Probing the Coevolution of Supermassive Black Holes and Quasar Host Galaxies

At low redshift, there are fundamental correlations between the mass of supermassive black holes (ℳBH) and the mass (ℳbulge) and luminosity of the host galaxy bulge. We investigate the same relation at z ≳ 1. Using virial mass estimates for 11 quasars at z ≳ 2 to measure their black hole mass, we find that black holes at high z fall nearly on the same ℳBH versus R-band magnitude (MR) relation (to ~0.3 mag) as low-redshift active and inactive galaxies, without making any correction for luminosity evolution. Using a set of conservative assumptions about the host galaxy stellar population, we show that at z ≳ 2 (10 Gyr ago), the ratio of ℳBH/ℳbulge was 3-6 times larger than today. Barring unknown systematic errors on the measurement of ℳBH, we also rule out scenarios in which moderately luminous quasar hosts at z ≳ 2 were fully formed bulges that passively faded to the present epoch. On the other hand, five quasar hosts at z ≈ 1 are consistent with the current-day ℳBH-MR relationship after taking into account evolution that is appropriate for E/S0 galaxies. Therefore, z ≈ 1 host galaxies appear to fit the hypothesis that they are fully formed early-type galaxies. We also find that most quasar hosts with absolute magnitudes brighter than MR = -23 cannot fade below L* galaxies today, regardless of their stellar population makeup, because their black hole masses are too high and they must arrive at the local ℳBH-MR relationship by z = 0.