On the cosmological evolution of the black hole - host galaxy relation in quasars

TL;DR

Quasar host galaxy observations suggest a significant increase in black hole to galaxy mass ratio from redshift 0 to 3, conflicting with models unless selection biases are properly accounted for.

Contribution

This study compares observational data of quasar hosts with semi-analytical models, highlighting the importance of accounting for selection biases in understanding black hole-galaxy evolution.

Findings

Observed data indicates no evolution in the local M(BH)-L(host) relation.

A significant increase in the mass ratio Gamma from z=0 to z=3.

Selection biases can reconcile models with observations.

Abstract

Quasars are useful tracers of the cosmological evolution of the black hole mass - galaxy relation. We compare the expectations of Semi-Analytical Models (SAM) of galaxy evolution, to the largest available datasets of quasar host galaxies out to z=3. Observed quasar hosts are consistent with no evolution from the local M(BH) - L(host) relation, and suggest a significant increase of the mass ratio Gamma = M(BH)/M(host) from z=0 to z=3. Taken at face value, this is totally at odds with the predictions of SAM, where the intrinsic Gamma shows little evolution and quasar host galaxies at high redshift are systematically overluminous (and/or have undermassive BH). However, since quasars preferentially trace very massive black holes (10^9-10^10 Msun) at the steep end of the luminosity and mass function, the ensuing selection biases can reconcile the present SAM with the observations. A proper interpretation of quasar host data thus requires the global approach of SAM so as to account for statistical biases.