Oral Presentation

Large-scale parameter search of stellar merger simulations for refinement of intermediate mass black holes formation model

Author(s): Toshinori Yamauchi (The University of Tokyo); Ataru Tanikawa (The University of Tokyo); Takeru Suzuki (The University of Tokyo); Kazumi Kashiyama (The University of Tokyo)

Presenter: Toshinori Yamauchi (The University of Tokyo)

A supermassive black hole (SMBH) has more than 1 billion solar masses. Recent observations have revealed that SMBHs exists in the early universe whose age is less than 1 billion years. The detail formation process of such SMBHs has been under debate. One possible scenario is that intermediate mass black holes (IMBHs), whose mass range is 100 to 10000 solar masses, are formed through runaway mergers of stars in dense star clusters, and grow to SMBHs. In order to investigate the formation of IMBHs, several N-body simulations have followed such runaway mergers in dense star clusters (e.g. Portegies Zwart et al. 2004; Fujii et al. 2009; Sakurai et al. 2017). However, they have oversimplified the merger conditions of two stars. Therefore, we clarified the merger conditions by performing fluid numerical simulations, and investigated features of merger remnants. We used SPH code parallelized to OpenMP and MPI by FDPS (Iwasawa et al. 2016). Also, we accelerated calculation of particle-particle interactions using GPU. We prepared two polytropic spheres that are dynamically stable as an initial condition. We performed a lot of merging experiments, adapting various values for stellar masses (M_1 and M_2), relative velocities at infinity (v_∞), periapsis distances between two stars (r_p), and polytropic indices (n), where we fix the mass ratio of two stars to be unity. We found that two stars merge when r_p=1.3(R_1+R_2) to r_p=1.9(R_1+R_2), depending on M_1 (or M_2), v_∞, and n, where R_1 and R_2 are radii of two stars. Two stars merge more easily with M_1 (or M_2) increasing, v_∞ decreasing, and n decreasing. We also investigate the density distribution of a merger remnant, and found that its radius is more than 10 times larger than a polytropic sphere with the same mass. In this presentation, we will report the results of our numerical simulations, and discuss the formation of IMBHs in dense star clusters.