Astronomy and Astrophysics, volume 640, L20-20 (2020/8-1)
Formation of low-spinning 100 M☉ black holes.
BELCZYNSKI K. and BANERJEE S.
Abstract (from CDS):
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 (∼50-135M☉). 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∼100M☉ 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 (≥200km/s) from gravitational-wave kick during formation that exceed typical escape speeds from globular clusters (∼50km/s). However, a very low-spinning (a∼0.1) and massive (∼100M☉) 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 (∼400km/s), they may avoid ejection since massive nuclear clusters have high escape velocities (∼300-500km/s). 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.