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 x 10(4)h(-1) M-circle dot and 3 x 10(5)h(-1)M(circle dot) 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 similar to 200 pc, we find that the in-simulation DF on large scales accounts for more than half of the total orbital decay time (similar to 500 Myr) due to DF. The binary hardening time is an order of magnitude longer than the DF time, especially for the seed-mass binaries (M-BH < 2M(seed)). As a result, only less than or similar to 20per cent 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-circle dot. 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 similar to 7 higher than the prediction using the circular orbit assumption. The Laser Interferometer Space Antenna events are expected at a similar rate, and comprise greater than or similar to 60 per cent seed-seed mergers, similar to 30 per cent involving only one seed-mass MBH, and similar to 10 per cent mergers of non-seed MBHs.

Automated summary

In this study, we investigate the merging of massive black holes (MBHs) and their associated gravitational wave signals using the cosmological simulation Astrid. We find that only a small percentage of seed MBH pairs merge at high redshifts after considering unresolved dynamical friction (DF) evolution and binary hardening. With the higher initial eccentricity predicted by Astrid, we estimate a higher merger rate for the z > 3 MBH population compared to previous predictions.