The Tidal Evolution of Local Group Dwarf Spheroidals

TL;DR

This study uses N-body simulations to explore how tidal forces from larger galaxies affect dwarf spheroidal galaxies, revealing their resilience, the impact on observable properties, and implications for their dark matter content and origins.

Contribution

The paper demonstrates that NFW-embedded King models are highly resilient to tides and that tidal evolution primarily depends on total mass loss, providing new insights into dSphs' dark matter dominance and formation scenarios.

Findings

dSphs retain King-like profiles after losing over 99% of stars

Tidal effects cause monotonic decreases in luminosity, velocity dispersion, and surface brightness

Ultra-faint dwarfs are unlikely to be tidal remnants of brighter systems

Abstract

(Abridged) 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 approximated by a King model embedded within an NFW halo. We find that these NFW-embedded King models are extraordinarily resilient to tides; the stellar density profile still resembles a King model even after losing more than 99% of the stars. As tides strip the galaxy, the stellar luminosity, velocity dispersion, central surface brightness, and core radius 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. Interestingly, tides tend to make dSphs more dark-matter dominated because the tightly bound central dark matter ``cusp'' is more resilient to disruption than the ``cored'' King profile. We examine whether the extremely large M/L ratios of the newly-discovered ultra-faint dSphs might have been caused by tidal stripping 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.