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 an MBH seed population between 3 × 104h-1M? and 3 × 105h-1M? and a sub-grid dynamical friction (DF) model to follow the MBH dynamics down to 1.5 ckpc h-1;. We calculate the 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 ? 200, we find that the in-simulation DF on large scales accounts for more than half of the total orbital decay time ?500 due to DF. The binary hardening time is an order of magnitude longer than the DF time, especially for the seed-mass binaries (MBH < 2Mseed). As a result, only ? 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 > 109M?. 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 ?7 higher than the prediction using the circular orbit assumption. The Laser Interferometer Space Antenna events are expected at a similar rate, and comprise ?60 per cent seed-seed mergers, ? 30 per cent involving only one seed-mass MBH, and ? 10 per cent mergers of non-seed MBHs.