Tracing black hole and galaxy growth across environments since cosmic noon

Abstract

The distribution of systems in the black hole (BH) mass-stellar mass ($M_\mathrm{BH}-M_\star$) plane encodes both the integrated growth of galaxies and their central black holes, and the processes that shape their evolution. Using the ASTRID and TNG300 cosmological simulations, we track massive BHs from cosmic noon ($z=2$) to $z=0.5$. TNG300 repositions BHs to the centers of their host galaxies at every time step, while ASTRID instead advances them under the resolved gravitational forces plus a subgrid dynamical friction model, allowing BHs to wander off-center when orbital decay is inefficient. We follow the BHs rather than their original host galaxies, thereby capturing central BHs, BHs in satellites, and wandering BHs. We find that central BHs in both simulations evolve along a tight, nearly redshift-invariant $M_\mathrm{BH}-M_\star$ relation that is broadly consistent with local empirical constraints. Departures from this relation trace distinct evolutionary channels in which mergers play a key role. Major galaxy mergers drive BH-BH coalescence that dominates the growth of the most massive central BHs. These BHs subsequently quench their hosts through active galactic nucleus (AGN) kinetic feedback. Minor mergers tidally strip satellites to lower $M_\star$ at nearly fixed $M_\mathrm{BH}$, producing weakly accreting, overmassive central BHs in gas-poor systems. In ASTRID, satellite accretion and inefficient dynamical friction generate wandering BHs that are undermassive relative to their new hosts and experience minimal accretion or merger-driven growth. A BH's location in the $M_\mathrm{BH}-M_\star$ and specific BH accretion rate-specific star formation rate planes is therefore a fossil record of its dynamical, accretion, merger, and feedback history.