Abstract
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.
Highlights
Mergers between galaxies are important drivers of evolution
Lower mass black hole (BH) dominate at higher redshifts, and higher mass BHs dominate at lower redshifts, consistent with expectations from hierarchical growth
We have developed models for active galactic nuclei (AGN) that are spatially or kinematically offset from their host galaxies owing to gravitational waves (GWs) recoil kicks following a major merger
Summary
Mergers between galaxies are important drivers of evolution. They grow stellar bulges, evolve galaxies along the Hubble sequence, and can trigger both star formation and accretion on to central supermassive black holes (BHs), which in turn influence the host via feedback processes (e.g. Sanders et al 1988; Barnes & Hernquist 1991, 1996; Mihos & Hernquist 1996; Wyithe & Loeb 2003; DiMatteo, Springel & Hernquist 2005; Hopkins et al 2006, 2008a). Mergers between galaxies are important drivers of evolution. They grow stellar bulges, evolve galaxies along the Hubble sequence, and can trigger both star formation and accretion on to central supermassive black holes (BHs), which in turn influence the host via feedback processes While the majority of active galactic nuclei (AGN) in the local Universe do not appear to be triggered by mergers (Cisternas et al 2011; Schawinski et al 2011; Kocevski et al 2012), a clear correlation between merging activity and AGN fuelling has been observed In addition to fuelling rapid growth, a major merger between comparable-mass galaxies, each containing a central BH, will inevitably lead to the formation of a BH binary.
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