Abstract
When galaxies merge to assemble more massive galaxies, their nuclear supermassive black holes (SMBHs) should form bound binaries. As these interact with their stellar and gaseous environments, they will become increasingly compact, culminating in inspiral and coalescence through the emission of gravitational radiation. Because galaxy mergers and interactions are also thought to fuel star formation and nuclear black hole activity, it is plausible that such binaries would lie in gas-rich environments and power active galactic nuclei (AGN). The primary difference is that these binaries have gravitational potentials that vary - through their orbital motion as well as their orbital evolution - on humanly tractable timescales, and are thus excellent candidates to give rise to coherent AGN variability in the form of outbursts and recurrent transients. Although such electromagnetic signatures would be ideally observed concomitantly with the binary's gravitational-wave signatures, they are also likely to be discovered serendipitously in wide-field, high-cadence surveys; some may even be confused for stellar tidal disruption events. I discuss several types of possible smoking gun AGN signatures caused by the peculiar geometry predicted for accretion disks around SMBH binaries.
Highlights
Given that galaxies assemble hierarchically and that nearly every massive galaxy appears to host a supermassive black hole (SMBH) in its nucleus [1, 2], it is inevitable that they go through stages in which they harbor multiple SMBHs
Because galaxy mergers and interactions are thought to fuel star formation and nuclear black hole activity, it is plausible that such binaries would lie in gas-rich environments and power active galactic nuclei (AGN)
I discuss several types of possible “smoking gun” AGN signatures caused by the peculiar geometry predicted for accretion disks around SMBH binaries
Summary
Given that galaxies assemble hierarchically and that nearly every massive galaxy appears to host a supermassive black hole (SMBH) in its nucleus [1, 2], it is inevitable that they go through stages in which they harbor multiple SMBHs. Dissipation of gravitational energy via dynamical friction should form bound SMBH binaries, which become increasingly compact as they interact with ambient stars and gas before inspiraling and coalescing by emitting gravitational radiation [3]. Dissipation of gravitational energy via dynamical friction should form bound SMBH binaries, which become increasingly compact as they interact with ambient stars and gas before inspiraling and coalescing by emitting gravitational radiation [3] Such binaries should be common throughout cosmic time [4,5,6,7,8] and are expected to be strong sources of gravitational radiation during inspiral and merger. The evolution of this disk spectrum during and after the merger of the binary (§3); and transient, quasiperiodic flares fueled by inward leakage of circumbinary gas into the cavity and onto one of the SMBHs (§4)
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