Controlled experiments on a Milky-Way-mass galaxy evolved with the EAGLE galaxy-formation model showed that changing the halo assembly history and binding energy without disruptive mergers leaves the central supermassive black hole (SMBH) almost unchanged, because a rotationally supported galactic disc decouples black-hole growth from halo properties, while introducing major mergers disrupts the disc, grows the black hole about fourfold, and quenches star formation. Two parts of that picture remain open: the specific merger characteristics (orbital configuration, gas richness, orbit geometry, morphology of the progenitors) that render a merger disruptive were never separately controlled, and the two-mode regulation picture — secular disc-mediated decoupling versus merger-driven coupling and quenching — has been tested only for a single ∼L⋆\sim L^\star halo, leaving its behaviour at group and cluster halo masses and its predicted high-mass analog, the observed super-spiral galaxies, unverified. The problem is to establish which merger parameters causally control disc disruption and SMBH growth, and whether the regulation modes generalize across halo mass.

Background

Supermassive black holes (SMBHs, black holes of 10610^6--101010^{10} solar masses found at galaxy centres) co-evolve with their host galaxies: their masses correlate with host properties such as stellar mass and velocity dispersion, and the energy released as they accrete — active galactic nucleus (AGN) feedback — is invoked to expel gas from galaxies and halt star formation (quenching). In cosmological hydrodynamical simulations such as EAGLE (a large suite of simulations of galaxy formation in a Lambda cold dark matter universe), population-level analyses find that SMBH masses correlate not only with stellar mass but, at fixed halo mass, with properties of the dark matter halo such as its binding energy (the energy needed to unbind it, denoted EbindE_{\rm bind}) and assembly time, suggesting halo properties govern black-hole growth. Yet galaxy mergers are also known to trigger black-hole growth and AGN activity, so the causal roles of halo properties and individual merger events are hard to separate, particularly since haloes that merge more tend to be more concentrated and more tightly bound.

A technique designed to separate such confounded causes is genetic modification: zoomed cosmological simulations (simulations that re-simulate one halo at high resolution within its cosmological environment) whose initial conditions are altered so that one chosen aspect of the halo's assembly history is changed while everything else is kept as close to the original as possible. Using this technique on a Milky-Way-like galaxy in an ∼L⋆\sim L^\star halo (a halo of roughly the characteristic luminosity scale of galaxies, M200∼1012 M⊙M_{200}\sim10^{12}\,M_\odot, where M200M_{200} is the mass within the radius enclosing 200 times the critical density of the Universe), Davies, Pontzen and Crain (2023) showed that accelerating the halo's assembly (raising its binding energy) without disruptive mergers barely changes the black hole's growth, because gas that cools from the halo settles into a rotationally supported disc whose angular momentum inhibits inflow to the centre. Introducing major mergers instead disrupts the disc, grows the black hole by a factor of about four, and lets AGN feedback expel roughly half the halo's baryons into the circumgalactic medium (the gas surrounding the galaxy within its dark matter halo) and quench star formation. They proposed a two-mode regulation picture: (i) co-regulation by stellar and weak AGN feedback in secularly evolving (changing slowly through internal processes rather than mergers), star-forming disc galaxies, and (ii) AGN-dominated regulation in discless, quenched systems, with mergers providing the transition.

The study, however, controlled only the stellar mass ratio of the mergers; the authors note that other characteristics plausibly governing how disruptive a merger is — the orbital configuration (prograde or retrograde relative to the disc spin), the gas richness of the progenitors, the orbit geometry (impact parameter, spiral-in versus head-on), and progenitor morphology — varied in coupled ways (for example, raising the mass ratio made mergers more head-on). Earlier idealized galaxy-pair simulations, cited in that work, indicate that even 1:1 mergers cause little disruption when gas-rich, prograde, or spiral-in. The technique needed to isolate these parameters — genetic modification of the angular momentum of the galaxy and its progenitors, which Cadiou, Pontzen and Peiris (2021, 2022) showed can predict and control the angular momentum assembled into a halo — was proposed but not yet applied to this question. In parallel, the two-mode picture was established only at the single ∼L⋆\sim L^\star mass scale. Whether it holds at group and cluster halo masses (M200∼1013M_{200}\sim10^{13}--1015 M⊙10^{15}\,M_\odot), and whether the rare observed population of massive (M⋆>1011.3 M⊙M_\star>10^{11.3}\,M_\odot), intensely star-forming super-spiral galaxies represents the predicted high-mass, disc-retaining secular mode, remain untested; the authors proposed a specific controlled experiment, making the assembly of a ∼1013 M⊙\sim10^{13}\,M_\odot halo hosting a quenched spheroid as secular as possible to see whether it becomes a star-forming super-spiral.

Problem Statement

Determine, in the framework of the controlled genetic-modification experiments of Davies, Pontzen and Crain (2023) on an EAGLE-simulated ∼L⋆\sim L^\star galaxy, (i) which characteristics of a galaxy merger — orbital configuration (prograde versus retrograde), gas richness of the progenitors, orbit geometry (impact parameter, spiral-in versus head-on), and progenitor morphology, held at fixed stellar mass ratio and fixed halo assembly history — render the merger disruptive for the galactic gas disc and the growth of the central supermassive black hole, and (ii) whether the resulting two-mode regulation picture (disc-mediated decoupling of black-hole growth from halo properties in secularly evolving systems, versus merger-driven disc disruption, rapid black-hole growth, circumgalactic gas expulsion, and quenching) applies at group and cluster halo masses, including whether observed super-spiral galaxies represent the high-mass analog of the secular, disc-retaining mode. The problem is to identify which merger parameters causally control disc disruption and black-hole growth, with evidence separating the coupled variables, and to establish the halo-mass range over which each regulation mode operates.

References

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