Black Hole Mergers from Star Clusters with Top-heavy Initial Mass Functions

Abstract

Recent observations of globular clusters (GCs) provide evidence that the stellar initial mass function (IMF) may not be universal, suggesting specifically that the IMF grows increasingly top-heavy with decreasing metallicity and increasing gas density. Noncanonical IMFs can greatly affect the evolution of GCs, mainly because the high end determines how many black holes (BHs) form. Here we compute a new set of GC models, varying the IMF within observational uncertainties. We find that GCs with top-heavy IMFs lose most of their mass within a few gigayears through stellar winds and tidal stripping. Heating of the cluster through BH mass segregation greatly enhances this process. We show that, as they approach complete dissolution, GCs with top-heavy IMFs can evolve into "dark clusters" consisting of mostly BHs by mass. In addition to producing more BHs, GCs with top-heavy IMFs also produce many more binary BH (BBH) mergers. Even though these clusters are short-lived, mergers of ejected BBHs continue at a rate comparable to, or greater than, what is found for long-lived GCs with canonical IMFs. Therefore, these clusters, though they are no longer visible today, could still contribute significantly to the local BBH merger rate detectable by LIGO/Virgo, especially for sources with higher component masses well into the BH mass gap. We also report that one of our GC models with a top-heavy IMF produces dozens of intermediatemass black holes (IMBHs) with masses, including one with. Ultimately, additional gravitational wave observations will provide strong constraints on the stellar IMF in old GCs and the formation of IMBHs at high redshift.

Matching Globular Cluster Models to Observations

“…However, it is also important to keep in mind that NGC 6624 exists in a more complex high-metallicity regime, and lies behind a moderate amount of extinction. Coupled with uncertainty in terms of the distance from Gaia (D = 7.19 ± 0.37 kpc, Baumgardt et al 2019b) and the initial mass function (which may play a significant role; see Weatherford et al 2021), it remains plausible that both observational and modeling uncertainties could account for the deficit in MSPs and XRBs in the best-fitting models.…”

Effects of Baryon-Antibaryon Annihilation in the Evolution of Antimatter Domains in Baryon Asymmetrical Universe

“…Neutralino clumps of mass M cl are estimated to survive the Galaxy evolution if their mass is within the range 10 −8 M ≤ M cl ≤ 10 −6 M in [13], [14]; further classifications of neutralino clumps also allow for further comparison with the case of antimattter domains [15]. Analyses of the limiting processes of disappearence of the antimatter domains are possible, after [16], and the characterization of the results follow [17]. The space-time evolution of antimatter domains separated in a small angular distance can be further studied through the Rubin-Limber correlation functions for small angles [18], [19].…”

The Merger Rate of Black Holes in a Primordial Black Hole Cluster

“…This possibility was studied for globular star clusters with black holes in Refs. [17][18][19][20][21][22]. In addition, these binaries might significantly affect the cluster evolution, stop the core collapse, and cause gravothermal oscillations [40][41][42][43][44].…”

“…[7,[10][11][12][13] in the context of PBH clusters with narrow mass spectra (with the number of PBH N PBH 10 4 ) and in Refs. [14][15][16][17][18][19][20][21][22] in the context of a dense globular star cluster. The second possibility was considered for globular star clusters [23][24][25][26][27][28] and galactic nuclei [29][30][31][32][33] in the loss-cone treatment [34,35].…”