Gravitational Waves from Massive Black Hole Mergers in ASTRID: Predictions for LISA

Abstract We use the ASTRID cosmological simulation to forecast massive black hole (MBH) mergers detectable by Laser Interferometer Space Antenna (LISA) down to z = 0. ASTRID directly models MBH dynamical friction, allowing a realistic tracking of their trajectory. It also incorporates relatively low-mass MBH seeds down to 5 × 10 4 M ⊙ , providing a more complete picture of LISA MBH mergers. We find that LISA MBH mergers initially have high eccentricities, peaking around e 0 = 0.8 across all redshifts. Accounting for this boosts the event rate from 5.6 yr −1 (if circular orbits are assumed) to 10.5 yr −1 . This enhancement is largely due to additional inspiral sources that will coalesce after LISA’s observation, which constitute 46% of detected events. This underscores the importance of LISA’s sensitivity to the early inspiral phase, especially for eccentric binaries that emit gravitational waves across a wider frequency band. Most LISA events in ASTRID arise from M BH ∼ 10 5−6 M ⊙ , low-redshift ( z < 2) and low-mass-ratio ( q ∼ 0.01–0.1) mergers. Accounting for eccentricity broadens the detectable MBH mass range up to 10 9 M ⊙ and shifts the peak of detectable mergers to a lower redshift z peak = 0.8. This implies that the most massive LISA events may also be PTA sources. We predict LISA events to be in various galaxy environments, including many low-mass satellite galaxies. The electromagnetic (EM) counterparts of most LISA sources have active galactic nuclei (AGN) luminosities L bol > 10 42 erg s −1 , albeit only 1% with >10 44 erg s −1 . The brightest AGN are those associated with the rare LISA/PTA events with M BH > 10 8 M ⊙ .