The Dark Side of QSO Formation at High Redshifts

TL;DR

This study uses constrained simulations to compare the expected and observed galaxy counts around high-redshift QSOs, revealing discrepancies that suggest different galaxy formation processes in QSO environments.

Contribution

It provides the first detailed simulation-based analysis of the environment of z~6 QSOs, highlighting differences between predicted and observed galaxy counts and evolution of QSO host halos.

Findings

Predicted about eight galaxies per QSO field, but observations show four.

QSO halos are the most massive at z~6 but not at z~3.

QSO halos do not evolve into the most massive galaxy clusters at z=0.

Abstract

Observed high-redshift QSOs, at z~6, may reside in massive dark matter (DM) halos of more than 10^{12} Msun and are thus expected to be surrounded by overdense regions. In a series of 10 constrained simulations, we have tested the environment of such QSOs. Comparing the computed overdensities with respect to the unconstrained simulations of regions empty of QSOs, assuming there is no bias between the DM and baryon distributions, and invoking an observationally-constrained duty-cycle for Lyman Break Galaxies, we have obtained the galaxy count number for the QSO environment. We find that a clear discrepancy exists between the computed and observed galaxy counts in the Kim et al. (2009) samples. Our simulations predict that on average eight z~6 galaxies per QSO field should have been observed, while Kim et al. detect on average four galaxies per QSO field compared to an average of three galaxies in a control sample (GOODS fields). While we cannot rule out a small number statistics for the observed fields to high confidence, the discrepancy suggests that galaxy formation in the QSO neighborhood proceeds differently than in the field. We also find that QSO halos are the most massive of the simulated volume at z~6 but this is no longer true at z~3. This implies that QSO halos, even in the case they are the most massive ones at high redshifts, do not evolve into most massive galaxy clusters at z=0.