The Most Dark Matter Dominated Galaxies: Predicted Gamma-ray Signals from the Faintest Milky Way Dwarfs

TL;DR

The paper investigates gamma-ray signals from dark matter annihilation in three ultra-faint Milky Way dwarfs (Willman 1, Coma Berenices, Ursa Major II) by constraining their dark matter halos with stellar kinematics. It combines Jeans-based mass modeling, a DM density profile formalism, and a CDM-derived V_max–r_max prior to predict the gamma-ray flux Phi, with the particle-physics factor P linking annihilation to observed flux. The results show these galaxies are extremely DM-dominated and can yield fluxes comparable to or higher than brighter satellites, with flux peaks near Phi0 ~ 1e-10 cm^-2 s^-1 for optimistic models; substructure boosts could elevate the flux by up to two orders of magnitude. Detection prospects with GLAST and VERITAS appear favorable, and the framework accommodates inner-slope variations and tidal effects, emphasizing the role of substructure and robust priors in predicting indirect DM signals.

Abstract

We use kinematic data from three new, nearby, extremely low-luminosity Milky Way dwarf galaxies (Ursa Major II, Willman 1, and Coma Berenices) to constrain the properties of their dark matter halos, and from these make predictions for the gamma-ray flux from annihilation of dark matter particles in these halos. We show that these 10^3 solar luminosity dwarfs are the most dark matter dominated galaxies in the Universe, with total masses within 100 pc in excess of 10^6 solar masses. Coupled with their relative proximity, their large masses imply that they should have mean gamma-ray fluxes comparable to or greater than any other known satellite galaxy of the Milky Way. Our results are robust to both variations of the inner slope of the density profile and the effect of tidal interactions. The fluxes could be boosted by up to two orders of magnitude if we include the density enhancements caused by surviving dark matter substructure.

Figures (4)

• Figure 1: The $90\%$ confidence level region in the $\rho_s-r_s$ (top) $\rho_s^2 r_s^3-\rho_s$ (bottom) parameter space for Coma , Ursa Major II, Willman 1, and Ursa Minor. We marginalize over the velocity anisotropy and have assumed an inner slope of $\gamma =1$. The best-fit values are indicated with points.
• Figure 2: The probability distributions of the angular size subtended by $r_s$ for each galaxy. We marginalize over the velocity anisotropy and $\rho_s$. The inner slope is fixed to $\gamma=1$.
• Figure 3: The probability distributions for the $\gamma$-ray fluxes from Coma, Ursa Major II, Willman 1, and Ursa Minor, marginalizing over the velocity anisotropy, $\rho_s$, and $r_s$. We assume $P= 10^{-28} \, {\rm cm}^{3} \, {\rm s}^{-1} \, {\rm GeV}^{-2}$ and an inner slope of $\gamma = 1.0$. We have assumed no boost from halo substructure, which increases these fluxes by a factor $\sim 10-100$.
• Figure 4: The $\gamma$-ray flux probability distributions for Coma and Ursa Minor for inner slopes of 0.8 (long-dashed), 1.0 (solid), and 1.2 (short-dashed). We marginalize over the same quantities as in Fig. (\ref{['fig:flux']}). The value of $P$ is the same as in Fig. (\ref{['fig:flux']}).