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Resonant stripping as the origin of dwarf spheroidal galaxies


Dwarf spheroidal galaxies are the most dark-matter-dominated systems in the nearby Universe1,2,3 and their origin is one of the outstanding puzzles of how galaxies form. Dwarf spheroidals are poor in gas and stars, making them unusually faint4,5,6, and those known as ultra-faint dwarfs7,8 have by far the lowest measured stellar content of any galaxy9,10. Previous theories11 require that dwarf spheroidals orbit near giant galaxies like the Milky Way, but some dwarfs have been observed in the outskirts of the Local Group12. 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 transforms them into dwarf spheroidals. This effect is distinct from other mechanisms proposed to form dwarf spheroidals, including mergers13, galaxy–galaxy harassment14, or tidal and ram pressure stripping, because it is driven by gravitational resonances. It may account for some of the observed properties of dwarf spheroidals in the Local Group. Within this framework, dwarf spheroidals should form and interact in pairs, leaving detectable long stellar streams and tails.

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Figure 1: Encounters between galaxies.
Figure 2: The time evolution of the dark-mass to luminous-mass ratio.
Figure 3: Radial stellar surface mass density profile of the smaller dwarf.
Figure 4: The time evolution of the structural properties of the small dwarf galaxy.


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This research was partly supported by an EU Marie Curie Intra-European Fellowship under contract MEIF-041569 and by an NSERC postgraduate fellowship. Numerical simulations were performed on the Odyssey supercomputer at Harvard University.

Author Contributions E.D. designed and led the study and analysis, ran the simulations, and wrote the paper; G.B. was involved in study design and contributed to text writing; T.J.C. designed the initial condition programs; and L.H. was involved in study design and contributed to text writing.

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Correspondence to Elena D'Onghia.

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D'Onghia, E., Besla, G., Cox, T. et al. Resonant stripping as the origin of dwarf spheroidal galaxies. Nature 460, 605–607 (2009).

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