Cause of conflicting overdensity measurements around z≳6 quasars

Determine whether the discrepancies in the observed abundance of companion galaxies around redshift z≳6 quasars are primarily due to observational limitations (limited sensitivity, velocity offsets between narrow-band filters and quasar systemic redshifts, or restricted field of view) or instead indicate that a subset of high-redshift quasars resides in less massive dark matter halos than predicted by ΛCDM galaxy–quasar formation scenarios.

Background

Luminous quasars at z≳6 are theoretically expected to trace the highest density peaks of the early universe and therefore to exhibit rich environments with many companion galaxies. However, past surveys have produced inconclusive and often contradictory results regarding galaxy overdensities in quasar fields—some report strong enhancements while others do not.

The authors note that recent JWST/NIRCam slitless spectroscopy has revealed overdense environments in some cases, yet the overall picture remains unsettled. They explicitly highlight uncertainty about whether conflicting findings stem from observational constraints (e.g., sensitivity, filter–redshift mismatches, small fields of view) or reflect genuine astrophysical diversity (quasars in less massive halos than anticipated). Resolving this uncertainty is vital for interpreting quasar environments and for inferring host halo masses and quasar duty cycles at the end of reionization.