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JWST lensed quasar dark matter survey IV: Stringent warm dark matter constraints from the joint reconstruction of extended lensed arcs and quasar flux ratios

Using JWST observations of 28 quadruple-image strong lenses, this study jointly reconstructs extended arcs and quasar flux ratios to derive the most stringent constraints on warm dark matter to date, setting a half-mode mass limit of $m_{\rm{hm}} < 10^{7.4} \mathrm{M}_{\odot}$ and providing the most precise measurement of subhalo abundance around strong lenses.

Original authors: D. Gilman, A. M. Nierenberg, T. Treu, C. Gannon, X. Du, H. Paugnat, S. Birrer, A. J. Benson, P. Mozumdar, K. C. Wong, D. Williams, R. E. Keeley, K. N. Abazajian, T. Anguita, V. N. Bennert, S. G. Djorg

Published 2026-06-05

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

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

The Big Picture: Weighing the Invisible

Imagine the universe is filled with a giant, invisible web called Dark Matter. We know it's there because its gravity pulls on visible things, like stars and galaxies, but we can't see it directly. Scientists have two main theories about what this web is made of:

Cold Dark Matter (CDM): Think of this as a web made of tiny, heavy pebbles. It's dense and clumpy, with lots of small pebbles (sub-halos) scattered everywhere.
Warm Dark Matter (WDM): Think of this as a web made of lighter, fluffier cotton balls. The "fluffiness" means the small clumps get smoothed out or washed away, leaving fewer tiny pebbles.

The goal of this paper is to figure out which type of web we actually have by looking at how gravity bends light.

The Tool: Cosmic Magnifying Glasses

The scientists used gravitational lenses. Imagine a massive galaxy sitting between us and a distant, bright quasar (a super-bright black hole). The gravity of the galaxy acts like a giant glass lens, bending the light from the quasar. Usually, this splits the quasar's image into four separate dots (like a diamond shape) around the galaxy.

If the invisible dark matter web has tiny clumps (sub-halos) near those four dots, they act like tiny pebbles on the glass lens. These pebbles distort the light, making some of the four dots appear brighter or dimmer than they should be. By measuring these brightness changes (called flux ratios), scientists can count how many tiny clumps exist.

The Problem: The "Fuzzy" Lens

In the past, scientists had a hard time getting a clear answer. It was like trying to hear a whisper in a noisy room.

The Noise: The main galaxy acting as the lens isn't a perfect, smooth glass. It has bumps, swirls, and irregular shapes (like a warped piece of glass).
The Confusion: When the four dots of light had weird brightness levels, scientists didn't know if it was caused by a tiny dark matter clump (the signal they wanted) or just the weird shape of the main galaxy (the noise).

The New Trick: Drawing the Arcs

This paper introduces a major upgrade. Instead of just looking at the four tiny dots of light, the team used the James Webb Space Telescope (JWST) to look at the lensed arcs.

The Analogy: Imagine you are trying to guess the shape of a hidden object by looking at the shadow it casts.

Old Method: You only looked at four tiny specks of light in the shadow. It was hard to tell if the shadow was weird because of the object's shape or because of a small pebble on the ground.
New Method: The JWST is so powerful it can see the entire curved shadow (the lensed arc) surrounding the galaxy. By mapping out the entire curve, the scientists can perfectly understand the shape of the "glass lens" (the main galaxy).

Once they know the shape of the main lens perfectly, they can subtract that "noise." Now, any remaining weirdness in the brightness of the four dots must be caused by the tiny dark matter clumps.

What They Found

By combining the four dots and the full arcs for 28 different systems, the team ran a massive simulation (creating over 174 million possible universes in their computer) to see which theory fit best.

Ruling out "Fluff": They found that the universe is not made of "fluffy cotton ball" dark matter. The data shows there are plenty of tiny clumps.
The Limit: They set a strict limit on how "warm" (fluffy) the dark matter can be. If dark matter were any warmer than a specific threshold, we wouldn't see as many tiny clumps as we do.
- They calculated that the dark matter particles must be at least 6.5 to 7.4 keV (a unit of mass). This is a very specific, heavy weight, ruling out lighter, "fluffier" versions.
Counting the Clumps: Assuming the standard "pebble" theory (Cold Dark Matter) is correct, they measured exactly how much mass is packed into these tiny clumps. Their count matches predictions from some computer models but is slightly higher than others, suggesting our models of how these clumps form might need a tiny tweak.

Why This Matters

This study is a breakthrough because it used the JWST to see the "whole picture" (the arcs) rather than just the "dots." This allowed them to separate the signal from the noise better than ever before.

They didn't just guess; they used a rigorous mathematical method to prove that the "Cold Dark Matter" theory (the pebbles) is the best fit for our universe, and they placed the tightest constraints yet on how "warm" or "fluffy" the dark matter could possibly be.

In short: By using the James Webb Space Telescope to trace the full curve of bent light, scientists confirmed that the invisible web of the universe is made of dense, clumpy "pebbles" rather than smooth, fluffy "cotton," and they measured exactly how heavy those pebbles must be.

Technical Summary: JWST Lensing Survey IV – Warm Dark Matter Constraints via Joint Reconstruction

Problem Statement
The nature of dark matter (DM) remains unknown, with the Cold Dark Matter (CDM) paradigm predicting a scale-free hierarchy of halos down to very small masses. Alternative theories, such as Warm Dark Matter (WDM), predict a suppression of structure below a specific free-streaming scale, characterized by the half-mode mass ( $m_{hm}$ ). Strong gravitational lensing of quadruply imaged quasars offers a unique probe of this substructure by measuring perturbations in image positions and flux ratios. However, previous analyses relying solely on point-source positions and flux ratios have been limited by degeneracies between the mass profile of the main deflector (macromodel) and the substructure population. Uncertainties in the macromodel (e.g., ellipticity, external shear, and angular structure) can mimic or mask the signatures of dark matter subhalos, leading to weaker constraints on DM properties. Furthermore, stellar microlensing has historically contaminated flux ratio measurements for compact sources, though this has been mitigated by recent JWST observations of extended warm dust regions.

Methodology
This paper presents a forward-modeling framework that simultaneously reconstructs extended lensed arcs, image positions, and flux ratios for a sample of 28 quadruply imaged quasars. The analysis utilizes a Bayesian inference framework to constrain hyper-parameters describing the DM substructure population.

Data Integration: The study combines JWST MIRI flux ratios (from the warm dust region of the quasar, avoiding stellar microlensing) with astrometric data and extended lensed arc imaging from JWST (NIRCam and MIRI) and HST.
Joint Modeling: Unlike previous works that treated lensed arcs and flux ratios separately, this analysis employs importance sampling weights ( $w_{img}$ ) to propagate constraints from the extended arcs onto the macromodel parameters. This breaks the covariance between the macromodel and the substructure population, tightening the constraints on model-predicted flux ratios.
Physical Modeling:
- Substructure: The framework generates full populations of subhalos, line-of-sight halos, and globular clusters using the pyHalo software. It incorporates a tidal evolution model (based on Du et al.) that predicts subhalo density profiles based on infall properties, accounting for mass loss and concentration changes.
- WDM Effects: The model modifies the CDM halo mass function and concentration-mass relation to account for free-streaming suppression in WDM, parameterized by the half-mode mass $m_{hm}$ .
- Macromodel: The main deflector is modeled using an elliptical power-law (EPL) profile augmented with elliptical multipole perturbations (m=1, 3, 4) to account for angular structure (e.g., boxiness/diskiness) that could otherwise masquerade as substructure.
Inference: The likelihood function is evaluated by generating millions of realizations (over 174 million for the full sample) of lens models. The analysis marginalizes over nuisance parameters (macromodel, source light, substructure realizations) to infer the posterior distributions of the hyper-parameters, specifically the half-mode mass $m_{hm}$ and the subhalo mass function amplitude $\Sigma_{sub}$ .

Key Contributions

Breaking Degeneracies: The primary methodological advancement is the simultaneous modeling of extended lensed arcs alongside flux ratios and image positions. This approach significantly reduces the uncertainty in the macromodel, thereby breaking the covariance between the subhalo abundance ( $\Sigma_{sub}$ ) and the free-streaming scale ( $m_{hm}$ ).
Improved Subhalo Modeling: The paper implements a more physically rigorous tidal evolution model that predicts subhalo properties at the time of infall, rather than at the lens redshift. This accounts for the dependence of tidal stripping on infall concentration, which is particularly relevant for WDM where halos are less concentrated.
Comprehensive Perturbation Sources: The analysis explicitly includes globular clusters and elliptical multipole perturbations as sources of flux-ratio noise, ensuring that constraints on WDM are not biased by unmodeled small-scale structures.
Large Sample Size: This work analyzes the largest sample of quadruply imaged quasars to date for DM inference (28 systems), leveraging the full capabilities of the JWST lensed quasar survey.

Results

Warm Dark Matter Constraints:
- Assuming a log-uniform prior on the subhalo mass function amplitude, the inclusion of lensed arcs improves the bound on the half-mode mass by 0.6 dex compared to using image positions and flux ratios alone.
- When applying informative priors on $\Sigma_{sub}$ derived from the semi-analytic model galacticus, the analysis infers $m_{hm} < 10^{7.4} M_\odot$ (Bayes factor of 10:1).
- When applying priors based on N-body simulations (Symphony suite), the constraint tightens to $m_{hm} < 10^{7.2} M_\odot$ .
- These limits correspond to lower bounds on the mass of a thermal relic WDM particle of 6.5 keV and 7.4 keV, respectively.
- The 95% exclusion limits are $m_{hm} < 10^{6.8} M_\odot$ and $m_{hm} < 10^{6.7} M_\odot$ , corresponding to particle masses $> 9.6$ keV and $> 10.2$ keV.
CDM Subhalo Abundance:
- Assuming the CDM limit ( $m_{hm} < 10^5 M_\odot$ ), the study measures the projected surface mass density of subhalos to be $1.7^{+2.6}_{-1.2} \times 10^7 M_\odot \text{kpc}^{-2}$ at 95% confidence.
- This corresponds to a projected mass fraction of $3.2^{+5.1}_{-2.3}\%$ .
- This measurement is statistically consistent with galacticus predictions but shows mild tension ( $\sim 1.8\sigma$ ) with recent N-body simulations, which predict a lower subhalo abundance.

Significance
The paper claims these results represent some of the strongest bounds on WDM to date and the most precise measurement of subhalo abundance around strong lenses. The significance lies in the demonstration that joint modeling of extended arcs and flux ratios effectively isolates the signal of dark matter substructure from uncertainties in the main deflector's mass profile. The authors note that their constraints surpass previous strong lensing bounds and are competitive with or stronger than other small-scale probes like the Lyman- $\alpha$ forest and dwarf galaxy counts. The work establishes a robust framework for future DM inferences using the large samples of lenses expected from upcoming surveys (Euclid, Rubin, Roman), highlighting the transformative potential of combining high-resolution space-based imaging with advanced forward modeling.