Repopulating the pair-instability mass gap without sustained growth to massive IMBHs: the case of 47,Tuc

Repopulating the pair-instability mass gap without sustained growth to massive IMBHs: the case of 47,Tuc
Debatri Chattopadhyay, Daniel Mar’in Pina, Mark Gieles, Fabio Antonini, Fotios Fronimos Pouliasis
arXiv:2604.09773v1 Announce Type: new
Abstract: We model the formation and retention of the most massive black hole (BH) in 47~Tuc using the semi-analytical code texttt{cBHBd}, coupling cluster evolution with binary BH dynamics and computing merger-remnant masses, spins, and gravitational-wave recoil kicks via numerical-relativity surrogate prescriptions. We evolve 80,000 cluster realisations spanning initial masses, densities, IMFs, and metallicities, in both a baseline scenario ($m_{rm max} = 130,mathrm{M}_{odot}$) and an extended-IMF scenario with ${sim},50-110$ primordial BH seeds above the pair-instability gap ($M_{rm BH} sim 130-700,mathrm{M}_{odot}$). Selecting models reproducing 47~Tuc’s present-day mass and half-mass radius, we find hierarchical mergers alone yield a most massive retained BH of $M_{rm BH} sim 45-70,mathrm{M}_{odot}$ with spin $chi_{rm BH} sim 0.65$, limited to ${sim},1-3$ mergers, as second-generation remnants acquire spin $chi sim 0.7$ that amplifies recoil kicks in subsequent generations. When primordial seeds are included, the retained-mass distribution becomes bimodal — in ${sim},90%$ of realisations all seeds are ejected, but in ${sim},10%$ a massive seed ($M_{rm BH} gtrsim 450,mathrm{M}_{odot}$) survives — while the joint mass-spin distribution is trimodal; seeds surviving via stellar-mass BH mergers retain low spin ($chi lesssim 0.3$), whereas seed-seed mergers produce high-mass, high-spin remnants ($chi sim 0.65-0.7$), yielding 90th-percentile retained masses of ${sim},500-1100,mathrm{M}_{odot}$. Both scenarios are consistent with the $3sigma$ dynamical upper limit of $578,mathrm{M}_{odot}$. Our results favour a dark-remnant subsystem over a single massive IMBH and provide a spin-mass diagnostic testable with LIGO-Virgo-KAGRA, the Einstein Telescope, Cosmic Explorer, and LISA.arXiv:2604.09773v1 Announce Type: new
Abstract: We model the formation and retention of the most massive black hole (BH) in 47~Tuc using the semi-analytical code texttt{cBHBd}, coupling cluster evolution with binary BH dynamics and computing merger-remnant masses, spins, and gravitational-wave recoil kicks via numerical-relativity surrogate prescriptions. We evolve 80,000 cluster realisations spanning initial masses, densities, IMFs, and metallicities, in both a baseline scenario ($m_{rm max} = 130,mathrm{M}_{odot}$) and an extended-IMF scenario with ${sim},50-110$ primordial BH seeds above the pair-instability gap ($M_{rm BH} sim 130-700,mathrm{M}_{odot}$). Selecting models reproducing 47~Tuc’s present-day mass and half-mass radius, we find hierarchical mergers alone yield a most massive retained BH of $M_{rm BH} sim 45-70,mathrm{M}_{odot}$ with spin $chi_{rm BH} sim 0.65$, limited to ${sim},1-3$ mergers, as second-generation remnants acquire spin $chi sim 0.7$ that amplifies recoil kicks in subsequent generations. When primordial seeds are included, the retained-mass distribution becomes bimodal — in ${sim},90%$ of realisations all seeds are ejected, but in ${sim},10%$ a massive seed ($M_{rm BH} gtrsim 450,mathrm{M}_{odot}$) survives — while the joint mass-spin distribution is trimodal; seeds surviving via stellar-mass BH mergers retain low spin ($chi lesssim 0.3$), whereas seed-seed mergers produce high-mass, high-spin remnants ($chi sim 0.65-0.7$), yielding 90th-percentile retained masses of ${sim},500-1100,mathrm{M}_{odot}$. Both scenarios are consistent with the $3sigma$ dynamical upper limit of $578,mathrm{M}_{odot}$. Our results favour a dark-remnant subsystem over a single massive IMBH and provide a spin-mass diagnostic testable with LIGO-Virgo-KAGRA, the Einstein Telescope, Cosmic Explorer, and LISA.