How do the LIGO-Virgo-KAGRA's Heavy Black Holes Form? No evidence for core-collapse Intermediate-mass black holes in GWTC-4

TL;DR

This study analyzes the properties of high-mass binary black holes from GWTC-4, finding no evidence for core-collapse intermediate-mass black holes and suggesting hierarchical mergers as their formation mechanism.

Contribution

First search for low-spin IMBHs in GWTC-4, setting upper limits and linking high-spin IMBHs to hierarchical merger formation.

Findings

No evidence for core-collapse IMBHs in GWTC-4.

Mass distribution truncates at ~65 M_sun, consistent with pair-instability gap.

High-spin IMBHs likely formed via hierarchical mergers.

Abstract

We investigate the population properties of binary black holes (BBHs) from the LIGO-Virgo-KAGRA collaboration, focusing especially on those in the high-mass range, using the newly released GWTC-4 catalog. For the first time, we search for a subpopulation of low-spin intermediate-mass black holes (IMBHs) that would indicate formation via stellar core collapse. With the currently available catalog, we find no evidence for such a subpopulation, and set a 90\% upper limit on the merger rate of collapse-formed IMBHs at $0.077~\mathrm{Gpc}^{-3}\,\mathrm{yr}^{-1}$. The mass distribution of low-spin (stellar-origin) black holes truncates at $65^{+23}_{-22}\,M_\odot$, consistent with the lower edge of the pair-instability mass gap (PIMG), although we cannot directly determine its upper boundary from current data. Informed by stellar evolution theory, we estimate the upper edge of the PIMG to be $150\pm24\,M_\odot$. We find that the observed IMBHs belong to a high-spin subpopulation, consistent with formation through successive hierarchical mergers.