Physical Properties of Tidal Features of Interacting Disk Galaxies: Three-dimensional Self-consistent Models

TL;DR

This study uses 3D N-body simulations to analyze how tidal interactions create and influence non-axisymmetric features like spiral arms and tails in disk galaxies, revealing their formation conditions, evolution, and underlying dynamics.

Contribution

It provides the first detailed 3D self-consistent models of tidal features, exploring the effects of interaction strength, and comparing 3D and thin-disk models on spiral arm properties.

Findings

Tidal tails form when P > 0.05 or S > 0.07.

Stronger interactions produce earlier, more loosely wound tails and arms.

Spiral arms are kinematic density waves with decreasing pattern speed.

Abstract

Using self-consistent three-dimensional (3D) N-body simulations, we investigate the physical properties of non-axisymmetric features in a disk galaxy created by a tidal interaction with its companion. The primary galaxy consists of a stellar disk, a bugle, and a live halo, corresponding to Milky-Way type galaxies, while the companion is represented by a halo alone. We vary the companion mass and the pericenter distance to explore situations with differing tidal strength parameterized by either the relative tidal force P or the relative imparted momentum S. We find that the formation of a tidal tail in the outer parts requires P > 0.05 or S > 0.07. A stronger interaction results in a stronger, less wound tail that forms earlier. Similarly, a stronger tidal forcing produces stronger, more loosely wound spiral arms in the inner parts. The arms are approximately logarithmic in shape, with both amplitude and pitch angle decaying with time. The derived pattern speed decreases with radius and is close to the Omega-kappa/2 curve at late time, with Omega and kappa denoting the angular and epicycle frequencies, respectively. This suggests that the tidally-induced spiral arms are most likely kinematic density waves weakly modified by self-gravity. Compared to the razor-thin counterparts, arms in the 3D models are weaker, have a smaller pitch angle, and wind and decay more rapidly. The 3D density structure of the arms is well described by the concentrated and sinusoidal models when the arms are in the nonlinear and linear regimes, respectively. We demonstrate that dynamical friction between interacting galaxies transfers the orbital angular momentum of one galaxy to the spin angular momentum of the companion halo.