Simple J-Factors and D-Factors for Indirect Dark Matter Detection

TL;DR

This paper introduces simple analytic formulas for calculating J- and D-factors in dark matter halos, improving accuracy and robustness in predicting signals for indirect dark matter detection, especially in dwarf spheroidal galaxies.

Contribution

It provides new, simple analytic expressions for J- and D-factors applicable to spherical dark matter models, validated against complex models, and identifies the most promising dwarf galaxies for detection.

Findings

Ursa Major II, Reticulum II, Tucana II, Horologium I have highest J-factors.

Classical dwarfs Draco, Sculptor, Ursa Minor also have high J-factors.

J-factor behavior can be inferred from halo parameters and is most robust at the half-light radius.

Abstract

J-factors (or D-factors) describe the distribution of dark matter in an astrophysical system and determine the strength of the signal provided by annihilating (or decaying) dark matter respectively. We provide simple analytic formulae to calculate the J-factors for spherical cusps obeying the empirical relationship between enclosed mass, velocity dispersion and half-light radius. We extend the calculation to the spherical Navarro-Frenk-White (NFW) model, and demonstrate that our new formulae give accurate results in comparison to more elaborate Jeans models driven by Markov Chain Monte Carlo methods. Of the known ultrafaint dwarf spheroidals, we show that Ursa Major II, Reticulum II, Tucana II and Horologium I have the largest J-factors and so provide the most promising candidates for indirect dark matter detection experiments. Amongst the classical dwarfs, Draco, Sculptor and Ursa Minor have the highest J-factors. We show that the behaviour of the J-factor as a function of integration angle can be inferred for general dark halo models with inner slope $\gamma$ and outer slope $\beta$. The central and asymptotic behaviour of the J-factor curves are derived as a function of the dark halo properties. Finally, we show that models obeying the empirical relation on enclosed mass and velocity dispersion have J-factors that are most robust at the integration angle equal to the projected half-light radius of the dSph divided by heliocentric distance. For most of our results, we give the extension to the D-factor which is appropriate for the decaying dark matter picture.