Multiple dynamical components in Local Group dwarf spheroidals

Abstract

Abstract The dwarf spheroidal (dSph) satellites of the Local Group have long been thought to be simple spheroids of stars embedded within extended dark matter haloes. Recently, however, evidence for the presence of spatially and kinematically distinct stellar populations has been accumulating. Here, we examine the influence of such components on dynamical models of dwarf galaxies embedded in cold dark matter haloes. We begin by constructing a model of Andromeda II, a dSph satellite of M31 which shows evidence for spatially distinct stellar components. We find that the two-component model predicts an overall velocity dispersion profile that remains approximately constant at ∼10−11 km s−1 out to ∼1 kpc from the centre; this is despite wide kinematic and spatial differences between the two individual components. This prediction can be validated by detailed spectroscopic analysis of this galaxy. The presence of two components may also help to explain oddities in the velocity dispersion profiles of other dSphs; we show that velocity dispersion profiles which appear to rise from the centre outwards before levelling off – such as those of Leo I, Draco, and Fornax – can result from the gradual transition from a dynamically cold, concentrated component to a second, hotter, and more spatially extended one, both in equilibrium within the same dark halo. Dwarf galaxies with two stellar components generally have a leptokurtic line-of-sight velocity distribution which is well described by a double Maxwellian. This may be contrasted with other dynamical explanations such as a radially dependent anisotropy in the stars' orbits. Interestingly, we find that multiple equilibrium components could also provide a potential alternative origin for ‘extra-tidal’ stars (normally ascribed to tidal effects) in situations where corroborating evidence for tides – such as elongation of the main body of the dwarf in the orbital direction or velocity gradients across its face driven by protruding tidal tails – may be lacking.