Cosmological hydrodynamical simulations show that mergers with stellar mass ratios above 1:10 drive essentially all of the morphological transformation that turns rotation-supported discs into dispersion-supported spheroids, but in dense cluster environments roughly one-third of the morphological change in intermediate-mass galaxies is not attributable to mergers and must be produced by other environmental processes such as galaxy harassment, ram-pressure stripping, and suppression of cosmological gas accretion. The open problem is to identify and quantify these non-merger mechanisms — which process contributes what fraction, acting how and when along satellite orbits — using simulations with adequate rich-cluster statistics and sufficient resolution, and to validate the merger-driven majority empirically through deep, wide-area, surface-brightness-limited imaging that detects minor mergers and faint tidal features.

Background

Galaxies come in two broad structural families: rotation-supported discs (like the Milky Way) and dispersion-supported spheroids (ellipticals and classical bulges), commonly distinguished by the ratio V/σV/\sigma of ordered rotational to random stellar motions. The Universe was disc-dominated at early epochs yet contains a large spheroid population today, so some physical processes must steadily transform discs into spheroids — the phenomenon called morphological transformation. Galaxy mergers are the classical candidate: a major merger (mass ratio ≥1:4\geq 1:4) violently relaxes the remnant into a spheroid, while a minor merger (ratio between 1:4 and 1:10) builds a dispersion-dominated component more gently. A galaxy's ex-situ stellar mass fraction is the fraction of its stars accreted from other galaxies rather than formed in situ, and is a fossil record of its merger history. Environmental processes offer merger-independent channels: galaxy harassment (repeated high-speed tidal encounters with cluster members), ram-pressure stripping (the pressure of the intracluster gas pushing a galaxy's own gas out), tidal stripping of stars, and suppression of the cosmological gas accretion that would otherwise regrow discs.

Horizon-AGN is a cosmological hydrodynamical simulation of a 100 h−1100\,h^{-1} Mpc box (Dubois et al. 2016) that reproduces the observed galaxy population well, and Martin et al. (2018) used it to quantify the merger contribution to morphological change in massive (M⋆>1010 M⊙M_\star > 10^{10}\,M_\odot) galaxies. They found that mergers with mass ratios above 1:10 drive essentially all the morphological evolution of galaxies that are spheroids at z=0z=0; that minor mergers account for about a third of all morphological transformation and dominate it after z∼1z\sim 1; that retrograde (counter-rotating-orbit) mergers transform more strongly than prograde ones; and that gas-rich mergers routinely regrow discs. However, in the densest environments — the top 10th percentile of local density, corresponding to clusters — mergers explain only about two-thirds of the morphological change in intermediate-mass (1010.510^{10.5}–1011 M⊙10^{11}\,M_\odot) galaxies, leaving a residual one-third that must arise from non-merger processes. Horizon-AGN contains only a single rich, Coma-like cluster, so this residual could not be characterized statistically. The same study acknowledged three further limitations: the origins of two outlier sub-populations (extremely massive discs with high ex-situ fractions, and massive slowly-rotating spheroids with low ex-situ fractions) were deferred to follow-up work; empirical detection of low mass-ratio mergers and faint tidal features is limited by survey surface-brightness depth; and the simulation's ‘∼‘`\sim`1 kpc resolution sets a minimum detectable galaxy mass and forces a sub-grid 'boost factor' in the black-hole accretion prescription to stand in for the unresolved cold interstellar medium. Prior cluster simulations (Choi & Yi 2017) had already concluded that non-merger mechanisms contribute to cluster-galaxy transformation, and subsequent Horizon-AGN work (Choi et al. 2018) showed that non-merger tidal perturbations, not mergers, best explain the spin-down of satellite early-type galaxies — but neither quantified the residual third's mechanism-level budget.

Problem Statement

Which physical processes, with what relative contributions, produce the roughly one-third of morphological change in intermediate-mass (1010.510^{10.5}–1011 M⊙10^{11}\,M_\odot) galaxies in dense (cluster) environments that is not attributable to mergers with stellar mass ratios above 1:10 — and can the merger-driven majority of disc-to-spheroid transformation be confirmed empirically? The question therefore has two linked parts: (1) identifying and quantifying the non-merger environmental mechanisms (galaxy harassment through repeated high-speed encounters, ram-pressure stripping, tidal stripping, and suppression of cosmological gas accretion) in simulations with statistically adequate rich-cluster samples and resolution sufficient to follow galaxy morphology and gas, attributing specific fractions of the morphological change to specific mechanisms with stated uncertainties; and (2) testing the merger-based accounting observationally, with deep, wide-area, surface-brightness-limited imaging and kinematic data and with quantified completeness for low mass-ratio mergers and faint tidal features, at low and high redshift. The simulation-based partition requires empirical confirmation, and the observational signatures require a theoretical attribution framework, so the two are mutually dependent.

References

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