Pixel-level modelling of group-scale strong lens CASSOWARY 19

TL;DR

The paper presents a high-precision, pixel-level strong-lensing model for the group-scale CSWA19 using HST data. By coupling a parametric light/mass framework with a brightness-adaptive pixelization for source reconstruction in PyAutoLens, it constrains the mass distribution to within the Einstein radius and reveals a total magnification of μ ≈ 103, while resolving merging background components. The results yield M_{θ_E} ≈ 1.41×10^{13} M_⊙, θ_E ≈ 8.07″, and a total density slope ̃γ(R_{eff}) ≈ 1.33, situating CSWA19 along the scale-dependent trend toward cluster-like profiles. The brightness-adaptive pixelization proves to be a scalable paradigm for group-/cluster-scale lensing, enabling efficient, accurate reconstructions and informing future studies with next-generation surveys and potential IFU or GPU-accelerated improvements.

Abstract

We present the first high-precision model for the group-scale strong lensing system CASSOWARY 19 (CSWA19), utilising images from the Hubble Space Telescope (HST). Sixteen member galaxies identified via the red-sequence method, and the main halo, all modelled as the dual Pseudo Isothermal Elliptical profile (dPIE), are incorporated into a parametric lens model alongside an external shear field. To model the system, we adopt the PyAutoLens software package, employing a progressive search chain strategy for realizing the transition of source model from multiple Sérsic profiles to a brightness-adaptive pixelization, which uses 1000 pixels in the source plane to reconstruct the background source corresponding to 177,144 image pixels in the image plane. Our results indicate that the total mass within the Einstein radius is $M_{θ_\mathrm{E}}$ $\approx 1.41\times10^{13}$M$_{\odot}$ and the average slope of the total mass density $ρ(r)\propto r^{-γ}$ is $\tildeγ=1.33$ within the effective radius. This slope is shallower than those measured in galaxies and groups but is closer to those of galaxy clusters. In addition, our approach successfully resolves the two merging galaxies in the background source and yields a total magnification of $μ=103.18^{+0.23}_{-0.19}$, which is significantly higher than the outcomes from previous studies of CSWA19. In summary, our research demonstrates the effectiveness of the brightness-adaptive pixelization source reconstruction technique for modelling group-scale strong lensing systems. It can serve as a technical reference for future investigations into pixel-level modelling of the group- and cluster-scale strong lensing systems.

Figures (9)

• Figure 1: Colour-composite image of the field of CASSOWARY 19 and its surrounding environment($1 \times 1 \text{ arcmin}^2$). The image is a combination of the F475W (blue), F606W+F814W (green), and F110W+F160W (red) images. We only consider member galaxies within a radius of $\sim 14{"}$ from the centre (marked with a white dashed circle). The two right panels show the two images formed by the background source. The lens galaxies are marked with red diamonds, and non-member galaxies within the circle are marked with blue diamonds. The yellow circles mark the peak positions of the lensed images of the source galaxy 1. Accordingly, the green circles mark the peak positions of the lensed images of the source galaxy 2, and the cyan circles mark the second peak positions of the lensed images of the source galaxy 2.
• Figure 2: Red-sequence fitting of the galaxy group. The red solid line is the final converged red sequence, and the shaded area is the 3$\sigma$ range. The 21 candidate galaxies are divided into two parts: member galaxies, i.e., BGG and L1$\sim$L15 (marked with red squares), and non-member galaxies, i.e.,G1$\sim$G5 (marked with blue squares). The yellow pentagrams denote the five member galaxies with spectroscopic redshifts used for the initial guess.
• Figure 3: Multi-Sérsic light profile fittings to the F475W and F160W images for the lens galaxies. The upper panels show the F475W band fitting results, where the panels from left to right show the observed image, the Sérsic model image, and the normalized residual image (range $\pm$5$\sigma$). The lower panels show the F160W band fitting results, and the panels from left to right are the the observed image, Sérsic model image, and the normalized residual image (range $\pm$10$\sigma$). Only the BGG, L1$\sim$L10, and G1 are modelled with the F475W band image, because the purpose of the F475W band modelling is to subtract the light contamination of the foreground galaxies, and the light contamination of L11$\sim$L15 and G2$\sim$G5 is small or outside the lens modelling image area.
• Figure 4: Schematic diagram of the construction of the brightness-adaptive grid. The left image shows the irregular grid point distribution on the image plane, constructed by the Hilbert space-filling curve, these 1000 points are distributed more densely in brighter areas and sparsely in unstructured areas, this grid point distribution comes from the best fitting values of Model 5-0, and we manually mask the area of G2; the right image shows the Delaunay triangle mesh on the source plane, the vertices of these triangles are mapped from the points on the image plane to the source plane through the mass model, the grid points shown here correspond to the mass distribution of the best fitting results of Model 5-1.
• Figure 5: Comparison of lens modelling results. The panels in the left part (\ref{['subfig:3Sérsic']}) show the 3-Sérsic model results, and the panels in the right part (\ref{['subfig:pixelized']}) present the pixelized model results using a Delaunay mesh visualization for the source. Both parts share common features: 95 radius circular mask (black circle) , tangential critical curves and caustics (white lines) and radial critical curves and caustics (yellow lines) are overlaid, and red markers indicate mass component positions (including external L11$\sim$L15 member galaxies not visible in frame).