VENUS: Strong-lensing model of MACS J1931.8-2635 -- revealing the farthest multiply imaged supernova

Abstract

We present a parametric strong-lensing model for the galaxy cluster MACS J1931.8-2635 ($z_l = 0.35$), accompanying the detection of the spectroscopically confirmed SN Eos at $z = 5.13$ (Coulter et al. 2026). We identify 10 new multiple-image systems in recent VENUS JWST/NIRCam imaging, so that the model is constrained with a total of 19 robust multiple-image systems -- nine of which also have a spectroscopic redshift. For the point-like source corresponding to SN Eos, our model predicts a total of five images, with the observed radial image pair having a similar magnification of $μ\simeq 25 - 30$ and a small time delay of $< 5$ days, in agreement with their simultaneous observation. According to the model, the other three predicted images arrived earlier, with time delays of $3.7 \pm 0.7$, $3.5 \pm 0.7$ and $54.0 \pm 10.8$ years prior to the two observed images, and with magnifications of $12.9 \pm 2.6$, $13.0 \pm 2.9$ and $2.2 \pm 0.4$, respectively. The absence of detections at the predicted positions, where the host galaxy's images are also visible, confirms the transient nature of the source. SN Eos and its host galaxy are studied in separate articles, and we here focus on the lens model. The final model reaches a very good $r.m.s.$ distance between model and observations of $0.44''$. We present the lens-modeling results, including newly identified systems such as a triply imaged, grand-design spiral galaxy candidate at $z \simeq 3.65_{-0.09}^{+0.04}$, and briefly discuss the potential of using high-redshift lensed SNe for cosmography.

Figures (6)

• Figure 1: Composite color image of MACS J1931.8-2635, using the summed VENUS-collected JWST NIRCam bands F115W and F150W for the blue channel, F200W and F277W for the green, and F356W and F444W for the red channel. All 20 strongly-lensed systems are represented with a unique shape-color couple. This includes candidate system 18, although it is not used as a constraint. We show the Eos galaxy host and the SN (respectively, systems 100 and 101) in a zoomed-in image on the top-left corner. The blue lines represent the $|\mu| > 100$ critical lines for SN Eos, i.e. a source redshift of $z_s = 5.13$. In the lens plane, $50"$ represent $248.1$ physical kpc.
• Figure 2: Left: Convergence $\kappa$ map; Right: Magnification $|\mu|$ map, both for the best-model at redshift $z_S = 5.13$, corresponding to the Eos system.
• Figure 3: Image reconstruction of the quadruply-lensed System 1 (left) and triply-lensed System 2 (right), at respective redshifts $z_{\rm 1, spec} = 1.178$ and $z_{\rm 2, spec} = 1.835$. The first row presents the extracted images from Fig. \ref{['fig:Colour_image_m1931']}; the second row the image reconstruction. All insets are $4"\times4"$.
• Figure 4: Reconstruction of System 14 Charybdis, the triply imaged spiral galaxy at redshift $z \simeq 3.5$, from our lens model. The first row presents the extracted observations from Fig. \ref{['fig:Colour_image_m1931']}; the second row the image reconstruction from the averaged source, for a redshift $z_{\rm 14, model} = 3.65_{-0.09}^{+0.04}$; the third row presents the source plane reconstruction for each individual image. The three images 14.1, 14.2 and 14.3 (first row, of respective magnifications $5.5 \pm 1.1$, $2.9 \pm 0.6$ and $2.4 \pm 0.5$) are projected in the source plane (third row) and averaged (see Fig. \ref{['fig:source_reconstruction_Sys14']}), and then reprojected in the lens plane (second row). The reproductions demonstrate the quality of the lens model.
• Figure 5: Averaged source reconstruction of the spiral galaxy Charybdis at redshift $z_{\rm 14, model} = 3.65_{-0.09}^{+0.04}$, i.e. the triply-lensed System 14. This image was obtained by centering and averaging the source reconstruction from each individual image (and is thus effectively smoothed; cf. third row in Fig. \ref{['fig:sys14']}). The source WCS coordinate position is at $(292.9579, 26.5768)$ J2000 degrees. A physical scale bar is overplotted, where at the presumed redshift of the source of $z=3.65$, $1" \simeq 7.2$ kpc.