The Co‐Formation of Spheroids and Quasars Traced in their Clustering

We compare observed clustering of quasars and galaxies as a function ofredshift, mass, luminosity, & color/morphology, to constrain models of quasarfueling and spheroid-BH co-evolution. High redshift quasars are shown to bedrawn from progenitors of local early-type galaxies, with the characteristicquasar luminosity L* reflecting a characteristic mass of 'active' BH/hostpopulations at each redshift. Evolving observed high-z quasar clustering to z=0predicts a trend of clustering in 'quasar remnants' as a function of stellarmass identical to that observed for early-types. However, quasar clusteringdoes not simply reflect observed early (or late)-type populations; at eachredshift, quasars cluster as an 'intermediate' population. Comparing with theage of elliptical stellar populations reveals that this 'intermediate'population represents those ellipticals undergoing or terminating their finalsignificant star formation at each epoch. Assuming that quasar triggering isassociated with the formation/termination epoch of ellipticals predicts quasarclustering at all observed redshifts without any model dependence orassumptions about quasar light curves, lifetimes, or accretion rates. This isnot true for disks or quasar halos: i.e. quasars do not generically trace starformation, disks, or halo assembly. Quasar clustering at all z is consistentwith a constant halo mass ~4x10^{12} M_sun, similar to local 'group scales.'The observations support a scenario in which major mergers trigger quasaractivity and dominate bright, high-z quasar populations. We show thatmeasurements of quasar clustering versus luminosity at z~1 can be used toconstrain different lower-luminosity AGN fueling mechanisms, and that high-zclustering is sensitive to whether or not systems 'shut down' growth at z>3.Comment: 21 pages, 14 figures. Accepted to ApJ. Minor revisions to match published versio