Intermediate Mass Black Hole Formation in compact Star Clusters Francesco Paolo Rizzuto, Thorsten Naab, Rainer Spurzem, Mirek Giersz, J. P. Ostriker, N. C. Stone, Long Wang, Peter Berczik, M. Rampp.

1. Compact Star Clusters 2. Direct N-body Simulations of Compact Star Clusters Compact star clusters are a promising environment for the formation of massive black holes and very massive stars. We explored the formation of massive objects in dense star clusters with the help of direct N-body simulations. We evolved 80 massive young Due to their small sizes and high central density, massive object (such as stellar black holes and massive stars) sink clusters, with different sizes and densities using the code NBODY6++GPU. With this code, we follow the stellar and dynamical evolution of each rapidly in the inner region where they can increase their mass through physical collisions and mass transfer events. star. Our simulations show that in a dense stellar environment, it is not uncommon to form black holes and very massive stars of a few hundred solar masses through runaway collisions.

R = 1.0 pc R = 0.6 pc 500 h h W0 = 7 W0 = 8 W0 = 9 W0 = 10 W0 = 6 W0 = 7 W0 = 8 W0 = 9

400

300 ] M [ M 200

100

01.0 × 105 4.0 × 105 3.0 × 106 1.8 × 107 1.1 × 105 5.0 × 105 3.0 × 106 3.0 × 107 3 3 c [M /pc ] c [M /pc ]

3. Triple Interactions: the trigger of runaway collisions 4. Intermediate Mass Black Hole Formation Through multiple black Hole Our most compact models register about 300 collisions within 300 Myr, 40% of which happen in the first 15 Myr. Most of these Collisions collisions are triggered by three-body scattering events between a hard binary and a third particle. These interactions have the In one of our simulations an intermediate mass black holes grows its mass almost entirely by swallowing other black holes. The overall effect of increasing the binding energy of the binaries and they can also raise their eccentricity. formation process occurred in about ∼ 90 Myr and it involves a chain of low mass BH mergers with masses of 17 : 28, 25 : 45, 68 : 70

e = 0.002 e = 0.002 e = 0.446 . This mechanism is particularly interesting because it provides a possible formation path of GW190521, the last LIGO/VIRGO 3.24 Myr 1 3.25 Myr 1 3.27 Myr 1 p1 = 52.3 R p1 = 67.6 R p1 = 37.5 R detection.

e2 = 1.020 e3 = 0.999

p2 = 20.1 R p3 = 31.8 R

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