The ASTRID simulation: the evolution of supermassive black holes

Abstract

ABSTRACTWe present the evolution of black holes (BHs) and their relationship with their host galaxies in Astrid, a large-volume cosmological hydrodynamical simulation with box size 250 h−1Mpc containing 2 × 55003 particles evolved to z = 3. Astrid statistically models BH gas accretion and active galactic nucleus (AGN) feedback to their environments, applies a power-law distribution for BH seed mass $\, M_{\rm sd}$, uses a dynamical friction model for BH dynamics, and executes a physical treatment of BH mergers. The BH population is broadly consistent with empirical constraints on the BH mass function, the bright end of the luminosity functions, and the time evolution of BH mass and accretion rate density. The BH mass and accretion exhibit a tight correlation with host stellar mass and star formation rate. We trace BHs seeded before z > 10 down to z = 3, finding that BHs carry virtually no imprint of the initial $\, M_{\rm sd}$ except those with the smallest $\, M_{\rm sd}$, where less than 50 per cent of them have doubled in mass. Gas accretion is the dominant channel for BH growth compared to BH mergers. With dynamical friction, Astrid predicts a significant delay for BH mergers after the first encounter of a BH pair, with a typical elapse time of about 200 Myr. There are in total 4.5 × 105 BH mergers in Astrid at z > 3, ∼103 of which have X-ray detectable EM counterparts: a bright $\, {\rm kpc}$ scale dual AGN with LX > 1043 erg s−1. BHs with $\, M_{\rm BH}\sim 10^{7\!-\!8} \, M_{\odot }$ experience the most frequent mergers. Galaxies that host BH mergers are unbiased tracers of the overall $\, M_{\rm BH}\!-\! M_*$ relation. Massive ($\gt 10^{11}\, M_{\odot }$) galaxies have a high occupation number (≳10) of BHs, and hence host the majority of BH mergers.