Published Article: The difficult path to coalescence: massive black hole dynamics in merging low-mass dark matter haloes and galaxies
In this article we systematically study the dynamical sinking of massive black holes in low mass galaxies using the KETJU code. Our idealized high-resolution study highlights the difficulty to bring in and keep low-mass MBHs in the centres of low-mass haloes/galaxies.
Abstract: We present a high-resolution numerical study of the sinking and merging of massive black holes (MBHs) with masses in the range of \(10^3 - 10^7 \, \mathrm{M}_\odot\) in multiple minor mergers of low-mass dark matter haloes without and with galaxies \((4\times 10^8 \, \mathrm{M}_\odot \lesssim {M}_{\mathrm{halo}} \lesssim 2\times 10^{10} \, \mathrm{M}_\odot)\). The KETJU simulation code, a combination of the GADGET tree solver with accurate regularized integration, uses unsoftened forces between the star/dark matter components and the MBHs for an accurate treatment of dynamical friction and scattering of dark matter/stars by MBH binaries or multiples. Post-Newtonian corrections up to order 3.5 for MBH interactions allow for coalescence by gravitational wave emission and gravitational recoil kicks. Low-mass MBHs \((\lesssim 10^5 \, \mathrm{M}_\odot)\) hardly sink to the centre or merge. Sinking MBHs have various complex evolution paths - binaries, triplets, free-floating MBHs, and dynamically or recoil ejected MBHs. Collisional interactions with dark matter alone can drive MBHs to coalescence. The highest mass MBHs of \(\gtrsim 10^6 \, \rm M_\odot\) mostly sink to the centre and trigger the scouring of dark matter and stellar cores. The scouring can transform a centrally baryon-dominated system into a dark-matter-dominated system. Our idealized high-resolution study highlights the difficulty to bring in and keep low-mass MBHs in the centres of low-mass haloes/galaxies - a remaining challenge for merger assisted MBH seed growth mechanisms.