Dynamical formation of dormant black holes: Predictions for Gaia DR4
Abstract
Dormant black holes (dormant BHs, i.e., X-ray quiet binaries comprising a BH and a companion star) are probes of the effect of cluster dynamics and binary stellar evolution on compact-object binaries. A dozen dormant BH binaries are known, a number that will increase substantially with the upcoming Gaia Data Release 4 (DR4). Their origin is uncertain, and dynamical formation is proposed as one possible channel. Here, we study the formation of dormant BHs in low-mass star clusters, accounting for the interplay of binary stellar evolution and cluster dynamics. We present new simulations with the N-body code PeTar, interfaced with the population synthesis code SEVN for single and binary star evolution. Our grid spans a range of cluster masses (300−4800M⊙), metallicities (1.4×10−2, 1.4×10−4), and natal kick models (with and without fallback). We find that almost all dormant BH binaries escape the parent cluster within ∼1Gyr (∼10Myr) when assuming weak (strong) natal kicks. Most (73%) dormant BHs form via the exchange of a BH (or its progenitor star) into a stellar binary, or of a star into a binary with a BH. Dormant BHs in low-mass clusters have a wide orbital period distribution (from a few hours to 108d), with an upper limit set by natal kicks, a bimodal eccentricity distribution (a first peak at zero eccentricity for circularised systems and a second at high eccentricity), and stellar masses that follow the (evolved) initial mass function. Large BH masses are a smoking gun of dynamical formation, via exchanges and stellar collisions. We expect Gaia DR4 to contain ∼1−64 high-metallicity and ∼0.4−48 low-metallicity dormant BHs, with a period distribution peaking at ∼103d, ≳90% of them with eccentricity e>0.1, and ∼20% with large BH masses (mBH ≳ 30M⊙).
Related
- Subpopulation of low-mass, spinning black holes: Signatures of dynamical assembly
- Compactness Peaks and Subpopulations: Probing Stellar Physics and Formation Channels of Merging Binary Black Holes
- Predictions on the abundance of primordial black holes: results from the SMILE VLBI milli-lensing sample
- Dynamical tidal response of Schwarzschild Black Holes
- Dynamical friction on black holes in scalar field environment
BibTeX
@misc{pina2026dynamical,
title = {{Dynamical formation of dormant black holes: Predictions for Gaia DR4}},
author = {Daniel Marín Pina and Michela Mapelli and Giuliano Iorio and Cecilia Sgalletta},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.18601},
eprint = {2610.18601},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.18601},
note = {CC BY-NC-SA 4.0},
}
RIS
TY - UNPB
TI - Dynamical formation of dormant black holes: Predictions for Gaia DR4
AU - Daniel Marín Pina
AU - Michela Mapelli
AU - Giuliano Iorio
AU - Cecilia Sgalletta
PY - 2026
DA - 2026/10/13
PB - arXiv
DO - 10.48550/arxiv.2610.18601
UR - https://arxiv.org/abs/2610.18601
AB - Dormant black holes (dormant BHs, i.e., X-ray quiet binaries comprising a BH and a companion star) are probes of the effect of cluster dynamics and binary stellar evolution on compact-object binaries. A dozen dormant BH binaries are known, a number that will increase substantially with the upcoming Gaia Data Release 4 (DR4). Their origin is uncertain, and dynamical formation is proposed as one possible channel. Here, we study the formation of dormant BHs in low-mass star clusters, accounting for the interplay of binary stellar evolution and cluster dynamics. We present new simulations with the N-body code PeTar, interfaced with the population synthesis code SEVN for single and binary star evolution. Our grid spans a range of cluster masses (300−4800M_⊙), metallicities (1.4×10^−2, 1.4×10^−4), and natal kick models (with and without fallback). We find that almost all dormant BH binaries escape the parent cluster within ∼1Gyr (∼10Myr) when assuming weak (strong) natal kicks. Most (73%) dormant BHs form via the exchange of a BH (or its progenitor star) into a stellar binary, or of a star into a binary with a BH. Dormant BHs in low-mass clusters have a wide orbital period distribution (from a few hours to 10^8d), with an upper limit set by natal kicks, a bimodal eccentricity distribution (a first peak at zero eccentricity for circularised systems and a second at high eccentricity), and stellar masses that follow the (evolved) initial mass function. Large BH masses are a smoking gun of dynamical formation, via exchanges and stellar collisions. We expect Gaia DR4 to contain ∼1−64 high-metallicity and ∼0.4−48 low-metallicity dormant BHs, with a period distribution peaking at ∼10^3d, ≳90% of them with eccentricity e>0.1, and ∼20% with large BH masses (m_BH ≳ 30M_⊙).
ER -