A Lens Finder Map to check claimed High-z Galaxies behind SMACS J0723.3-7327

TL;DR

This paper develops a lens model for the galaxy cluster SMACS J0723.3-7327 using spectroscopic and photometric data, creating a map to identify high-redshift galaxy candidates and challenging some claims of galaxies at redshifts up to 20.

Contribution

It introduces a new lens finder map for galaxy clusters that helps verify high-redshift galaxy candidates without relying solely on spectroscopy.

Findings

The lens model accurately predicts positions and properties of multiple images.

The lens finder map suggests some claimed high-z galaxies are likely at lower redshifts.

No evidence found for galaxies at redshifts 10-20 in the examined regions.

Abstract

The first science image released by the JWST reveals numerous galaxies in the distant background of the galaxy cluster SMACS J0723.3-7327. Some have claimed redshifts of up to $z \simeq 20$, challenging standard cosmological models for structure formation. Here, we present a lens model for SMACS J0723.3-7327 anchored on five spectroscopically-confirmed systems at $1.38 \leq z \leq 2.21$ that are multiply lensed, along with twelve other systems with proposed image counterparts sharing common colours, spectral energy distributions, and morphological features, but having unknown redshifts. Constrained only by their image positions and, where available, redshifts, our lens model correctly reproduces the positions and correctly predicts the morphologies and relative brightnesses of all these image counterparts, as well as providing geometrically-determined redshifts spanning $1.4 \lesssim z \lesssim 6.7$ for the twelve candidate multiply-lensed galaxies lacking spectroscopic measurements. From this lens model, we create a lens finder map that defines regions over which galaxies beyond a certain redshift are predicted to be multiply lensed. Applying this map to three galaxies claimed to be at $10 \lesssim z \lesssim 20$, we find no image counterparts at locations (with an uncertainty of $\sim$$0.^{\prime\prime}5$) where they ought to be sufficiently magnified to be detectable - suggesting instead that these galaxies lie at $z \lesssim 1.7-3.2$. In lieu of spectroscopy, the creation of reliable lens finder maps for cluster fields are urgently needed to test and constrain redshifts inferred from photometry for a rapidly increasing number of candidate high-$z$ galaxies found with the JWST.