The Co-formation of Spheroids and Quasars Traced in Their Clustering

We compare the 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 (z . 3) quasars are shown to be drawn from the progenitors of local z = 0 early-type galaxies, with the characteristic quasar luminosity L∗ at a given redshift reflecting a characteristic mass of the “active” BH/host population at that epoch. In detail, evolving observed high-redshift quasar clustering to z = 0 predicts a trend of clustering in “quasar remnants” as a function of stellar mass and/or luminosity, identical to that observed for early-type populations. The same is true at intermediate redshifts between z = 0.1 − 1.5, although the observed galaxy clustering is less well-constrained. Inverting this, the clustering of ellipticals as a function of mass can be used to well-predict quasar clustering as a function of redshift. However, the data suggest that active quasar clustering does not reflect a constant early-type host population, nor does it reflect the observed early-type (or late-type) populations at the same redshift. At all redshifts, 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 which are undergoing their final significant star formation (or terminating that star formation) 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 spiral/disk populations or the quasar halos (by any definition of their ages); i.e. quasars do not generically trace star formation, disk systems/instabilities, or halo assembly/growth processes. Interestingly, however, quasar clustering at all redshifts is consistent with a constant host halo mass ∼ 4×1012 h−1 M⊙, similar to the local “group scale.” The observations support a scenario in which major mergers, responsible for the morphological transformation of disks to ellipticals and powerful starbursts, also trigger quasar activity and dominate the bright, high-redshift quasar population. We consider quasar clustering as a function of quasar luminosity, which reinforces the robustness of our results, and show that future measurements of quasar clustering at low luminosities (hard X-ray L . 1043 ergs−1) at z ∼ 1 can be used to constrain different quasar fueling mechanisms (for example, various fueling mechanisms in unperturbed, gas-rich disks which may dominate AGN populations at low luminosities). We use our empirical methodology to predict quasar clustering at high redshifts z = 3 − 6, and find that the results here are sensitive to whether or not systems “shut down” after their quasar epoch; i.e. whether or not feedback can act efficiently at high redshift. Future measurements of this high-redshift clustering should be able to break the degeneracy between these scenarios and answer this unresolved question in galaxy formation theories. Subject headings: quasars: general — galaxies: active — galaxies: evolution — cosmology: theory