Massive Black Hole Mergers with Orbital Information: Predictions from
  the ASTRID Simulation
  We examine massive black hole (MBH) mergers and their associated
gravitational wave signals from the large-volume cosmological simulation
Astrid. Astrid includes galaxy formation and black hole models recently updated
with a MBH seed population between $3\times 10^4M_{\odot}/h$ and $3\times
10^5M_{\odot}/h$ and a sub-grid dynamical friction (DF) model to follow the MBH
dynamics down to $1.5\;\text{ckpc}/h$. We calculate initial eccentricities of
MBH orbits directly from the simulation at kpc-scales, and find orbital
eccentricities above $0.7$ for most MBH pairs before the numerical merger.
After approximating unresolved evolution on scales below ${\sim
200\,\text{pc}}$, we find that the in-simulation DF on large scales accounts
for more than half of the total orbital decay time ($\sim 500\,\text{Myrs}$)
due to DF. The binary hardening time is an order of magnitude longer than the
DF time, especially for the seed-mass binaries ($M_\text{BH}<2M_\text{seed}$).
As a result, only $\lesssim20\%$ of seed MBH pairs merge at $z>3$ after
considering both unresolved DF evolution and binary hardening. These $z>3$
seed-mass mergers are hosted in a biased population of galaxies with the
highest stellar masses of $>10^9\,M_\odot$. With the higher initial
eccentricity prediction from Astrid, we estimate an expected merger rate of
$0.3-0.7$ per year from the $z>3$ MBH population. This is a factor of $\sim 7$
higher than the prediction using the circular orbit assumption. The LISA events
are expected at a similar rate, and comprise $\gtrsim 60\%$ seed-seed mergers,
$\sim 30\%$ involving only one seed-mass MBH, and $\sim 10\%$ mergers of
non-seed MBHs.
