Spherical Jeans analysis for dark matter indirect detection in dwarf spheroidal galaxies - impact of physical parameters and triaxiality

Abstract

Dwarf spheroidal (dSph) galaxies are among the most promising targets for the indirect detection of dark matter (DM) from annihilation and/or decay products. Empirical estimates of their DM content-and hence the magnitudes of expected signals-rely on inferences from stellar-kinematic data. However, various kinematic analyses can give different results and it is not obvious which are most reliable. Using extensive sets of mock data of various sizes (mimicking 'ultra-faint' and 'classical' dSphs) and an MCMC engine, here we investigate biases, uncertainties, and limitations of analyses based on parametric solutions to the spherical Jeans equation. For a variety of functional forms for the tracer and DM density profiles, as well as the orbital anisotropy profile, we examine reliability of estimates for the astrophysical J-and D-factors for annihilation and decay, respectively. For large (N 1000) stellar-kinematic samples typical of 'classical' dSphs, errors tend to be dominated by systematics, which can be reduced through the use of sufficiently general and flexible functional forms. For small (N 100) samples typical of 'ultrafaints', statistical uncertainties tend to dominate systematic errors and flexible models are less necessary. We define an optimal strategy that would mitigate sensitivity to priors and other aspects of analyses based on the spherical Jeans equation. We also find that the assumption of spherical symmetry can bias estimates of J (with the 95% credibility intervals not encompassing the true J-factor) when the object is mildly triaxial (axis ratios b/a = 0.8, c/a = 0.6). A concluding table summarises the typical error budget and biases for the different sample sizes considered.

Dark matter annihilation and decay from non-spherical dark halos in galactic dwarf satellites

“…For Segue I,Segue II, Boötes I, Hercules and Leo V, we adopt data from Simon et al (2011), Kirby et al (2013), Koposov et al (2011), Adén et al (2009) and Walker et al (2009c), respectively. The J factors of the above dSphs were estimated by GS15, Bonnivard et al (2015a), and some related papers. Thus, we can compare J values estimated by spherical and axisymmetric mass models, as we discuss later.…”

“…These effects are not negligible even for classical dSphs and UFD galaxies. For the effects of contamination, as an example, Bonnivard et al (2015a) investigated how estimations of J-factors are sensitive to foreground contamination of the stellar-kinematic data of the UFD galaxy Segue I. They found that Milky Way halo stars as contamination can induce the astrophysical factors to be overestimated by orders of magnitude.…”

“…Meanwhile, the non-sphericity of actual stellar systems and their dark haloes is not taken much into consideration with respect to the evaluation of astrophysical factors, even though some groups have examined this effect using a mock data catalogue (Bonnivard et al 2015a). As mentioned above, most previous studies have treated the dSphs and their dark haloes as spherical systems for simplicity, despite that the observed luminous parts of the dSphs are actually non-spherical (e.g., Irwin & Hatzidimitriou 1995;McConnachie 2012) and the concordance Λ-cold dark matter models predict non-spherical virialized dark haloes (e.g., Jing & Suto 2002;Kuhlen et al 2007;Vera-Ciro et al 2014).…”

Simple J-factors and D-factors for indirect dark matter detection

“…The spherical Jeans equations are often used to relate the velocities of the stars to the underlying dark matter distribution. Nowadays, this is often explored in a Bayesian framework using Monte Carlo techniques, so that the calculation of the J-factors requires significant computational effort [8][9][10][11][12][13].…”

Macro Dark Matter Selfgravitating Halos around Galaxies

“…In order to determine the parameters that best reproduce the observed properties of RetII, we run a simulation of 8 · 10 4 MCMC points with the CLUMPY 2 software [26,27] on the member stars of RetII [20], according to the prescriptions listed in [29,30] and assuming an Einasto profile [31] for the DM distribution…”