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

Abstract

Spatially resolved stellar kinematics has become a key ingredient in time-delay cosmography to break the mass-sheet degeneracy in the mass profile and in turn provide a precise constraint on the Hubble constant and other cosmological parameters. In this paper, we present the first measurements of 2D resolved stellar kinematics for the lens galaxy in the quadruply lensed quasar system RXJ1131$-$1231 using integral field spectroscopy from JWST's Near-Infrared Spectrograph (NIRSpec), marking the first such measurement conducted with JWST. In extracting robust kinematic measurements from this first-of-its-kind dataset, we have made methodological improvements both in the data reduction and kinematic extraction. In our kinematic extraction procedure, we performed joint modeling of the lens galaxy, the quasar, and its host galaxy's contributions in the spectra to deblend the lens galaxy component and robustly constrain its stellar kinematics. Our improved methodological frameworks are released as software pipelines for future use: squirrel, for extracting stellar kinematics, and RegalJumper, for JWST-NIRSpec data reduction. We incorporated additional artifact cleaning beyond the standard JWST pipeline. We compared our measured stellar kinematics from the JWST NIRSpec with previously obtained ground-based measurements from the Keck Cosmic Web Imager integral field unit and find that the two datasets are statistically consistent at a $\sim$1.1$σ$ confidence level. Our measured kinematics will be used in a future study to improve the precision of the Hubble constant measurement.

Figures (16)

• Figure 1: Imaging of the system RXJ1131$-$1231. Left panel: HST-ACS image in the F814W band. The four quasar images are labeled A, B, C, and D. The central deflector is marked with G, of which we are measuring the spatially resolved velocity dispersion. An arrow points to the nearby satellite S. The North and East directions, along with a 1 scale, are also illustrated. The $3\farcs2\times3\farcs1$ yellow squares represent the dithered $2\times2$ mosaic pattern of the NIRSpec IFU, which covers a $6\farcs1\times5\farcs55$ field of view over the system. Right panel: "White-light" image of the system from the JWST-NIRSpec datacube, summed within 8255--8890 Å in the lens galaxy's rest frame. The white bar represents 1, and the red and yellow arrows point to the east and north, respectively. The blue contours show the region within which the spaxels are summed to obtain a high-$S/N$ spectrum of the host galaxy. The black regions around the quasar images were used to extract summed spectra for modeling the quasar's emission line components. HST and JWST logo credits: NASA.
• Figure 2: Lens modeling with the image obtained by summing the datacube within 8700--8800 Å in the lens galaxy's rest frame. This wavelength range covers the Ca triplets of the lens galaxy. Top row, from left to right: Illustration of the data, the optimized-model-based reconstruction of the data, and the normalized residuals. Bottom row, from left to right: Illustration of the initial PSF model from stpsf, the iteratively reconstructed PSF, and the fraction of the lens light compared to the total light in the image. The reconstructed PSF will be necessary for dynamical modeling, and we use the lens light fraction to obtain the $S/N$ of the lens galaxy for Voronoi binning.
• Figure 3: Voronoi binning map. Left panel: Voronoi bins outlined over the white-light image of the system. Middle panel: Bin numbering. Right panel: Total s$S/N$ for each Voronoi bin. The horizontal dashed line marks 90 $\AA^{-1/2}$, which we set as the minimum target for each bin. Only the last Voronoi bin, which contains the satellite galaxy, is slightly below this s$S/N$ target.
• Figure 4: Fitting of the Ca ii region of the quasar host galaxy. The gray bars represent the data points, with the width of each bar corresponding to the pixel size and the height representing the original $\pm 1\sigma$ noise level. The gray lines attached to the bars represent the boosted uncertainty levels to achieve $\chi^2_{\rm red} = 1$. The red line illustrates the best-fit model. The Ca ii triplet features are marked with blue lines.
• Figure 5: Fitting of the quasar lines at images A, B, C, and D, respectively, from top to bottom. The observed spectra are shown with gray bars, with the width corresponding to the bin width and the height representing the $\pm1\sigma$ noise range. The red lines show the best fit. The individual narrow and emission lines from the best fit are shown in purple, and the lens galaxy's stellar continuum is shown in emerald. We adopted the best-fit gas lines with fixed relative amplitudes for all three images as free linear components in our kinematic fitting of the spectra across the entire field of view.