Transformations of galaxies. III. Encounter dynamics and tidal response as functions of galaxy structure

TL;DR

This study investigates how galaxy structure influences tidal interactions and encounter dynamics, revealing that halo properties and velocity ratios significantly affect orbital evolution, tidal features, and merger outcomes.

Contribution

It introduces a comprehensive grid of galaxy models and simulations to elucidate the impact of structure on tidal responses and encounter dynamics.

Findings

Halo mass and extent influence orbit evolution.

Reversal of orbital motion can occur due to halo dynamics.

Tidal morphology constrains progenitor galaxy structure.

Abstract

Tidal interactions between disc galaxies depend on galaxy structure, but the details of this relationship are incompletely understood. I have constructed a three-parameter grid of bulge/disc/halo models broadly consistent with $\Lambda$CDM, and simulated an extensive series of encounters using these models. Halo mass and extent strongly influence the dynamics of orbit evolution. In close encounters, the transfer of angular momentum mediated by the dynamical response of massive, extended haloes can reverse the direction of orbital motion of the central galaxies after their first passage. Tidal response is strongly correlated with the ratio $v_\mathrm{e} / v_\mathrm{c}$ of escape to circular velocity within the participating discs. Moreover, the same ratio also correlates with the rate at which tidal tails are reaccreted by their galaxies of origin; consequently, merger remnants with `twin tails', such as NGC 7252, may prove hard to reproduce unless $(v_\mathrm{e} / v_\mathrm{c})^2 \lesssim 5.5$. The tidal morphology of an interacting system can provide useful constraints on progenitor structure. In particular, encounters in which halo dynamics reverses orbital motion exhibit a distinctive morphology which may be recognized observationally. Detailed models attempting to reproduce observations of interacting galaxies should explore the likely range of progenitor structures along with other encounter parameters.