On merger bias and the clustering of quasars

TL;DR

This study investigates whether recently merged haloes and galaxies exhibit stronger large-scale clustering than their non-merged counterparts, finding minimal merger bias for massive systems and implications for quasar clustering interpretations.

Contribution

The paper provides the first detailed analysis of merger bias in both dark matter haloes and galaxies using large simulations and semi-analytic models, challenging assumptions about quasar clustering.

Findings

No significant excess bias for recently merged haloes at 2<z<5.

20-30% excess bias for recently merged galaxies at fixed stellar mass.

Massive systems show weak merger bias, affecting quasar clustering interpretations.

Abstract

We use the large catalogues of haloes available for the Millennium Simulation to test whether recently merged haloes exhibit stronger large-scale clustering than other haloes of the same mass. This effect could help to understand the very strong clustering of quasars at high redshift. However, we find no statistically significant excess bias for recently merged haloes over the redshift range 2 < z < 5, with the most massive haloes showing an excess of at most ~5%. We also consider galaxies extracted from a semianalytic model built on the Millennium Simulation. At fixed stellar mass, we find an excess bias of ~ 20-30% for recently merged objects, decreasing with increasing stellar mass. The fact that recently-merged galaxies are found in systematically more massive haloes than other galaxies of the same stellar mass accounts for about half of this signal, and perhaps more for high-mass galaxies. The weak merger bias of massive systems suggests that objects of merger-driven nature, such as quasars, do not cluster significantly differently than other objects of the same characteristic mass. We discuss the implications of these results for the interpretation of clustering data with respect to quasar duty cycles, visibility times, and evolution in the black hole-host mass relation.