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_therm > 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.
🎯 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) × 10^7 M⊙/kpc^2
- Number of gravitational lenses in sample: 28