Tidal debris of dwarf spheroidals as a probe of structure formation models

Abstract

Recent observations suggest that Carina and other nearby dwarf spheroidal galaxies are surrounded by unbound stars tidally stripped by the Milky Way. We run high-resolution N-Body simulations of dwarf galaxies orbiting within the Milky Way halo to determine if such observations can be explained with dark matter potentials as those implied by current structure formation models. We show that tidal forces acting on dwarfs with constant density cores or with cuspy profiles having a low concentration parameter ($c < 5$) lead to flat outer stellar density profiles like that of Carina for a variety of orbital configurations. On the contrary, it is more difficult to remove stars from cuspy dark matter halos with concentrations as high as predicted by CDM models at the mass scale of dwarf galaxies ($c \simgt 10$) and the data can only be reproduced assuming nearly radial orbits. Our simulations show that Carina is losing mass at a fractional rate $< 0.1$ Gyr$^{-1}$ and its mass-to-light ratio could be inflated by at most a factor of 2 due to unbound stars projected along the line of sight. We follow the evolution of the tidal debris within a triaxial clumpy cold dark matter Milky Way halo which causes differential precession and small scale heating of the stellar streams. This renders their use as a dynamical tracer of the Galactic potential practically useless, but does provide a novel test of the nature of the dark matter. Models with warm dark matter (WDM) or fluid dark matter (FDM) produce dwarf halos with lower central densities than CDM and would be consistent with the observed tidal tails even for orbits with eccentricities as low as indicated by current data on nearby dwarf spheroidals. Galactic halos in FDM are smooth and spherical and would be favored by the detection of coherent streams.