Hedorah, the first yellow supergiant Kaiju star candidate at $z\approx3$ revealed by behind AS1063

TL;DR

The paper presents a new free-form lens model for the AS1063 cluster using JWST GLIMPSE and spectroscopic lens constraints to identify counterimages, predict geometric redshifts, time delays, and magnifications, and to discover new lensed systems. It highlights three notable discoveries: (i) a singly imaged, highly magnified galaxy at $z \approx 1.85$ with extreme magnification; (ii) a caustic-crossing arc at $z=0.73$ showing strong microlensing prospects; and (iii) a triply imaged high‑redshift galaxy at $z \approx 7.5$ with strong H$\beta$–OIII emission. The most intriguing object, Hedorah, is a candidate lensed yellow supergiant at $z \approx 3.1$, whose nature is explored through microlensing and dark matter scenarios, with a favored interpretation as a yellow monster star undergoing magnification. The work demonstrates JWST’s power for detailed lensing studies and provides public lens-model products to support cosmology and microlensing investigations.

Abstract

We present a new free-form lens model for the $z=0.348$ galaxy cluster AS1063, based on previously spectroscopically confirmed lensed galaxies and new images from the GLIMPSE program. We use the ultra-deep data to identify new counterimages for previously confirmed (spectroscopically) lensed systems. We use the full set of spectroscopically confirmed systems to derive a new lens model, which is later used to confirm many of the previous lensed system candidates and discover new lensed system candidates in the images. We compute the geometric redshifts, time delays, and magnification for all counterimages (confirmed and not confirmed). Among the new systems we find a peculiar multiply lensed galaxy with a strong emission line at $\approx 4\, μ$m that likely corresponds to H$-β$ and/or OIII at $z\approx 7.5$. This galaxy could be a little-red-dot or an extreme emission line galaxy. We also identify a yellow supergiant lensed star candidate at $z\approx 3.1$. This star shows some similarities with previous Kaiju stars and we nickname it "Hedorah", in honor of the famous yellow-eyed Kaiju. Previous lensed stars at $z>0.1$ are either blue supergiants or red supergiants, making Hedorah the first yellow supergiant discovered beyond $z=0.1$ and confirming that, despite their rarity, they can also be found at these redshifts. Since many Cepheid stars are yellow supergiants, we consider the possibility that Hedorah could also be the first Cepheid discovered at cosmological distances, but we conclude that Hedorah is more likely a hypergiant yellow star approaching the end of its life. Alternatively, Hedorah could be a small group of stars, although this is less likely based on Hedorah's peculiar colors and additionally may require the more exotic fuzzy dark matter to help explain the lack of counterimage.

Figures (10)

• Figure 1: Central region of AS1063 ($2'.25\times2'.25$) combining 12 HST and JWST filters (range 0.4--5 micron). Yellow dots show the spectroscopically confirmed lensing constraints, or rank A constraints (see Table \ref{['tab_arcs']}), used to derive the lens model. Cyan dots show the rank B constraints (Table \ref{['tab_arcs']}) not used in the lens model but highly consistent with it and for which we also derive geometric redshifts, time delays and magnification. The critical curve correponds to $z_s=3$.
• Figure 2: Mass profile in the central region of AS1063 and centered on the BCG. The projected mass from the lens model is shown in black (in units of $\kappa=\Sigma/\Sigma_{\rm crit}$ at $z_s=3$) with the two blue vertical dotted lines marking the range of distances with lensing constraints. The red line shows a NFW profile with parameters shown in the top right corner.
• Figure 3: The giant arc in the north west. Lack of counterimages for this arc and large magnification can be explained only if the arc lies in a narrow redshift range around $z=1.85$. The critical curve for this redshift is shown as a yellow curve, and in white for a slightly larger redshift of $z=1.9$, or red for $z=3$, where counterimages start to form. The predicted magnification at different points along the giant arc is marked with yellow arrows and numbers. The source plane reconstruction is shown in the bottom-right corner. The entire arc delenses into a merging galaxy system less than 1" in length. South of the giant arc the lens model predicts another unconfirmed system to be at $z\approx3$ (white ellipses) with two symmetric images on each side of the critical curve at $z=3$ (red curve).
• Figure 4: Color image of the caustic crossing arc at $z=0.73$ (system 19). The top panel shows the color image after combining F090W, F150W and F200W. The bottom image is a color version using a high-pass filter version image of the same three bands. We show the cluster CC at $z=0.73$ as a white curve and mark with circles two regions near the CC where distinctively redder sources are found. The yellow ellipse shows a region where the macromodel magnification alone exceeds 700. The parity of the images is indicated in the bottom panel with plus and minus symbols.
• Figure 5: Multiply lensed ELG (new system 78) found after searching for high-z lensed galaxies near the $z=10$ CC. The galaxy is not detected in F090W and shows prominent emission in F410M. The three counterimages are consistent in color an predicted position if $z\approx 7.5$. The predicted magnification at this redshift is also consistent with the flux ratio specially if one accounts for the possibly biased low magnification of image c. At this redshift, the emission observed in F410M would correspond to H$\beta$--OIII.