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 $5\times10^{4} M_{\odot}$, 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 $M_{\mathrm{BH}}\sim10^{5-6}\ M_\odot$, low-redshift ($z<2$) and low mass-ratio ($q\sim0.01$-$0.1$) mergers. Accounting for eccentricity broadens the detectable MBH mass range up to $10^{9} M_\odot$ and shifts the peak of detectable mergers to a lower redshift $z_{\rm peak}=0.8$. 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 $L_{\rm bol}>10^{42}$ 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 $M_{\rm BH}>10^{8} M_\odot$.