Probing the coevolution of supermassive black holes and galaxies using gravitationally lensed quasar hosts

In the present-day universe, supermassive black hole masses (M _BH) appear to be strongly correlated with their galaxy's bulge luminosity, among other properties. In this study we explore the analogous relationship between M_BH, derived using the virial method, and the stellar (L_R)-band bulge luminosity (L_R) or stellar bulge mass (M*) at epochs of 1 ≲ z ≲ 4.5, using a sample of 31 gravitationally lensed AGNs and 20 nonlensed AGNs. At redshifts z > 1.7 (10-12 Gyr ago), we find that the observed M_BH-L_R relation is nearly the same (to within ∼0.3 mag) as it is today. When the observed L_R are corrected for luminosity evolution, this means that the black holes grew in mass faster than their hosts, with the M_BH/M* mass ratio being a factor of ≳4_-1⁺² times larger at z > 1.7 than it is today. By the redshift range 1 ≲ z ≲ 1.7 (8-10 Gyr ago), the M_BH/M* ratio is at most 2 times higher than today, but it may be consistent with no evolution. Combining the results, we conclude that the ratio M_BH/M* rises with look-back time, although it may saturate at ≈6 times the local value. Scenarios in which moderately luminous quasar hosts at z ≳ 1.7 were fully formed bulges that passively faded to the present epoch are ruled out.