The Destruction of Thin Stellar Disks Via Cosmologically Common Satellite Accretion Events

TL;DR

This study uses high-resolution simulations to show that common satellite accretion events in a cosmological context tend to thicken and heat thin galactic disks, challenging their long-term survival without additional stabilizing factors.

Contribution

It demonstrates through simulations that typical satellite mergers in LCDM cosmology destroy thin stellar disks unless a stabilizing gas component is present.

Findings

Thin disks become roughly three times thicker after satellite impacts.

Disks become more than twice as kinematically hot post-impact.

Survival of thin disks likely requires a stabilizing gas component.

Abstract

Most Galaxy-sized systems (M_host ~ 10^12 M_sun) in the LCDM cosmology are expected to have accreted at least one satellite with a total mass M_sat ~ 10^11 M_sun = 3M_disk in the past 8 Gyr. Analytic and numerical investigations suggest that this is the most precarious type of merger for the survival of thin galactic disks because more massive accretion events are relatively rare and less massive ones preserve thin disk components. We use high-resolution, dissipationless N-body simulations to study the response of an initially-thin, fully-formed Milky-Way type stellar disk to these cosmologically common events and show that the thin disk does not survive. Regardless of orbital configuration, the impacts transform the disks into structures that are roughly three times as thick and more than twice as kinematically hot as the observed dominant thin disk component of the Milky Way. We conclude that if the Galactic thin disk is a representative case, then the presence of a stabilizing gas component is the only recourse for explaining the preponderance of disk galaxies in an LCDM universe; otherwise, the disk of the Milky Way must be uncommonly cold and thin for its luminosity, perhaps as a consequence of an unusually quiescent accretion history.