Dark matter free-streaming length measured using 28 JWST gravitational lenses ✛ ✦
Context
Understanding the nature of dark matter remains one of the biggest challenges in modern physics. Since the days of Fritz Zwicky and Vera Rubin, gravitational methods have proven the existence of hidden mass, but its microscopic properties—particle mass, formation mechanism, possible self-interactions—are unknown. A key observable is the free-streaming length, which characterizes the suppression of density fluctuations on small scales in the early expanding universe. In warm dark matter (WDM) models, this scale is directly related to the particle mass: the lighter the particles, the more they smear out the seeds of galaxies and their halos.
Methods
The study is based on data from the JWST space telescope for 28 systems where a background quasar is lensed by a massive elliptical galaxy into four images. The uniqueness of the approach lies in using the MIRI instrument, which detects emission from warm cosmic dust (size ~1–10 pc) around the quasar, avoiding contamination from microlensing by stars while retaining sensitivity to millilensing by halos down to $10^6$ M⊙. For two systems without MIRI, flux ratios in narrow emission lines were used, measured via spectroscopy on the Keck telescope. Precise photometric measurements (image fluxes) were combined with modeling of extended arcs, yielding simultaneous constraints on the macro- and microstructure of the gravitational potential.
Results
The analysis ruled out deviations from cold dark matter (CDM) predictions on half-mode mass scales $m_{hm}$ > $10^7$.2 M⊙ for subhalos predicted by the Symphony simulations, and $m_{hm}$ > $10^7$.4 M⊙ for predictions from the semi-analytic Galacticus model. This corresponds to upper limits on the effective free-streaming length $λ_{FS}$ < 6.0 kpc and $λ_{FS}$ < 7.0 kpc, and lower bounds on the mass of a thermal relic particle with spin 1/2: $m_{ther}$m > 7.4 keV and > 6.5 keV, respectively. For spin-3/2 particles, the limits are > 6.2 keV and > 5.4 keV. Assuming CDM (negligible free-streaming length), the projected surface mass density in dark subhalos with masses $10^6$ – $10^{10}$.7 M⊙ was measured: $1.7^{+2.6}_{-1.2} × 10^7$ M⊙/kpc^2 (95% confidence interval). This amounts to about 3.2% of the mass in subhalos near the images.
Implications
These constraints are among the strongest independent tests of the CDM paradigm on subgalactic scales. They significantly narrow the allowed parameter space for warm dark matter models, making scenarios with light (less than ~7 keV) particles unlikely. This confirms the picture of hierarchical clustering, where small-scale structure persists down to very small masses.
Future development
Further improvement of the method will come with the growth of the lensed quasar sample in upcoming surveys (LSST, Euclid) and improved models of subhalo evolution. Combining strong lensing with other small-scale probes—stellar streams, dwarf galaxies—will help break remaining degeneracies and possibly reveal the first signs of deviation from CDM.
Impact
The results have a direct impact on particle physics beyond the Standard Model, cosmology, and the theory of structure formation.
Next steps
Next steps include analyzing additional lenses, accounting for subtler effects (dark matter self-interactions, non-Gaussian initial perturbations), and preparing for joint analysis of data from JWST and the upcoming Nancy Grace Roman telescope.
Key open problems
The work is directly connected to the unsolved problem of the nature of dark matter: whether it is cold, warm, or has additional properties such as self-interactions or non-thermal origin. Limits on the free-streaming length translate into constraints on particle mass and scenarios of their production in the early universe.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors used JWST data for 28 quadruply lensed quasar systems to measure the dark matter free-streaming length. A joint analysis of image positions, relative brightnesses, and extended arcs allowed them to place upper limits on the half-mode scale and lower bounds on the mass of thermal relic particles. They also measured the surface density of dark subhalos, consistent with CDM.
Where its strength is
The strength of the work lies in using MIRI to observe the quasar's dust emission, which suppresses microlensing by stars and enhances sensitivity to millilensing. This yielded some of the strongest constraints on warm dark matter at subgalactic scales.
Where it could go next
- It might be interesting to compare the obtained constraints with predictions from models where dark matter has a wave nature and forms quantum waves.based on: Ripples in the Dark Ocean: Quantum Waves Point to the Nature of Dark Matter arXiv:2607.20614v1
- A related approach uses galaxies with a deficit of dark matter to probe its properties; comparing with such systems could broaden the range of constraints.based on: Galaxies without dark matter: a sieve for the invisible arXiv:2607.26085v1
- It might be worth exploring the connection with laboratory dark matter searches, for example using qubits, to translate astrophysical limits into particle parameters.based on: Hunting Dark Matter with Qubits arXiv:2601.02474
Worth trying
- Test the method's sensitivity to self-interacting dark matter models using subhalo surface density data.
- Apply a similar analysis to more systems from future JWST surveys to improve statistics and reduce systematics.
- Combine free-streaming length constraints with data on small satellite galaxies to refine the dark matter particle mass.
Why this may matter
Such constraints are important for narrowing the range of viable dark matter models: they rule out a significant portion of warm dark matter parameter space and stimulate the development of alternative scenarios, such as wave dark matter or self-interactions.
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Original abstract
as written by the authors arXiv:2606.05277v1
The formation of gravitationally bound overdensities of dark matter (DM), or \textit{halos}, is a generic prediction of theories with particle DM. We present a measurement of halo properties in 28 quadruple image strong lens systems recently observed by JWST, and use these observations to constrain the free-streaming length, $$\lambda_{\rm{FS}}$$, of DM, a quantity that depends on the DM particle mass and formation mechanism. We improve on previous lensing analyses by simultaneously reconstructing extended lensed arcs with image positions and relative magnifications, enhancing sensitivity to perturbations by halos. Our analysis rules out deviations from the predictions of cold dark matter (CDM) on scales above $$10^{7.2} M_{\odot}$$ and $$10^{7.4} M_{\odot}$$ for subhalo abundance predicted by cosmological $$N$$-body simulations and semi-analytic models, respectively. These bounds correspond to upper limits $$\lambda_{\rm{FS}}<6.0 \ \rm{kpc}$$ and $$\lambda_{\rm{FS}}<7.0 \ \rm{kpc}$$, and lower limits on the mass of a spin--1/2 thermal relic DM particle $$m_{\rm{therm}}>7.4 \ \rm{keV}$$ and $$m_{\rm{therm}}>6.5 \ \rm{keV}$$. Conversely, assuming a negligible free-streaming length, as predicted by CDM, we measure a projected mass in subhalos around elliptical galaxies $$1.7_{-1.2}^{+2.6} \times 10^7 \ \mathrm{M}_{\odot} \ \rm{kpc^{-2}}$$ at $$95 \%$$ confidence. These inferences confirm key predictions of the CDM paradigm.
To obtain the final constraints, 176 million realizations of dark structure were generated; for each lens, between half a million and 22 million possible halo configurations were explored.
Key numbers
- Upper limit on free-streaming length (N-body): 6.0 kpc
- Upper limit on free-streaming length (semi-analytic): 7.0 kpc
- Lower bound on thermal particle mass (spin 1/2): 7.4 keV
- Measured subhalo surface density: 1.7 (+2.6 −1.2) × 107 M⊙/kpc^2
- Number of gravitational lenses in sample: 28