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

Quasars are useful tracers of the cosmological evolution of the black hole mass-galaxy relation. We compare the expectations of semi-analytical models (SAMs) of galaxy evolution to the largest available data sets of quasar host galaxies out to z≃ 3. Observed quasar hosts are consistent with no evolution from the local MBH-Lhost relation and suggest a significant increase of the mass ratio ? from z= 0 to 3. Taken at face value, this is totally at odds with the predictions of SAMs, where the intrinsic Γ shows little evolution and quasar host galaxies at high redshift are systematically overluminous (and/or have an undermassive BH). However, since quasars preferentially trace very massive black holes (109-1010 M&sun;) at the steep end of the luminosity and mass function, the ensuing selection biases can reconcile the present SAMs with the observations. A proper interpretation of quasar host data thus requires the global approach of SAMs so as to account for statistical biases