Preliminary Evidence for Lensing-Induced Alignments of High-Redshift Galaxies in JWST-CEERS

TL;DR

This study investigates whether gravitational lensing explains the elongated shapes of high-redshift galaxies in JWST-CEERS data, finding weak lensing signals that are insufficient to account for the observed elongations, suggesting other factors or structures may be involved.

Contribution

It provides the first evidence of lensing-induced alignments of high-redshift galaxies using JWST data and discusses the potential influence of large-scale structures like protoclusters.

Findings

Weak lensing shear detected at $ ightarrow10 ext{%}$ level.

Null hypothesis of random orientations rejected at $ ightarrow99 ext{%}$ significance.

Maximum cosmic shear contribution estimated at only a few percent.

Abstract

The majority of low-mass ($\log_{10} M_*/M_{\odot}=9-10$) galaxies at high redshift ($z>1$) appear elongated in projection. We use JWST-CEERS observations to explore the role of gravitational lensing in this puzzle. The typical galaxy-galaxy lensing shear $\gamma\sim1\%$ is too low to explain the predominance of elongated early galaxies with ellipticity $e\approx0.6$. However, non-parametric quantile regression with Bayesian Additive Regression Trees reveals hints of an excess of tangentially-aligned source-lens pairs with $\gamma>10\%$. On larger scales, we also find evidence for weak lensing shear. We rule out the null hypothesis of randomly oriented galaxies at $\gtrsim99\%$ significance in multiple NIRCam chips, modules and pointings. The number of such regions is small and attributable to chance, but coherent alignment patterns suggest otherwise. On the chip scale, the average complex ellipticity $\langle e\rangle\sim10\%$ is non-negligible and beyond the level of our PSF uncertainties. The shear variance $\langle\overline{\gamma}^2\rangle\sim10^{-3}$ is an order of magnitude above the conventional weak lensing regime but is more sensitive to PSF systematics, intrinsic alignments, cosmic variance and other biases. Taking it as an upper limit, the maximum implied ``cosmic shear'' is only a few percent and cannot explain the elongated shapes of early galaxies. The alignments themselves may arise from lensing by a protocluster or filament at $z\sim0.75$ where we find an overabundance of massive lens galaxies. We recommend a weak lensing search for overdensities in ``blank'' deep fields with JWST and the Roman Space Telescope.