Black Hole Growth from Cosmological N-body Simulations

TL;DR

This study uses high-resolution cosmological N-body simulations to trace the growth and merger history of intermediate to supermassive black holes from early universe to present, highlighting observable signatures and growth mechanisms.

Contribution

It introduces an analytic model for black hole growth combining galaxy mergers, gas accretion, and coalescence, and provides detailed predictions for black hole merger rates and observable signatures.

Findings

Black hole merger rate peaks at ~25 per year between redshifts 8 and 11.

Most supermassive black hole mass assembly occurs by redshift 4.7 driven by major mergers.

Numerous black holes fail to coalesce, orbiting within dark matter halos and detectable as X-ray sources.

Abstract

(Abridged) We use high resolution cosmological N-body simulations to study the growth of intermediate to supermassive black holes from redshift 49 to zero. We track the growth of black holes from the seeds of population III stars to black holes in the range of 10^3 < M < 10^7 Msun -- not quasars, but rather IMBH to low-mass SMBHs. These lower mass black holes are the primary observable for the Laser Interferometer Space Antenna (LISA). The large-scale dynamics of the black holes are followed accurately within the simulation down to scales of 1 kpc; thereafter, we follow the merger analytically from the last dynamical friction phase to black hole coalescence. We find that the merger rate of these black holes is R~25 per year between 8 < z < 11 and R = 10 per year at z=3. Before the merger occurs the incoming IMBH may be observed with a next generation of X-ray telescopes as a ULX source with a rate of about ~ 3 - 7 per year for 1 < z < 5. We develop an analytic prescription that captures the most important black hole growth mechanisms: galaxy merger-driven gas accretion and black hole coalescence. Using this, we find that SMBH at the center of Milky Way type galaxy was in place with most of its mass by z = 4.7, and most of the growth was driven by gas accretion excited by major mergers. Hundreds of black holes have failed to coalesce with the SMBH by z=0, some with masses of 10000 Msun, orbiting within the dark matter halo with luminosities up to ~ 30000 Lsun. These X-ray sources can easily be observed with Chandra at ~ 100 kpc.