Simulating Core Formation Through Supermassive Black Hole Mergers

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Title: Simulating Core Formation Through Supermassive Black Hole Mergers
Author: Suortti, Joonas
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2020
Language: eng
Thesis level: master's thesis
Degree program: Alkeishiukkasfysiikan ja astrofysikaalisten tieteiden maisteriohjelma
Master's Programme in Particle physics and Astrophysical Sciences
Magisterprogrammet i elementarpartikelfysik och astrofysikaliska vetenskaper
Specialisation: Astrofysikaaliset tieteet
Astrophysical Sciences
De astrofysikaliska vetenskaperna
Discipline: none
Abstract: Core galaxies are bright elliptical galaxies that contain a shallow central surface brightness profile. They are expected to form in mergers of massive gas-poor elliptical galaxies that contain supermas- sive black holes (SMBHs) in their respective centres. During the merger process, these black holes form a coalescing binary, which causes the ejection of stars from the centre of the galaxy merger in complex three-body interactions, resulting in the creation of a low-luminosity core. I have studied whether core galaxies can form according to the formation model described above. I analysed the results of seven galaxy merger simulations done using KETJU, a simulation code specifically made for studying the dynamics of supermassive black holes in galaxies. KETJU is a regularised tree-code, combining both the GADGET-3 tree-code and an AR-CHAIN integrator. This allows for the simultaneous simulation of both general galactic dynamics and accurate particle motion near black holes, respectively. All seven simulations consisted of a merger of two identical galaxies. Six of the simulations had galaxies with equal mass central SMBHs, where the mass of the black holes changed from one simulation to another, and ranged from 8.5 × 10 8 M to 8.5 × 10 9 M . For the sake of comparison, the galaxies in the seventh simulation did not contain SMBHs. The other properties of the merged galaxies were determined in such a way, that the resulting merger remnants would be as similar as possible to the well studied core galaxy NGC 1600. Naturally, these properties were identical across all of the simulation runs. By calculating the surface brightness profiles of the merger remnants in the simulation results, I found out that only simulations that contained SMBHs produced remnants with cores. Furthermore, I identified a clear positive correlation between the size of the core and the mass of the coalescing binary SMBH. Both of these results corroborate the theory, that the cores are formed by interacting SMBH binaries. This interpretation of the results was further enforced by the fact that, according to their velocity anisotropy profiles, stellar orbits near the centre of the remnants were tangentially dominated, implying that stellar particles on more radial orbits had been ejected from the system. I also generated 2D maps of the stellar line-of-sight velocity distributions in the simulated merger remnants. These maps showed kinematic properties similar to observed core galaxies, such as "kinematically distinct cores". Finally, I compared both photometric and kinematic properties of the simulated merger remnant containing the largest SMBH binary to the observed properties of NGC 1600. I found that the simulation and the observations agree well with each other. Since the properties of the simulated merger remnants follow theoretical expectations and is in general good agreement with the obser- vations, I conclude that the formation of the cores in bright elliptical galaxies is likely caused by coalescing binary black holes in dry mergers of elliptical galaxies.

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