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 similar or equal to 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-star(host) from z = 0 to 3. Taken at face value, this is totally at odds with the predictions of SAMs, where the intrinsic Gamma 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 (10(9)-10(10) M-circle dot) 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.