SCYON Abstract

Received on December 10 2007

The Tidal Evolution of Local Group Dwarf Spheroidals

Authors	Jorge Penarrubia, Julio F. Navarro, and Alan W. McConnachie
Affiliation	Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Rd., Victoria, BC, V8P 5C2, Canada
Accepted by	Astrophysical Journal
Contact	jorpega@uvic.ca
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Abstract

We use N-body simulations to study the evolution of dwarf spheroidal galaxies (dSphs) driven by galactic tides. We adopt a cosmologically-motivated model where dSphs are dark matter-dominated systems on eccentric orbits whose stellar component may be approximated by a King model embedded within an NFW halo. We find that these NFW-embedded King models are extraordinarily resilient to tides; the density profile of the stellar component still resembles a King model even after losing more than 99% of the stars. As tides strip the galaxy, the stellar luminosity, L, velocity dispersion, σ₀, central surface brightness, Σ₀, and core radius, R_c, decrease monotonically. Remarkably, we find that the evolution of these parameters is solely controlled by the total amount of mass lost from within the luminous radius. Of all parameters, the core radius is the least affected: after losing 99% of the stars, R_c decreases by just a factor of ~2, implying that even in the event of extreme mass loss the core radius is a robust measure of the original size of the system. Contrary to naive expectations, tides tend to make dSphs more dark-matter dominated. This is because the tightly bound central dark matter "cusp" is more resilient to disruption than the comparatively more loosely bound "cored" King profile. We examine whether tidal effects may help to explain the extremely large mass-to-light ratios of some of the newly-discovered ultra-faint Milky Way dwarfs as tidal remnants of once brighter systems. Although dSph tidal evolutionary tracks parallel the observed scaling relations in the luminosity-radius plane, they predict too steep a change in velocity dispersion compared with the observational estimates hitherto reported in the literature. The ultra-faint dwarfs are thus unlikely to be the tidal remnants of systems like Fornax, Draco, or Sagittarius. Despite spanning four decades in luminosity, dSphs appear to inhabit halos of comparable peak circular velocity, lending support to scenarios that envision dwarf spheroidals as able to form only in halos above a certain mass threshold.