Dark matter cores and cusps: the case of multiple stellar populations in dwarf spheroidals

Abstract

A number of dwarf spheroidal (dSph) galaxies are known to contain a more extended, metalpoor population with a flattish velocity dispersion profile, and a more concentrated, metal-rich population with a velocity dispersion declining with radius. The two populations can be modelled with Michie-King distribution functions (DFs) in the isothermal and in the sharply truncated limits, respectively. We argue that the truncation of the metal-rich population can be traced back to the spatial distribution of the star-forming gas. Suppose δ is the exponent of the first non-constant term in the Taylor expansion of the total potential at the centre (δ = 1 for Navarro-Frenk-White or NFW haloes, δ = 2 for cored haloes). Then, we show that the ratio of the half-light radii of the populations R δ/2 h,2 /R h,1 δ/2 must be smaller than the ratio of the line-of-sight velocity dispersions σ los,2 (R h,2 )/σ los,1 (R h,1 ).Specializing to the case of the Sculptor dSph, we develop a technique to fit simultaneously both populations with Michie-King DFs. This enables us to determine the mass profile of the Sculptor dSph with unprecedented accuracy in the radial range 0.2 < r < 1.2 kpc. We show that cored halo models are preferred over cusped halo models, with a likelihood ratio test rejecting NFW models at any significance level higher than 0.05 per cent. Even more worryingly, the best-fitting NFW models require concentrations with c 20, which is not in the cosmologically preferred range for dwarf galaxies. We conclude that the kinematics of multiple populations in dSphs provides a substantial new challenge for theories of galaxy formation, with the weight of available evidence strongly against dark matter cusps at the centre.

A discrete chemo-dynamical model of the dwarf spheroidal galaxy Sculptor: mass profile, velocity anisotropy and internal rotation

“…There is still considerable debate as to whether the central regions of such halos are cored or cusped (e.g. Kleyna et al 2002;Koch et al 2007;Walker & Peñarrubia 2011;Agnello & Evans 2012;Amorisco & Evans 2012b;Breddels et al 2013). dSphs provide an unprecedented opportunity to test the inner structure of dark matter (DM) halos as they are DM-dominated at all radii and they are dust free (e.g.…”

“…Amorisco & Evans 2012b;Battaglia et al 2008). Battaglia et al (2008) also constructed surface number density profiles of RHB and BHB stars; these are shown as orange and green diamonds respectively in Figure 10.…”

“…Following a hard separation of the two populations, a few two-component dynamical studies were carried out to constrain the underlying DM halo by fitting the binned velocity dispersion profiles of the two populations simultaneously (e.g. Amorisco & Evans 2012b;Battaglia et al 2008). …”

Orbit-based dynamical models of the Sculptor dSph galaxy

“…For example, Lokas (2002) proposed to use higher moments to constrain the internal dynamics of dSphs since the kurtosis profile depends mostly on anisotropy while the velocity dispersion depends both on mass and anisotropy Lokas et al (2005), hence this lifts some of the degeneracies. Other possibilities would be to use parametrised phase-space distribution functions as pioneered by Kleyna et al (2001Kleyna et al ( , 2002; Wilkinson et al (2002a) (see also Amorisco & Evans 2012a), or the madeto-measure technique (Syer & Tremaine 1996;Long & Mao 2010).…”

Made-to-measure dark matter haloes, elliptical galaxies and dwarf galaxies in action coordinates

“…In the case of dwarf spheroidal galaxies, many studies have been carried out in order to determine the nature of the dark matter density law. Some results favour cores (e.g., Walker & Peñarrubia 2011;Amorisco & Evans 2012;Agnello & Evans 2012), while others offer a different view (e.g., Breddels & Helmi 2013;Strigari et al 2014;Richardson & Fairbairn 2014). As such, it is clear that dynamical models of both cored and cuspy dark matter haloes are required to resolve the controversy.…”