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 (script M sign _BH) and the mass (script M sign _bulge) 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 script M sign_BH versus R-band magnitude (M_R) 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 script M sign_BH/script M sign_bulge was 3-6 times larger than today. Barring unknown systematic errors on the measurement of script M sign_BH, 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 script M sign _BH-M_R 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 M_R = -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 script M sign_BH-M_R relationship by z = 0.