Internal kinematics of dwarf spheroidal galaxies.

Abstract

In a Universe that grows its galactic structure via mergers, the smallest galaxies serve as raw materials. The dwarf spheroidal (dSph) galaxies in the Local Group are the smallest and faintest galaxies known, and are likely the surviving members of the population of primordial galaxies that merged to build the Milky Way. Their resolved stellar populations provide not only a wealth of information regarding the formation and evolution of the Local Group, but also the best available window for studying the distribution of dark matter at its (empirically) smallest scale. We present observations, results and analysis from a large spectroscopic survey we have undertaken using the Michigan/MIKE Fiber System (MMFS) at the Magellan/Clay 6.5 m telescope at Las Campanas Observatory. Our sample is the largest of its kind, including 6415 high-resolution (<italic>R</italic> = 20000--25000, near the magnesium triplet &sim; 5160 A) spectra from 5180 stars in four dSphs: Carina, Fornax, Sculptor and Sextans. We measure line-of-sight velocity and [Fe/H] abundance with median precision +/-2.0 km s<super>-1</super> and +/-0.2 dex, respectively, for stars as faint as <italic>V</italic> &sim; 20.5. Despite order-of-magnitude variation in baryonic content, all the measured dSphs have stellar velocity dispersions of order 10 km s<super>-1</super> that remain approximately constant with radius. Equilibrium models imply dSph masses of &sim; 10<super>8</super> M&odot; , with no obvious correlation to luminous mass, and M/L<italic>_V</italic> ratios that range from 10--1000. The universal dark matter density profile motivated by cosmological simulations produces acceptable fits to the velocity data and implies mass profiles similar to those we derive using a non-parametric estimation technique, but the lack of a constraint on anisotropy prevents us from ruling out competing models. Our results are consistent with the notion that most, and perhaps all dSphs reside in dark halos of similar mass. We demonstrate that any kinematically significant perturbation due to tidal forces is limited to the &sim; 10% of sampled stars farthest from the dSph center. The ratio of metal-rich to metal-poor stars decreases with radius in each dSph, and metal-poor stars tend to have slightly higher velocity dispersion than metal-rich stars.