Kavli Affiliate: Mark Vogelsberger
| First 5 Authors: Aklant K. Bhowmick, Laura Blecha, Luke Z. Kelley, Aneesh Sivasankaran, Paul Torrey
| Summary:
We analyze the dynamics of low-mass black hole (BH) seeds in the
high-redshift ($zgtrsim5$) Universe using a suite of $[4.5~mathrm{Mpc}]^3$
and $[9~mathrm{Mpc}]^3$ BRAHMA cosmological hydrodynamic simulations. The
simulations form seeds with mass $M_{mathrm{seed}}=2.2times10^3~M_{odot}$ in
halos that exceed critical thresholds of dense & metal-poor gas mass
($5-150~M_{mathrm{seed}}$) and the halo mass ($1000-10000~M_{mathrm{seed}}$).
While the initial BRAHMA boxes pinned the BHs to the halo centers, here we
implement a sub-grid dynamical friction (DF) model. We also compare simulations
where the BH is allowed to wander without the added DF. We investigate the
spatial and velocity offsets of BHs in their host subhalos, as well as BH
merger rates. We find that subgrid DF is crucial to ensure that a significant
fraction of BHs effectively sink to halo centers by $zsim5$, thereby enabling
them to get gravitationally bound and merge with other BHs at separations close
to the spatial resolution ($sim0.2-0.4~rm kpc$) of the simulation. For the
BHs that merge, the associated merger time scales lag between
$sim100-1000~mathrm{Myr}$ after their host halos merge. Compared to
predictions using BH repositioning, the overall $zgtrsim5$ BH merger rates
under subgrid DF decrease by a factor of $sim4-10$. Under subgrid DF, the
different seed models predict merger rates between $sim100-1000$ events per
year at $zgtrsim5$. These mergers dominate early BH growth, assembling BHs up
to $sim10^4-10^5~M_{odot}$ by $zsim5$, wherein $lesssim2~%$ of their mass
is assembled via gas accretion. Our results highlight the promise for
constraining seeding mechanisms using gravitational waves from future
facilities such as the Laser Interferometer Space Antenna.
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