MAGICS I. The First Few Orbits Encode the Fate of Seed Massive Black Hole Pairs
Nianyi Chen, Diptajyoti Mukherjee, Tiziana Di Matteo, Yueying Ni, Simeon Bird, Rupert A. C. Croft
Abstract
The elusive massive black hole (MBH) seeds stand to be revealed by the Laser Space Antenna Interferometer through mergers. As an aftermath of galaxy mergers, MBH coalescence is a vastly multi-scale process connected to galaxy formation. We introduce the “Massive black hole Assembly in Galaxies Informed by Cosmological Simulations” (MAGICS) suite, with galaxy/MBH properties and orbits recovered from large-volume cosmological simulation ASTRID. The simulations include subgrid star formation, supernovae feedback, and MBH accretion/feedback. In this first suite, we extract fifteen representative galaxy mergers with seed MBHs to examine their dynamics at an improved mass and spatial resolution (by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>2000</mml:mn> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>20</mml:mn> </mml:mrow> </mml:math> ) and follow MBH orbits down to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>10</mml:mn> <mml:mspace width="0.167em"/> <mml:mtext mathvariant="normal">pc</mml:mtext> </mml:mrow> </mml:math> . We find that the seed MBH energy loss and orbital decay are largely governed by global torques induced by the galaxy merger process on scales resolvable by cosmological simulations. Specifically, pairs sink quickly if their orbits shrink rapidly below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mspace width="0.167em"/> <mml:mtext mathvariant="normal">kpc</mml:mtext> </mml:mrow> </mml:math> during the first <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>200</mml:mn> <mml:mspace width="0.167em"/> <mml:mtext mathvariant="normal">Myr</mml:mtext> </mml:mrow> </mml:math> of pairing due to effective energy loss in major galaxy mergers, whereas MBHs gaining energy in minor galaxy mergers with head-on collisions are likely to stall. High initial eccentricities ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mi>e</mml:mi> <mml:mtext mathvariant="normal">init</mml:mtext> </mml:msub> <mml:mo>></mml:mo> <mml:mn>0.5</mml:mn> </mml:mrow> </mml:math> ) and high stellar densities at kpc scales ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mi>ρ</mml:mi> <mml:mtext mathvariant="normal">star</mml:mtext> </mml:msub> <mml:mo>></mml:mo> <mml:mn>0.05</mml:mn> <mml:mspace width="0.167em"/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mi>/</mml:mi> <mml:msup> <mml:mtext mathvariant="normal">pc</mml:mtext> <mml:mn>3</mml:mn> </mml:msup> </mml:mrow> </mml:math> ) also lead to most efficient decays. <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>50</mml:mn> <mml:mi>%</mml:mi> </mml:mrow> </mml:math> high-redshift seed MBH pairs experience consecutive galaxy mergers and are more likely to stall at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>1</mml:mn> <mml:mspace width="0.167em"/> <mml:mtext mathvariant="normal">kpc</mml:mtext> </mml:mrow> </mml:math> . For a subset of systems, we carry out N-Body re-simulations until binary formation and find that some stalled systems merge at high-z when embedded in sufficient nuclear star clusters.