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

We compare observed clustering of quasars and galaxies as a function of redshift, mass, luminosity, and color/morphology, to constrain models of quasar fueling and the co-evolution of spheroids and supermassive black holes (BHs). High-redshift quasars are shown to be drawn from the progenitors of local early-type galaxies, with the characteristic quasar luminosity L_* reflecting a characteristic mass of "active" BH/host populations at each epoch. Evolving observed high-z quasar clustering to z = 0 predicts a trend of clustering in "quasar remnants" as a function of stellar mass identical to that observed for early types. However, quasar clustering does not simply reflect observed early (or late) type populations; at each redshift, quasars cluster as an "intermediate" population. Comparing with the age of elliptical stellar populations as a function of mass reveals that this "intermediate" population represents those ellipticals undergoing or terminating their final significant star formation activity at the given epoch. Assuming that quasar triggering is associated with the formation/termination epoch of ellipticals predicts quasar clustering at all observed redshifts without any model dependence or assumptions about quasar light curves, lifetimes, or accretion rates. This is not true for disks or quasar halos; i.e., quasars do not generically trace star formation or halo assembly. Quasar clustering at all redshifts is consistent with ~4 × 10¹² h^-1 M_☉, similar to group scales. This supports scenarios in which major mergers dominate the bright, high-redshift quasar populations. We show how improved clustering measurements can be used to constrain lower luminosity AGN fueling and whether or not accretion/star formation can "shut down" at z > 3.