. Hoenig, R. E. Keeley, A. Kusenko, H. R. Larsson, M. Malkan, T. Morishita, V. Motta, L. A. Moustakas, P. Mozumdar, H. Paugnat, W. Sheu, D. Sluse, D. Stern, M. Stiavelli, D. Williams, K. C. Wong

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe is filled with an invisible, ghostly substance called Dark Matter. We can't see it, touch it, or smell it, but we know it's there because its gravity acts like a cosmic glue, holding galaxies together.

For decades, scientists have debated what this "ghost" is made of. Is it heavy and sluggish (like a slow-moving elephant), or is it light and speedy (like a swarm of hyperactive bees)? This paper uses the James Webb Space Telescope (JWST) to settle a major part of that debate.

Here is the story of how they did it, explained simply:

1. The Cosmic Magnifying Glass

The researchers used a natural phenomenon called gravitational lensing. Imagine a massive galaxy sitting between us and a distant, bright quasar (a super-bright black hole). The gravity of that galaxy bends the light from the quasar, acting like a giant magnifying glass. Usually, this creates four distinct images of the same quasar, arranged in a cross shape.

If the universe were perfectly smooth, these four images would be perfectly symmetrical. But the universe isn't smooth. It's filled with clumps of dark matter (called subhalos) that act like tiny, invisible pebbles in the lens. These pebbles distort the light, making some images brighter or dimmer than they should be.

2. The Problem: The "Static" in the Signal

In the past, trying to spot these tiny dark matter clumps was like trying to hear a whisper in a rock concert. The main galaxy (the lens) had its own messy features, and stars within that galaxy would flicker the light (a phenomenon called "microlensing"), creating static that hid the subtle signals from the dark matter clumps.

3. The Solution: A New Pair of Glasses

This team used JWST's Mid-Infrared Instrument (MIRI). Think of this as putting on special glasses that see heat rather than visible light.

The Trick: They looked at the "warm dust" surrounding the distant quasar. This dust cloud is large enough (like a small city) to be immune to the flickering of individual stars (the static), but small enough to be nudged by the invisible dark matter clumps (the whisper).
The Sample: They analyzed 28 of these cosmic crossroads (strong lenses), doubling the number of previous studies.

4. The Detective Work: Weighing the Ghosts

The team built a massive computer simulation. They asked: "If dark matter is heavy and slow (Cold Dark Matter), what would the light look like? If it's light and fast (Warm Dark Matter), how would the light change?"

They compared their real JWST observations against millions of simulated scenarios.

The "Warm" Scenario: If dark matter particles were light and fast, they would have "streamed" away from small areas in the early universe, leaving the small clumps (subhalos) empty or missing.
The "Cold" Scenario: If dark matter is heavy and slow, those small clumps should be everywhere, just like the simulations predict.

5. The Verdict: The Ghost is Heavy

The results were clear. The data ruled out the idea that dark matter is "warm" and light.

The Limit: They found that dark matter cannot be "warm" on scales smaller than about 10 million times the mass of our Sun.
The Analogy: Imagine trying to build a sandcastle. If the sand grains are too big and heavy (Cold Dark Matter), you can build tiny, detailed towers. If the sand grains are too light and airy (Warm Dark Matter), the tiny towers collapse, and you only get big, smooth mounds. The universe still has the tiny towers. Therefore, the "sand grains" (dark matter particles) must be heavy.

6. What This Means

Confirmation: The findings strongly support the standard theory that dark matter is "Cold" (slow-moving and heavy).
The Mass Limit: They calculated that if dark matter is a specific type of particle (a "thermal relic"), it must weigh at least 6.5 to 7.4 keV (a specific unit of mass). Anything lighter would have washed out the small structures they observed.
The Abundance: They also measured how many of these dark matter clumps exist around galaxies. They found about 1.7 × 10⁷ solar masses of dark matter clumps per square kiloparsec. This number matches what computer simulations predicted for Cold Dark Matter, confirming that our models of how the universe builds structure are correct.

In a nutshell: By using JWST to look at the "flicker" of light from 28 distant quasars, astronomers proved that the invisible scaffolding of our universe is made of heavy, slow-moving particles, not light, fast ones. The "Cold Dark Matter" theory remains the champion.

Technical Summary: The Free-Streaming Length of Dark Matter from JWST Observations of 28 Strong Gravitational Lenses

Problem and Motivation
The physical properties of dark matter (DM), specifically its particle mass and formation mechanism, remain unresolved. While Cold Dark Matter (CDM) predicts a specific abundance of dark matter halos across all mass scales, alternative models such as Warm Dark Matter (WDM) predict a suppression of halo formation below a characteristic scale due to "free-streaming." This free-streaming length, $\lambda_{FS}$ , depends on the DM particle's velocity distribution in the early universe. Detecting deviations from CDM predictions on small scales (comparable to dwarf galaxies) offers a direct probe of these particle physics properties. However, previous analyses using strong gravitational lensing faced limitations in sensitivity and sample size, often struggling to distinguish small-scale halo perturbations from uncertainties in the main deflector's mass profile or contamination from stellar microlensing.

Methodology
This study presents a joint analysis of 28 quadruply imaged quasar systems (strong lenses) to constrain the DM free-streaming length. The analysis leverages two primary observational and methodological advances:

Observational Data: The authors utilize data from the James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI) for 26 systems, supplemented by 2 systems observed with Keck OSIRIS and HST WFC3. The JWST MIRI observations target mid-infrared emission from the warm dust region surrounding the background quasars (1–10 pc in size). This specific emission region is large enough to avoid contamination from optically thick stellar microlensing (which affects compact sources) yet compact enough to be sensitive to perturbations from dark subhalos as small as $10^6 M_\odot$ .
Modeling Framework: The analysis employs a forward-modeling approach using lenstronomy2 and pyHalo. A key innovation is the simultaneous reconstruction of extended lensed arcs (image positions and surface brightness) alongside the relative magnifications (flux ratios) of the point-source images. This joint modeling breaks degeneracies between small-scale substructure perturbations and the azimuthal features of the main deflector's mass profile, which previously limited sensitivity.
Statistical Inference: The authors infer the joint posterior distribution of hyper-parameters describing the DM population: the normalization of the subhalo mass function ( $\Sigma_{sub}$ ), the half-mode mass ( $m_{hm}$ ), the logarithmic slope of the subhalo mass function ( $\alpha$ ), and a line-of-sight rescaling factor ( $\delta_{LOS}$ ). The likelihood is computed by marginalizing over millions of possible halo realizations for each lens system. The half-mode mass is converted to constraints on the free-streaming length ( $\lambda_{FS}$ ) and the mass of thermal relic particles ( $m_{therm}$ ).

Key Contributions

Sample Expansion: The study doubles the sample size of lens systems suitable for dark matter analysis compared to previous work by utilizing JWST MIRI data.
Enhanced Sensitivity: By simultaneously modeling extended arcs and flux ratios, the analysis isolates small-scale perturbations more effectively than previous studies relying solely on image positions and flux ratios.
Robust Constraints: The analysis provides the most precise measurement of subhalo abundance around strong lenses to date and places stringent bounds on WDM models.

Results
The analysis yields the following constraints on dark matter properties:

Free-Streaming Length and Particle Mass: The data rule out deviations from CDM predictions on scales above $10^{7.2} M_\odot$ (for subhalo abundance predicted by N-body simulations) and $10^{7.4} M_\odot$ (for semi-analytic models). These correspond to upper limits on the free-streaming length of $\lambda_{FS} < 6.0$ kpc and $\lambda_{FS} < 7.0$ kpc, respectively.
Thermal Relic Mass: Assuming a spin-1/2 thermal relic particle, the lower mass limits are $m_{therm} > 7.4$ keV (N-body prior) and $m_{therm} > 6.5$ keV (semi-analytic prior). For spin-3/2 particles, the limits are $m_{therm} > 6.2$ keV and $m_{therm} > 5.4$ keV, respectively.
Subhalo Abundance: Assuming a negligible free-streaming length (the CDM limit), the authors measure a projected mass in subhalos around elliptical galaxies of $1.7^{+2.6}_{-1.2} \times 10^7 M_\odot \text{ kpc}^{-2}$ at 95% confidence. This corresponds to a subhalo mass fraction of $f_{sub} = 3.2^{+5.1}_{-2.3}\%$ .
Comparison with Simulations: The measured subhalo abundance is consistent with predictions from the galacticus semi-analytic model and the Symphony N-body simulations at the $2\sigma$ level, though a tentative $\sim 1.8\sigma$ tension is noted with the Symphony simulations.

Significance
The paper claims that these results provide some of the strongest bounds on the free-streaming length of dark matter derived from a single independent inference. The findings confirm key predictions of the CDM paradigm regarding the abundance of substructure around massive galaxies. The authors emphasize that this analysis demonstrates the capability of combining JWST observations with advanced lens modeling to probe dark matter physics on scales previously inaccessible, setting a precedent for future large cosmological surveys that will discover hundreds of new quadruply imaged quasars. The techniques presented are also noted as applicable to testing other DM models, such as self-interacting dark matter or modifications to the primordial power spectrum.