The Carousel Lens II: Cosmological Constraints with GIGA-Lens

Abstract

The nature of dark matter and dark energy are among the central questions in cosmology. Strong gravitational lenses with multiple source planes provide a geometric probe of cosmology: the ratio of deflection angles at different redshifts depends only on angular-diameter distances, constraining the matter density $Ω_m$ and the dark energy equation of state $w$. However, constraints from this technique have historically lagged behind those from the CMB, SNe Ia, and BAO. In this work, we present new cosmological constraints from the Carousel Lens, a cluster-scale lens with more than 40 extended images from 11 spectroscopically confirmed sources. Its relaxed core and rich set of extended images behind the main halo make it particularly suitable for cosmological inference. Using the GIGA-Lens pipeline, we construct a pixel-level lens model including six HST-detected sources and four mass components. From this model, we obtain $w$CDM constraints of $Ω_m = 0.34^{+0.16}_{-0.13}$ and $w = -1.31^{+0.35}_{-0.32}$ from the Carousel Lens alone, accounting for both statistical and systematic uncertainties. We further project that including four additional known higher-redshift sources, assuming similar fractional uncertainties, could improve the constraining power by ~80%, bringing the precision close to that of the CMB and SNe Ia. For an evolving dark energy model ($w_0w_a$CDM), the Carousel Lens alone yields constraints comparable to the CMB, providing an independent and complementary probe alongside SN Ia and BAO. While currently systematic uncertainties dominate, which we quantify through simulations, our results demonstrate that relaxed multi-source-plane cluster lenses can deliver competitive cosmological constraints. Further improvements are expected from reductions in systematics and from incorporating higher-redshift sources (known and new) with high-resolution imaging.

Figures (20)

• Figure 1: All currently known lensed sources in the Carousel Lens system. The images of the same source galaxy are labeled with the same number and color, with the letters indicating the image multiplicity of each source family. 7d$^*$ is the suspected radial image, but it has not been spectroscopically confirmed. Sources with Ly$\alpha$ emission detected in the MUSE data are shown with contours. The most prominent cluster members are labeled in red. This figure is reproduced from Paper I.
• Figure 2: Strong lensing surface-brightness model. Top left: $800 \times 800$ pixel HST/F140W cutout of the cluster core used as a constraint in the lens modeling. Top right: Best-fit lensed surface-brightness model. Bottom left: Noise-normalized residuals between the model and the data, with cluster member galaxies and non–strongly lensed objects masked. Bottom right: Reconstructed unlensed source-plane surface-brightness, overlaid with the lens caustics for each source redshift; individual sources are labeled and color-coded consistently with the caustics.
• Figure 3: Critical curves and predicted source positions. The image is an RGB composite of the cluster core, constructed from HST observations, where the F140W filter is mapped to the red channel, the F200LP filter to the blue channel, and the average of the two filters to the green channel. Colored curves indicate the critical lines of the best-fit lens model for the different source planes. Colored circles mark the model-predicted positions of multiple images for each source. Cluster member galaxies included in the lens model are labeled in red. The inset shows a zoomed-in view of the region around images 4c and 6c, highlighting a foreground galaxy at $z = 0.09$ whose light was not modeled in this work (see Section \ref{['sec:discussion_lens_model']}).
• Figure 4: Convergence ($\kappa$) contours of the best-fit lens mass model overlaid on the RGB composite image of the cluster core.
• Figure 5: $w$CDM constraints from the Carousel Lens under different uncertainty treatments. Panels: (a) 95% probability contours from source 1 (pink), source 3 (green), source 6 (red), source 7 (sky blue), and their joint constraints (black), considering only statistical uncertainties; (b) Comparison of joint constraints with statistical and statistical plus systematic uncertainties, contours are 68, 95, and 99% probability levels; the dashed line marks the concordance model ($\Omega_m=0.3$, $w=-1$).