Recoiling black holes: prospects for detection and implications of spin
  alignment
  Supermassive black hole (BH) mergers produce powerful gravitational wave (GW)
emission. Asymmetry in this emission imparts a recoil kick to the merged BH,
which can eject the BH from its host galaxy altogether. Recoiling BHs could be
observed as offset active galactic nuclei (AGN). Several candidates have been
identified, but systematic searches have been hampered by large uncertainties
regarding their observability. By extracting merging BHs and host galaxy
properties from the Illustris cosmological simulations, we have developed a
comprehensive model for recoiling AGN. Here, for the first time, we model the
effects of BH spin alignment and recoil dynamics based on the gas-richness of
host galaxies. We predict that if BH spins are not highly aligned,
seeing-limited observations could resolve offset AGN, making them promising
targets for all-sky surveys. For randomly-oriented spins, less than about 10
spatially-offset AGN may be detectable in HST-COSMOS, and > 10^3 could be found
with Pan-STARRS, LSST, Euclid, and WFIRST. Nearly a thousand velocity-offset
AGN are predicted within the SDSS footprint; the rarity of large broad-line
offsets among SDSS quasars is likely due in part to selection effects but
suggests that spin alignment plays a role in suppressing recoils. Nonetheless,
in our most physically motivated model where alignment occurs only in gas-rich
mergers, hundreds of offset AGN should be found in all-sky surveys. Our
findings strongly motivate a dedicated search for recoiling AGN.
