Breaching the Limit: Formation of GW190521-like and IMBH Mergers in Young Massive Clusters

Abstract

The LIGO-Virgo-Kagra collaboration (LVC) discovered recently GW190521, a gravitational wave (GW) source associated with the merger between two black holes (BHs) with mass 66 M and > 85 M . GW190521 represents the first BH binary (BBH) merger with a primary mass falling in the "upper mass-gap" and the first leaving behind a ∼ 150 M remnant. So far, the LVC reported the discovery of four further mergers having a total mass > 100 M , i.e. in the intermediate-mass black holes (IMBH) mass range. Here, we discuss results from a series of 80 N -body simulations of young massive clusters (YMCs) that implement relativistic corrections to follow compact object mergers. We discover the development of a GW190521-like system as the result of a 3rd-generation merger, and four IMBH-BH mergers with total mass (300 − 350) M . We show that these IMBH-BH mergers are low-frequency GW sources detectable with LISA and DECIGO out to redshift z = 0.01 − 0.1 and z > 100, and we discuss how their detection could help unravelling IMBH natal spins. For the GW190521 test case, we show that the 3rd-generation merger remnant has a spin and effective spin parameter that matches the 90% credible interval measured for GW190521 better than a simpler double merger and comparably to a single merger. Due to GW recoil kicks, we show that retaining the products of these mergers require birth-sites with escape velocities 50 − 100 km s −1 , values typically attained in galactic nuclei and massive clusters with steep density profiles.

Compact Binary Coalescences: Astrophysical Processes and Lessons Learned

“…First, they can accelerate the merging of compact object binaries, allowing them to merge within the age of the Universe. Secondly, they excite very high eccentricity in the inner compact binary, which can affect the GW emission waveform, and therefore can be potentially inferred by GW observations at different frequencies [446][447][448]. The evolution in triple systems can also be affected by all the processes that occur in binary stars, and more.…”

Recurrent X-ray flares of the black hole candidate in the globular cluster RZ 2109 in NGC 4472

“…A further possibility is that the IMBH growth is regulated by both processes above: first an IMBH seed with a mass > 100 M forms via stellar collisions or accretion onto stellar BHs and later the IMBH grows further via repeated BH mergers (e.g., Arca-Sedda et al 2021;Rizzuto et al 2021b). Assuming that a population of IMBH seeds with masses in the range 100 − 500 M formed in globular clusters at redshift z = 2 − 6 (Katz & Ricotti 2013), recently Arca have shown that the overall population of IMBHs at redshift z < 1 would be characterized by a mass distribution that extends between 250 − 1500 M , with a clear peak at around M IMBH ∼ 750 M .…”

Hydrodynamics of Collisions and Close Encounters between Stellar Black Holes and Main-sequence Stars

“…Furthermore, a number of recent observations have linked TDEs of white dwarfs by massive black holes with massive star clusters (e.g., Krolik & Piran 2011;Jonker et al 2013;Lin et al 2018) Aside from their intrinsic interest as electromagnetic transient sources, close encounters between black holes and stars may have an important effect on the overall demographics of black holes in clusters. If a significant fraction of the disrupted stellar material is accreted by the black hole, these TDEs may lead to significant black hole mass growth, potentially enabling formation of black holes within or above the pair-instability mass gap (e.g., Giersz et al 2015;Rizzuto et al 2021;Arca-Sedda et al 2021;Rose et al 2021). Furthermore, if a star is disrupted by a binary black hole, the disrupted material may alter the spin magnitude and orientation of the binary black hole components (e.g., Lopez et al 2019).…”