The Infrared Afterglow of Supermassive Black Hole Mergers

We model the spectra and light curves of circumbinary accretion disks during the time after the central black holes merge. The most immediate effect of this merger is the deposition of energy in the disk due to the gravitational wave energy and linear momentum flux released at merger. This has the effect of perturbing the circular orbits of gas in the disk, which then intersect and radiate the dissipated energy. Because the disk is expected to be very optically thick, the radiation emerges predominantly in the infrared and lasts for tens of thousands of years when the total black hole mass is M ∼ 10⁸ M_☉. On the basis of a simple cosmological merger model in which a typical supermassive black hole undergoes a few major mergers during its lifetime, we predict that ~10⁴-10⁵ of these afterglow sources should be observable today. We also discuss the possibility of identifying them with existing multiwavelength surveys such as SWIRE/XMM-LSS/XBootes and COSMOS.