Massive black hole mergers with orbital information: predictions from the ASTRID simulation. Issue 2 (30th May 2022)

• Record Type: Journal Article
• Authors: Chen, Nianyi; Ni, Yueying; Holgado, A Miguel; Matteo, Tiziana Di; Tremmel, Michael; DeGraf, Colin; Bird, Simeon; Croft, Rupert; Feng, Yu
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 514:Issue 2(2022)
• Page Start: 2220
• Page End: 2238
• Publication Date: 2022-05-30
• Subjects: gravitational waves -- methods: numerical -- quasars: supermassive black holes; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac1432
• Languages: English
• ISSNs: 0035-8711

Abstract

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 an MBH seed population between 3 × 10 4 h −1 M⊙ and 3 × 10 5 h −1 M⊙ 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 ${\sim 200\, \text{pc}}$, we find that the in-simulation DF on large scales accounts for more than half of the total orbital decay time ($\sim 500\, \text{Myr}$ ) 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 < 2 M seed ). As a result, only $\lesssim 20{{\rm per \, cent}}$ 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 $\gt 10^9\, {\rm M}_\odot$ . 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 ∼7 higher than the prediction using the circular orbit assumption. The Laser Interferometer Space Antenna events are expected at a similar rate, and comprise $\gtrsim 60\, {\rm{per\, cent}}$ seed-seed mergers, $\sim 30\, {\rm{per\, cent}}$ involving only one seed-mass MBH, and $\sim 10\, {\rm{per\, cent}}$ mergers of non-seed MBHs.