Massive Black Hole Mergers with Orbital Information: Predictions from the ASTRID Simulation

Abstract

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 $3\times 10^4{M}_{\odot }/h$ and $3\times 10^5{M}_{\odot }/h$ 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 $\lesssim 20\%$ 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.

17 pages, 13 Figures; comments are welcome

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• Modelling the accretion and feedback of supermassive black hole binaries in gas-rich galaxy mergers
• Orbital and Radiative Properties of Wandering Intermediate-Mass Black Holes in the ASTRID Simulation
• RABBITS -- II. The impact of AGN feedback on coalescing supermassive black holes in disc and elliptical galaxy mergers
• Wandering and escaping: recoiling massive black holes in cosmological simulations
• The ASTRID Simulation at z=0: From Massive Black Holes to Large-scale Structure
• Gravitational Waves from Massive Black Hole Mergers in ASTRID: Predictions for LISA
• A Sea of Black Holes: Characterizing the LISA Signature for Stellar-Origin Black Hole Binaries
• Premature supermassive black hole mergers in cosmological simulations of structure formation
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• First Light And Reionization Epoch Simulations (FLARES) -- XIX: Supermassive black hole mergers in the early Universe and their environmental dependence