Gravitational waves from supermassive black hole binaries in ultra-luminous infrared galaxies

Gravitational waves (GWs) in the nanohertz band are great tools for understanding the cosmological evolution of supermassive black holes (SMBHs) in galactic nuclei. We consider SMBH binaries in high-z ultraluminous infrared galaxies (ULIRGs) as sources of a stochastic GW background (GWB). ULIRGs are likely associated with gas-rich galaxy mergers containing SMBHs that possibly occur at most once in the life of galaxies, unlike multiple dry mergers at low redshift. Adopting a well-established sample of ULIRGs, we study the properties of the GWB due to coalescing binary SMBHs in these galaxies. Since the ULIRG population peaks at z > 1.5, the amplitude of the GWB is not affected even if BH mergers are delayed by as long as similar to 10 Gyr. Despite the rarity of the high-z ULIRGs, we find a tension with the upper limits from pulsar timing array experiments. This result suggests that if a fraction f(m, gal) of ULIRGs are associated with SMBH binaries, then no more than 20f(m,gal) (lambda(Edd)/0.3)(5/3)(t(life)/30 Myr) % of the binary SMBHs in ULIRGs can merge within a Hubble time, for plausible values of the Eddington ratio of ULIRGs (lambda(Edd)) and their lifetime (t(life)).