Preliminary Evidence for Lensing-Induced Alignments of High-Redshift Galaxies in JWST-CEERS
arxiv(2024)
Abstract
The majority of low-mass (log_10 M_*/M_⊙=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 γ∼1% is too low to explain the
predominance of elongated early galaxies with ellipticity e≈0.6.
However, non-parametric quantile regression with Bayesian Additive Regression
Trees reveals hints of an excess of tangentially-aligned source-lens pairs with
γ>10%. On larger scales, we also find evidence for weak lensing shear.
We rule out the null hypothesis of randomly oriented galaxies at ≳99%
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
⟨ e⟩∼10% is non-negligible and beyond the level of our PSF
uncertainties. The shear variance
⟨γ^2⟩∼10^-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∼0.75 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.
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