Observations of Milky Way Dwarf Spheroidal galaxies with the Fermi-LAT detector and constraints on Dark Matter models

TL;DR

This study uses 11 months of Fermi-LAT observations of 14 Milky Way dwarf spheroidal galaxies to search for gamma rays from WIMP annihilation. It implements both generic power-law and DM-motivated spectral models, derives flux upper limits, and, by combining these limits with Bayesian, Jeans-based dark matter density modeling, places constraints on the WIMP pair-annihilation cross-section ⟨σv⟩ for several DM frameworks (mSUGRA, MSSM, AMSB, UED). The analysis finds no gamma-ray excess and shows that, for plausible density profiles, LAT data begin to challenge low-thermal-density SUSY scenarios and AMSB wino-like models below about 300 GeV, while also restricting interpretations of the PAMELA/Fermi e+e− anomalies depending on diffusion assumptions. The work underscores the utility of dSphs as low-background laboratories for indirect DM detection and complements ground-based gamma-ray searches, all without invoking substructure boosts or Sommerfeld enhancements.

Abstract

We report on the observations of 14 dwarf spheroidal galaxies with the Fermi Gamma-Ray Space Telescope taken during the first 11 months of survey mode operations. The Fermi telescope provides a new opportunity to test particle dark matter models through the expected gamma-ray emission produced by pair annihilation of weakly interacting massive particles (WIMPs). Local Group dwarf spheroidal galaxies, the largest galactic substructures predicted by the cold dark matter scenario, are attractive targets for such indirect searches for dark matter because they are nearby and among the most extreme dark matter dominated environments. No significant gamma-ray emission was detected above 100 MeV from the candidate dwarf galaxies. We determine upper limits to the gamma-ray flux assuming both power-law spectra and representative spectra from WIMP annihilation. The resulting integral flux above 100 MeV is constrained to be at a level below around 10^-9 photons cm^-2 s^-1. Using recent stellar kinematic data, the gamma-ray flux limits are combined with improved determinations of the dark matter density profile in 8 of the 14 candidate dwarfs to place limits on the pair annihilation cross-section of WIMPs in several widely studied extensions of the standard model. With the present data, we are able to rule out large parts of the parameter space where the thermal relic density is below the observed cosmological dark matter density and WIMPs (neutralinos here) are dominantly produced non-thermally, e.g. in models where supersymmetry breaking occurs via anomaly mediation. The gamma-ray limits presented here also constrain some WIMP models proposed to explain the Fermi and PAMELA e^+e^- data, including low-mass wino-like neutralinos and models with TeV masses pair-annihilating into muon-antimuon pairs. (Abridged)

Figures (4)

• Figure 1: Spectral fits to the counts (left panels) and the corresponding residuals (right panels) for the ROIs around two dwarf spheroidal galaxies, Willman 1 (top panels) and Draco (bottom panels). The lines in the spectral plots (left panels) are point sources (black), the Galactic diffuse component (blue) and the isotropic component (red). The black line overlaid to the data points is the best-fit total spectrum in the respective ROIs. The best-fit power-law models (with $\Gamma=2$ here) for the dwarfs are below the lower bound of the ordinates. Willman 1 is the worst residual obtained in our sample, while Draco is illustrative of the fit quality for most ROIs.
• Figure 2: Derived upper limits on fluxes for all selected dwarfs and for various branching ratios: 100$\%$$b\bar{b}$ (upper left), 100$\%$$\tau^+\tau^-$ (upper right) and mixed 80$\%\ b\bar{b}$ + 20$\%\tau^+\tau^-$ (lower left) final state. Lower right plot gives an illustration of how the upper limits on the fluxes can change depending on the selected final state (here for the Ursa Minor dSph).
• Figure 3: mSUGRA (upper left), MSSM (upper right), Kaluza-Klein UED (lower left) and Anomaly mediated (lower right) models in the ($m_{{\rm wimp}}$,$<\sigma v >$) plane. All mSUGRA and MSSM plotted models are consistent with all accelerator constraints and red points have a neutralino thermal relic abundance corresponding to the inferred cosmological dark matter density (blue points have a lower thermal relic density, and we assume that neutralinos still comprise all of the dark matter in virtue of additional non-thermal production processes). The lines indicate the Fermi 95% upper limits obtained from likelihood analysis on the selected dwarfs given in Table \ref{['DMdwarf']}.
• Figure 4: Constraints on the annihilation cross-section for a $\mu^+\mu^-$ final state based on the 95% confidence limits on the $\gamma$-ray flux compared to dark matter annihilation models which fit well either the PAMELA 2009Natur.458..607A or Fermi $e^++e^-$ measurements 2009PhRvL.102r1101A. The left panel shows the constraints considering $\gamma$-ray emission from final state radiation only. The right panel shows the constraints for the Ursa Minor dwarf including both $\gamma$-ray emission from IC scattering and final state radiation. Here we consider two different diffusion coefficients, and show the effect of the uncertainties in the Ursa Minor density profile.