Resonant stripping as the origin of dwarf spheroidal galaxies

Elena D'Onghia,Thomas J Cox,Lars Hernquist,Gurtina Besla

doi:10.1038/nature08215

Elena D'Onghia, Thomas J Cox + Show 2 more

Open Access

https://doi.org/10.1038/nature08215

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Abstract

Dwarf spheroidal galaxies are the most dark matter dominated systems in the nearby Universe and their origin is one of the outstanding puzzles of how galaxies form. Dwarf spheroidals are poor in gas and stars, making them unusually faint, and those known as ultra-faint dwarfs have by far the lowest measured stellar content of any galaxy. Previous theories require that dwarf spheroidals orbit near giant galaxies like the Milky Way, but some dwarfs have been observed in the outskirts of the Local Group. Here we report simulations of encounters between dwarf disk galaxies and somewhat larger objects. We find that the encounters excite a process, which we term ``resonant stripping'', that can transform them into dwarf spheroidals. This effect is distinct from other mechanisms proposed to form dwarf spheroidals, including mergers, galaxy-galaxy harassment, or tidal and ram pressure stripping, because it is driven by gravitational resonances. It may account for the observed properties of dwarf spheroidals in the Local Group, including their morphologies and kinematics. Resonant stripping predicts that dwarf spheroidals should form through encounters, leaving detectable long stellar streams and tails.

Highlights

Previous theories[11] require that dwarf spheroidals orbit near giant galaxies like the Milky Way, but some dwarfs have been observed in the outskirts of the Local Group.[12]
We find that the encounters excite a process, which we term “resonant stripping”, that can transform them into dwarf spheroidals
This effect is distinct from other mechanisms proposed to form dwarf spheroidals, including mergers,[13] galaxy-galaxy harassment,[14] or tidal and ram pressure stripping, because it is driven by gravitational resonances

Summary

Numerical Methods

The simulations were carried out with GADGET3, a parallel TreePM-Smoothed particle hydrodynamics (SPH) code developed to compute the evolution of stars and dark matter, which are treated as collisionless fluids. GADGET3 is a cosmological code in which the gravitational field on large scales is calculated with a particle-mesh (PM) algorithm, while the short-range forces are computed using a tree-based hierarchical multipole expansion, resulting in an accurate and fast gravitational solver. The scheme combines the high spatial resolution and relative insensitivity to clustering of tree algorithms with the speed and accuracy of the PM method to calculate the long range gravitational field. We measure the total mass of the dark halo of each galaxy using the SUBFIND algorithm.[32] This code identifies as halo members particles in groups that are gravitationally self-bound and are overdense with respect to the local background

Setting the initial conditions

12 D’Onghia et al 3 Dark Halo

Findings

Numerical Tests