A vast population of wandering and merging IMBHs at cosmic noon

Abstract Massive black holes in the centers of galaxies today must have grown by several orders magnitude from seed formed at early times. Detecting a population intermediate mass (IMBHs) can provide constraints on these elusive BH seeds. Here we use large volume, cosmological hydrodynamical simulation Astrid, which includes IMBH seeds and dynamical friction to investigate Dynamical is largely inefficient sinking merging IMBHs high-z. This leads an extensive (several hundred per galaxy) wandering halos z ∼ 2. A small fraction are detectable as HLXs, Hyper Luminous X-ray sources. Importantly, 2, mergers produce peak GW events. We find close million events Astrid between = 2 − 3 involving mergers. These (almost all LISA) cosmic noon should strong models their formation mechanisms. At center massive galaxies, where number be high 10-100, SMBH-IMBH pairs form. Intermediate ratio inspirals (IMRIs) extreme (EMRIs), will require next generation milli-μHz space-based interferometers detected. Large populations around probe environments MBH causal structure.