The Observability of Recoiling Black Holes as Offset Quasars
The merger of two supermassive black holes (SMBHs) imparts a gravitational-wave (GW) recoil kick to the remnant SMBH. In extreme cases these kicks may be thousands of km/s -- enough to easily eject them from their host galaxies. Moderate recoil kicks may also cause substantial displacements of the SMBH, however. An actively-accreting, recoiling SMBH may be observable as an offset quasar. Prior to the advent of a space-based GW observatory, detections of these offset quasars may offer the best chance for identifying recent SMBH mergers, and observational searches for recoiling quasars have already identified several promising candidates. I will review this progress and discuss the prospects for dedicated searches for recoils. Such searches are currently hampered by large theoretical uncertainties regarding how often offset quasars should be observable, where they are most likely to be found, and whether BH spin alignment prior to merger is efficient at suppressing large recoils. Motivated by this, we have developed a model for the observable population of recoiling quasars in a cosmological framework, utilizing detailed information about the progenitor galaxies from state-of-the-art cosmological hydrodynamic simulations (the Illustris Project). I will describe the implications of our results for the design of systematic searches for offset quasars, as well as the indirect constraints that detections of such objects might place on the distribution of SMBH spins.