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TDCOSMO XXIV. First spatially resolved kinematics of the lens galaxy obtained using JWST-NIRSpec to improve time-delay cosmography

This paper presents the first spatially resolved 2D stellar kinematics of the lens galaxy in RXJ1131-1231 using JWST-NIRSpec integral field spectroscopy, introducing improved data reduction and kinematic extraction methods to break mass-sheet degeneracy and refine Hubble constant measurements.

Original authors: Anowar J. Shajib, Tommaso Treu, Sherry H. Suyu, David Law, Akın Yıldırım, Michele Cappellari, Aymeric Galan, Shawn Knabel, Han Wang, Simon Birrer, Frédéric Courbin, Christopher D. Fassnacht, Joshua A.

Published 2026-03-18

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CC BY 4.0

Original authors: Anowar J. Shajib, Tommaso Treu, Sherry H. Suyu, David Law, Akın Yıldırım, Michele Cappellari, Aymeric Galan, Shawn Knabel, Han Wang, Simon Birrer, Frédéric Courbin, Christopher D. Fassnacht, Joshua A. Frieman, Alejandra Melo, Takahiro Morishita, Pritom Mozumdar, Dominique Sluse, Massimo 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: Solving the Universe's "Speed Limit" Mystery

Imagine the universe is a giant race car speeding away from us. Astronomers want to know exactly how fast it is going (a number called the Hubble Constant). But here's the problem: when they look at the "baby pictures" of the universe (the early universe), they get one speed. When they look at the "adult pictures" (the nearby universe), they get a slightly different, faster speed.

This disagreement is called the "Hubble Tension." It's like two people measuring the same road trip and getting different arrival times. If the difference is real, it means our current understanding of physics is missing a piece of the puzzle.

To fix this, astronomers use a special trick called Time-Delay Cosmography. They look at a cosmic "funhouse mirror" (a gravitational lens) where a distant quasar (a super-bright black hole) is bent into four images by a galaxy in front of it. Because the light takes different paths, the images flicker at different times. By measuring these time delays, they can calculate the distance to the galaxy and, in turn, the speed of the universe.

The Problem: To get an accurate distance, they need to know the exact shape and weight of the "mirror" (the lens galaxy). But there's a trick in the math called the Mass-Sheet Degeneracy. It's like trying to guess the weight of a suitcase without knowing if it's filled with feathers or lead; you can't tell the difference just by looking at the size.

The Solution: You need to know how the stars inside the lens galaxy are moving. If you can map their speeds, you can figure out the galaxy's true weight and shape, breaking the "feathers vs. lead" confusion.

The New Tool: JWST's Super-Eye

For years, astronomers tried to map these star speeds using ground-based telescopes (like Keck in Hawaii). But Earth's atmosphere is like a wavy, dirty window. It blurs the image, making it hard to see the fine details of how stars move in different parts of the galaxy.

This paper introduces a game-changer: The James Webb Space Telescope (JWST).

Think of JWST as a high-definition camera floating in the vacuum of space, far above the dirty window. It has a new instrument called NIRSpec that acts like a super-powerful prism, splitting light into a rainbow to analyze the chemistry and speed of stars.

What they did:
They pointed JWST at a specific lens system called RXJ1131−1231. They didn't just take a picture; they took a "movie" of the light coming from every tiny pixel of the galaxy. This allowed them to create a 2D map of star speeds with a clarity six times better than previous ground-based attempts.

The Challenge: The "Cosmic Cocktail"

Getting this data wasn't easy. Imagine trying to listen to a single violinist (the lens galaxy) playing in a concert hall while a rock band (the bright quasar) and a choir (the host galaxy) are playing loudly right next to them.

The light from the lens galaxy, the quasar, and the host galaxy all got mixed together in the telescope's detector. If you just looked at the noise, you'd hear a mess.

How they solved it:
The team built a new software pipeline (a digital recipe) called RegalJumper to clean up the data. Then, they used another tool called Squirrel to act like a master audio engineer. They mathematically separated the "violin" from the "rock band" and the "choir."

They modeled all three components simultaneously:

The Lens Galaxy: The star we want to weigh.
The Quasar: The bright background light.
The Host Galaxy: The galaxy the quasar lives in.

By separating these signals, they could measure the lens galaxy's star speeds with incredible precision, free from the "noise" of the other objects.

The Results: A Clearer Picture

The Map: They produced the first-ever detailed map of how stars move in this specific lens galaxy. They found the galaxy is a "slow rotator," meaning the stars aren't spinning wildly; they are mostly jiggling in place.
The Comparison: They compared their new, crystal-clear JWST map with the old, blurry Keck map. The results matched up perfectly (statistically speaking), which proves their new method works.
The Breakthrough: Because the JWST map is so sharp, it breaks the "Mass-Sheet Degeneracy" much better than before. It's like going from guessing the weight of the suitcase to actually putting it on a scale.

Why This Matters

This paper is a "methodology" paper. It's not the final answer to the Hubble Tension yet, but it provides the perfect tool to find the answer.

The Analogy: If measuring the universe's speed is like trying to hit a bullseye on a dartboard, previous methods were throwing darts with a blindfold and a shaky hand. This paper gives us a laser sight and a steady hand.
The Future: The authors will use these new, precise star-speed maps in a follow-up paper to calculate the Hubble Constant with much higher precision. If the "Hubble Tension" is real, this new data will help us discover the "New Physics" needed to explain it.

In short: Astronomers used the sharpest eye in the solar system (JWST) to untangle a messy cosmic signal, creating a high-definition map of a galaxy's heartbeat. This map is the key to finally solving the mystery of how fast our universe is expanding.

1. Problem Statement

The primary motivation for this study is the "Hubble Tension," the statistically significant discrepancy between the Hubble constant ( $H_0$ ) measured by late-Universe probes (like the local distance ladder) and early-Universe probes (like the Cosmic Microwave Background).

Time-Delay Cosmography (TDCOSMO): This method measures $H_0$ using time delays between multiple images of a lensed quasar. However, its precision is limited by the Mass-Sheet Degeneracy (MSD) and the Mass-Anisotropy Degeneracy (MAD).
The Limitation: Breaking these degeneracies requires precise measurements of the lens galaxy's spatially resolved stellar kinematics (velocity dispersion and mean velocity maps). Previous ground-based observations (e.g., using Keck/KCWI) were limited by atmospheric seeing, resulting in insufficient spatial resolution to fully constrain the anisotropy parameter, leaving significant systematic uncertainties in $H_0$ measurements.
The Goal: To obtain the first high-resolution, space-based 2D stellar kinematic map of a time-delay lens galaxy (RXJ1131−1231) using JWST-NIRSpec to significantly improve the precision of cosmological parameter inference.

2. Methodology

The authors developed a comprehensive pipeline for data reduction, modeling, and kinematic extraction, introducing several methodological innovations.

A. Data Acquisition and Reduction

Observation: JWST-NIRSpec Integral Field Unit (IFU) observations of RXJ1131−1231 (Cycle 1, Program 1794) using the G140M grating and F100LP filter (0.97–1.84 µm).
Custom Pipeline (RegalJumper): Built on the standard JWST pipeline, this custom tool added specific steps to clean non-random noise:
- Removal of 1/f noise using NSClean.
- Enhanced Cosmic Ray (CR) cleaning using lacosmic.
- Correction for MSA (Multi-Shutter Array) leakage.
- Removal of "wiggles" (resampling noise artifacts) using the raccoon package.
Lens Modeling: A white-light image (summed 8700–8800 Å) was modeled using lenstronomy to reconstruct the Point Spread Function (PSF) and deblend the lens galaxy's light from the quasar and its host galaxy. This provided the necessary light fraction maps for Voronoi binning.

B. Kinematic Extraction (The `squirrel` Pipeline)

The authors developed the squirrel pipeline (based on pPXF) to extract kinematics from the complex, multi-component spectra.

Voronoi Binning: Spectra were binned to achieve a target specific Signal-to-Noise ratio (sS/N) of 90 Å $^{-1/2}$ .
Multi-Component Spectral Fitting: The spectra contain contributions from three distinct sources at different redshifts:
1. Lens Galaxy: Stellar continuum and absorption features (Ca II triplet).
2. Quasar: Emission lines (H $\alpha$ , [N II], [S II]) with broad and narrow components.
3. Quasar Host Galaxy: Stellar continuum and narrow emission lines.
Template Libraries: Two libraries were used to mitigate systematic errors: Indo-US and X-shooter Spectral Library (XSL).
Systematic Uncertainty Marginalization: To account for choices in template libraries, polynomial orders for continuum fitting, spike detection thresholds, and wavelength ranges, the authors ran 48 different model setups. These were combined using Bayesian Information Criterion (BIC) weighting and equal weighting across wavelength ranges to produce a final covariance matrix.
Spurious Feature Handling: An iterative process identified and modeled "spurious spikes" (instrumental artifacts or faint background sources) as emission lines to prevent them from biasing the kinematic fit.

3. Key Contributions

First JWST Kinematics: This is the first measurement of 2D resolved stellar kinematics for a time-delay lens galaxy using JWST.
Resolution Improvement: The JWST data offers a sixfold improvement in angular resolution compared to previous Keck/KCWI ground-based data, allowing for much tighter constraints on the inner regions of the lens galaxy.
Methodological Frameworks:
- RegalJumper: A public pipeline for robust JWST-NIRSpec data reduction, specifically addressing artifacts like "wiggles" and MSA leakage.
- Squirrel: A public pipeline for joint modeling of lens, quasar, and host galaxy spectra to extract robust kinematics.
Joint Modeling: The first application of simultaneous spectral modeling for all three components (lens, quasar, host) to deblend their contributions in a single IFU dataset.

4. Key Results

Kinematic Maps: The authors produced 2D maps of velocity dispersion ( $\sigma_{los}$ $σ_{l os}$ ) and mean line-of-sight velocity ( $v_{los}$ $v_{l os}$ ) across 32 Voronoi bins.
- The lens galaxy is confirmed as a slow rotator with mean velocities $\lesssim 50$ km s $^{-1}$ .
- The satellite galaxy (Bin 32) shows a distinct velocity offset ( $-59 \pm 23$ km s $^{-1}$ relative to the system).
Consistency with Ground Data: The JWST measurements are statistically consistent with previous Keck/KCWI measurements at a $\sim 1.1\sigma$ confidence level.
Breaking Degeneracies:
- Mass-Anisotropy Degeneracy (MAD): The high-resolution JWST data significantly constrains the stellar anisotropy parameter ( $\sigma_t/\sigma_r$ ), a parameter that was largely unconstrained by the seeing-limited KCWI data.
- Likelihood Ratio Test: A joint dynamical modeling of JWST and KCWI data yielded a p-value of 0.22, confirming the datasets are consistent.
Precision: While the paper focuses on the kinematic extraction, the authors note that the improved constraints on anisotropy and the breaking of the MSD will lead to a more precise $H_0$ measurement (targeting $<2\%$ precision) in a companion paper (Wang et al., in preparation).

5. Significance

This work represents a critical milestone in observational cosmology:

Solving the Hubble Tension: By providing the highest-quality spatially resolved kinematics to date, this study removes a major source of systematic uncertainty in time-delay cosmography. This allows for a more robust test of the Hubble tension, potentially confirming it as a sign of new physics beyond the standard $\Lambda$ CDM model.
Future-Proofing: The release of the RegalJumper and squirrel pipelines establishes a standard for analyzing future JWST time-delay lens datasets, enabling the TDCOSMO collaboration to achieve sub-2% precision on $H_0$ in the near future.
Technical Demonstration: It demonstrates the capability of JWST to perform complex, multi-component spectral analysis in crowded fields, overcoming challenges like PSF spikes and resampling noise that were previously prohibitive.