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

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.

6 pages, 4 figures; accepted by ApJL; high-resolution version and example movie of simulation available at http://web.me.com/purcellgalaxies/workshop/log/Entries/2008/10/15_The_Destruction_of_thin_stellar_disks_via_cosmologically_common_mergers.html

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• The Dawn of Disk Formation in a Milky Way-sized Galaxy Halo: Thin Stellar Disks at $z>4$
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• Resolved SPLASH Chemodynamics in Andromeda's PHAT Stellar Halo and Disk: On the Nature of the Inner Halo Along the Major Axis
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• The fragility of thin discs in galaxies -- II. Thin discs as tracers of the assembly history of galaxies
• On the Size Evolution of a Galactic Disk in Hierarchical Merging of Cold Dark Matter Halos
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