Probing the Coevolution of Supermassive Black Holes and Galaxies Using Gravitationally Lensed Quasar Hosts

In the present-day universe, supermassive black hole masses (ℳBH) appear to be strongly correlated with their galaxy's bulge luminosity, among other properties. In this study we explore the analogous relationship between ℳBH, derived using the virial method, and the stellar R-band bulge luminosity (LR) 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 ℳBH-LR relation is nearly the same (to within ~0.3 mag) as it is today. When the observed LR are corrected for luminosity evolution, this means that the black holes grew in mass faster than their hosts, with the ℳBH/ℳ* mass ratio being a factor of ≳4 times larger at z > 1.7 than it is today. By the redshift range 1 ≲ z ≲ 1.7 (8-10 Gyr ago), the ℳBH/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 ℳBH/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.