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

TL;DR

This paper estimates the gravitational-wave background from intermediate mass black holes in AGN disks, linking stellar-mass black hole mergers to supermassive black hole growth, and discusses its potential detectability with future observatories.

Contribution

It introduces a population-agnostic formalism to connect black hole populations and derives bounds on the GW background from AGN disks, highlighting its detectability prospects.

Findings

The GW background amplitude is estimated at $A_{IMR} \,\simeq\ 1.2\times 10^{-21}$ at 3 mHz.

The background is subdominant but distinguishable by non-Gaussian features and higher frequency cutoff.

Future decihertz detectors could measure this background, constraining SMBH growth efficiency.

Abstract

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.