How do the LIGO-Virgo-KAGRA's Heavy Black Holes Form? No evidence for core-collapse Intermediate-mass black holes in GWTC-4
Fan-Xiao-Yu Xia, Yuan-Zhu Wang, Ying Qin
TLDR
GWTC-4 data reveals no evidence for core-collapse intermediate-mass black holes, suggesting heavy black holes primarily form through hierarchical mergers.
Key contributions
- No evidence for low-spin, core-collapse intermediate-mass black holes in the GWTC-4 catalog.
- Sets a 90% upper limit on the merger rate of collapse-formed IMBHs at 0.077 Gpc⁻³ yr⁻¹.
- Low-spin black hole mass distribution truncates at ~65 M☉, consistent with the pair-instability mass gap.
- Observed intermediate-mass black holes are high-spin, supporting hierarchical merger formation.
Why it matters
This study significantly refines our understanding of how massive black holes form, challenging the core-collapse model for intermediate-mass black holes. It provides crucial observational constraints on black hole formation pathways. The findings strongly support hierarchical mergers as the dominant mechanism for the observed heavy black holes.
Original 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.
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