Letter to the Editor

Formation of low-spinning 100 M_⊙ black holes

¹ Nicolaus Copernicus Astronomical Centre of the Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warszawa, Poland
e-mail: chrisbelczynski@gmail.com
² Helmholtz-Instituts für Strahlen- und Kernphysik, Nussallee 14-16, 53115 Bonn, Germany
³ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 15 May 2020
Accepted: 2 August 2020

Abstract

Aims. It is speculated that a merger of two massive stellar-origin black holes in a dense stellar environment may lead to the formation of a massive black hole in the pair-instability mass gap (∼50−135 M_⊙). Such a merger-formed black hole is expected to typically have a high spin (a ∼ 0.7). If such a massive black hole acquires another black hole it may lead to another merger detectable by LIGO/Virgo in gravitational waves. Acquiring a companion may be hindered by gravitational-wave kick/recoil, which accompanies the first merger and may quickly remove the massive black hole from its parent globular or nuclear cluster. We test whether it is possible for a massive merger-formed black hole in the pair-instability gap to be retained in its parent cluster and have low spin. Such a black hole would be indistinguishable from a primordial black hole.

Methods. We employed results from numerical relativity calculations of black hole mergers to explore the range of gravitational-wave recoil velocities for various combinations of merging black hole masses and spins. We compared merger-formed massive black hole speeds with typical escape velocities from globular and nuclear clusters.

Results. We show that a globular cluster is highly unlikely to form and retain a ∼100 M_⊙ black hole if the spin of the black hole is low (a ≲ 0.3). Massive merger-formed black holes with low spins acquire high recoil speeds (≳ 200 km s⁻¹) from gravitational-wave kick during formation that exceed typical escape speeds from globular clusters (∼ 50 km s⁻¹). However, a very low-spinning (a ∼ 0.1) and massive (∼100 M_⊙) black hole could be formed and retained in a galactic nuclear star cluster. Even though such massive merger-formed black holes with such low spins acquire high speeds during formation (∼ 400 km s⁻¹), they may avoid ejection since massive nuclear clusters have high escape velocities (∼ 300−500 km s⁻¹). A future detection of a massive black hole in the pair-instability mass gap with low spin would therefore not be proof of the existence of primordial black holes, which are sometimes claimed to have low spins and arbitrarily high masses.