Accretion is all you need: Black Hole spin alignment in merger GW231123 indicates accretion pathway Bartos, Imre Haiman, Zoltán ; https://orcid.org/0000-0003-3633-5403 ddc:520 GW231123 represents the most massive binary–black hole merger detected to date, lying firmly within, or even above, the pair-instability mass gap. The component spins are both exceptionally high (a1 = 0.90 +0.10/-0.19, a2 = 0.80 +0.20/-0.51), which is difficult to explain with repeated mergers. Here we show that the black hole spin vectors are closely aligned with each other while significantly tilted relative to the binary’s orbital angular momentum, pointing to a common accretion-driven origin. We examine astrophysical formation channels capable of producing near-equal, high-mass, and mutually aligned spins consistent with GW231123—particularly binaries embedded in AGN disks and Population III remnants, which grew via coherent misaligned gas accretion. We further argue that other high-mass, high-spin events, e.g., GW190521, may share a similar evolutionary pathway. These findings underscore the critical role of sustained, coherent accretion in shaping the most extreme black hole binaries. IOP Publishing 2026 info:eu-repo/semantics/article doc-type:article text http://purl.org/coar/resource_type/c_2df8fbb1 https://research-explorer.ista.ac.at/record/21713 https://research-explorer.ista.ac.at/download/21713/21788 Bartos I, Haiman Z. Accretion is all you need: Black Hole spin alignment in merger GW231123 indicates accretion pathway. <i>The Astrophysical Journal Letters</i>. 2026;996(2). doi:<a href="https://doi.org/10.3847/2041-8213/ae2bff">10.3847/2041-8213/ae2bff</a> eng info:eu-repo/semantics/altIdentifier/doi/10.3847/2041-8213/ae2bff info:eu-repo/semantics/altIdentifier/issn/2041-8205 info:eu-repo/semantics/altIdentifier/e-issn/2041-8213 info:eu-repo/semantics/altIdentifier/arxiv/2508.08558 info:eu-repo/semantics/openAccess