The difficult path to coalescence: massive black hole dynamics in merging low-mass dark matter haloes and 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 103 - 107 M☉ in multiple minor mergers of low-mass dark matter haloes without and with galaxies (4 × 108 M☉ ≲ Mhalo ≲ 2 × 1010 M☉). 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 (≲ 105 M☉) 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 ≳ 106 M☉ 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.