Tidal evolution of disky dwarf galaxies in the Milky Way potential: the formation of dwarf spheroidals

TL;DR

This study uses high-resolution N-body simulations to explore how dwarf galaxies evolve tidally in the Milky Way, transforming from disks into spheroidals and maintaining dark matter dominance, providing insights into their morphological changes.

Contribution

It offers a detailed, quantitative explanation of the tidal transformation mechanism of dwarf galaxies, including the formation of dwarf spheroidals through bar shortening, based on simulations with varying initial disk inclinations.

Findings

Dwarfs undergo significant mass loss and morphological transformation over 10 Gyr.

The evolution is fastest when the disk is coplanar with the orbit.

Dwarfs remain dark matter dominated and stellar velocity dispersion traces halo properties.

Abstract

We conduct high-resolution collisionless N-body simulations to investigate the tidal evolution of dwarf galaxies on an eccentric orbit in the Milky Way (MW) potential. The dwarfs originally consist of a low surface brightness stellar disk embedded in a cosmologically motivated dark matter halo. During 10 Gyr of dynamical evolution and after 5 pericentre passages the dwarfs suffer substantial mass loss and their stellar component undergoes a major morphological transformation from a disk to a bar and finally to a spheroid. The bar is preserved for most of the time as the angular momentum is transferred outside the galaxy. A dwarf spheroidal (dSph) galaxy is formed via gradual shortening of the bar. This work thus provides a comprehensive quantitative explanation of a potentially crucial morphological transformation mechanism for dwarf galaxies that operates in groups as well as in clusters. We compare three cases with different initial inclinations of the disk and find that the evolution is fastest when the disk is coplanar with the orbit. Despite the strong tidal perturbations and mass loss the dwarfs remain dark matter dominated. For most of the time the 1D stellar velocity dispersion, \sigma, follows the maximum circular velocity, V_{\rm max}, and they are both good tracers of the bound mass. Specifically, we find that M_{\rm bound} \propto V_{\rm max}^{3.5} and V_{\rm max} \sim \sqrt{3} \sigma in agreement with earlier studies based on pure dark matter simulations. The latter relation is based on directly measuring the stellar kinematics of the simulated dwarf and may thus be reliably used to map the observed stellar velocity dispersions of dSphs to halo circular velocities when addressing the missing satellites problem.