Objects in JWST's mirrors are closer than they appear

TL;DR

JWST's photometric redshift estimates for early galaxies are systematically biased high due to a statistical effect similar to Eddington bias, requiring downward correction for accurate cosmic epoch measurements.

Contribution

This paper identifies and explains the bias in JWST's photometric redshift estimates, linking it to Eddington bias, and provides a correction heuristic for ultra-high redshift measurements.

Findings

Photometric redshifts are biased high at early cosmic epochs.

The bias is due to a statistical effect similar to Eddington bias.

A heuristic correction of downward adjustment by up to one standard deviation is recommended.

Abstract

The James Webb Space Telescope (JWST) has revealed extremely distant galaxies at unprecedentedly early cosmic epochs from its deep imaging using the technique of photometric redshift estimation, with its subsequent spectroscopy confirming their redshifts unambiguously, demonstrating the ability of JWST to probe the earliest galaxies, one of its major scientific goals. However, as larger samples continue to be followed up spectroscopically, it has become apparent that nearly all photometric redshifts at these epochs are biased high with confidence >>99%, for as yet unclear reasons. Here we show that this is the same statistical effect that was predicted in different contexts by Sir Arthur Eddington in 1913, in that there exist more lower redshift galaxies to be scattered upwards than the reverse. The bias depends on the shape of the intrinsic redshift distribution, but as an approximate heuristic, all ultra-high photometric redshift estimates must be corrected downwards by up to one standard deviation.