A Gravitational Wave Background from Intermediate Mass Black Holes in AGN Disks

Intermediate mass black holes (IMBHs) formed in active galactic nucleus (AGN) disks are expected to inspiral into their central supermassive black holes (SMBHs), generating a stochastic gravitational-wave (GW) background in the mHz–decihertz band. Using the population-agnostic energetic formalism, we treat the AGN-disk channel as a mass-flow pipeline connecting the stellar-mass black hole population observed by LIGO/Virgo/KAGRA (LVK) to the SMBH mass reservoir via IMBHs. By anchoring this estimate to the LVK merger rate densities and the cosmic SMBH mass density derived from scaling relations, we derive a limit on the background amplitude. We show that the total energy density of the background is bounded by the global mass budget of SMBH growth. For fiducial parameters consistent with the fourth Gravitational-Wave Transient Catalog (GWTC-4), this yields a characteristic strain $A_{\rm IMR} \simeq (1.2_{-0.2}^{+0.2})\times 10^{-21}$ at $3,{\rm mHz}$. While this fiducial amplitude is subdominant to the Galactic white dwarf foreground and the stellar-mass Extreme Mass Ratio Inspiral (EMRI) background, we show it can be distinguished by its non-Gaussian statistics and higher frequency cutoff. This new background may be detectable in the decihertz band where proposed detectors such as the Big Bang Observer or long-baseline lunar interferometers can measure it cleanly. A detection would provide a direct, model-independent constraint on the efficiency with which AGN disks process stellar remnants into SMBH mass growth, linking the LVK and LISA bands.

💡 Research Summary

The paper presents a quantitative estimate of a stochastic gravitational‑wave (GW) background generated by intermediate‑mass black holes (IMBHs) that form and grow inside active‑galactic‑nucleus (AGN) disks and eventually inspiral into their central supermassive black holes (SMBHs). The authors adopt a “mass‑flow pipeline” perspective, linking two robust observational inputs: (i) the binary‑black‑hole (BBH) merger rate density measured by ground‑based detectors (LIGO/Virgo/KAGRA, hereafter LVK) and (ii) the local SMBH mass density derived from galaxy‑SMBH scaling relations. By treating the AGN‑disk channel as a conduit that processes a fraction of the stellar‑mass black‑hole population observed by LVK into the SMBH mass reservoir, they derive a GW background whose amplitude is bounded by global energy‑conservation arguments.

Key elements of the model are three dimensionless efficiencies: (a) (f_{\rm AGN}), the fraction of LVK BBH mergers that occur inside AGN disks; (b) (\kappa_{\rm IM}), the fraction of the mass processed in those mergers that ends up as retained IMBHs and is eventually captured by the central SMBH; and (c) (\epsilon_{\rm gw}), the radiative efficiency of the IMBH–SMBH inspiral (the fraction of the binary’s rest‑mass energy emitted as GWs, taken to be ~5%). The comoving IMBH mass density is then \