Multi-tracer Correlated Stacking: A Novel Way to Discover Anisotropy in nHz Stochastic Gravitational-wave Background

Abstract The isotropic stochastic gravitational-wave background (SGWB) generated by a population of supermassive black hole binaries (SMBHBs) provides a unique window into their cosmic evolution. In addition to the isotropic power spectrum, the anisotropic component of the signal carries additional information about the supermassive black holes (BHs) and host galaxy connection. The measurement of this signal is usually carried out by angular power spectra, which is only a sufficient measure for a Gaussian and statistically isotropic distribution of SMBHBs, where the statistical properties of a field remain unchanged across the sky. In contrast, the contribution from SMBHBs in nHz SGWB will be hosted by fewer massive galaxies, making the nHz background anisotropic and non-Gaussian. As a result, the performance of angular power spectra in extracting the underlying physics is limited. In this work, we propose a novel technique called multi-tracer correlated stacking, which enables the detection of anisotropies in the SGWB by stacking the signal from regions of the sky with tracers of BHs such as active galactic nuclei (AGN), quasars, bright galaxies, etc., that can be mapped up to high redshift. We demonstrate this technique on a simulated population of SMBHBs using an AGN catalog, which maps the underlying matter distribution approximately up to redshift z = 5. This stacking technique uniquely distinguishes between isotropic and anisotropic distributions of SGWB sources, surpassing the capabilities of angular power spectrum-based methods in detecting anisotropic signals. This highlights the effectiveness of this technique in detecting anisotropic SGWB signals, and in the future, this technique can play a crucial role in its discovery.