Accretion is All You Need: Black Hole Spin Alignment in Merger GW231123 Indicates Accretion Pathway

TL;DR

This paper suggests that the high spins and alignment of black holes in GW231123 are best explained by a common accretion-driven formation pathway, emphasizing the role of coherent accretion in extreme black hole mergers.

Contribution

It introduces the idea that accretion processes can produce high-spin, aligned black hole binaries, challenging previous merger-based explanations.

Findings

Black hole spins are highly aligned with each other.

Component spins are significantly tilted relative to orbital angular momentum.

Accretion-driven formation channels can explain the observed properties.

Abstract

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 ($a_1 = 0.90^{+0.10}_{-0.19}$, $a_2 = 0.80^{+0.20}_{-0.51}$), 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 Pop~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.