Challenges in Revealing Dark Matter from the High Energy Gamma-Ray
Background (Continuum)
Ranga-Ram CharySpitzer Science Center, Caltech
The Different Galactic Components
p-p component
IC Component
EB component
GALACTIC PLANE l~0, b~0
GALACTIC ANTICENTER l~180, b~75
Isotropic Component
Hunter et al., Bertsch et al.
Gas Scale Height is smaller than Radiation Field Scale Height
Contribution of Dark Matter to the E~150 MeV Background
Strong Dependence on Emissivity and Halo Core Radius
Current Constraints on the Spectrum of the Residual Halo Emission:
Looks more like γ-ray pulsars
Isotropic EBL ~ E-2.1
Anisotropic Halo Component ~ E-1.7
Uncertainty I: Anisotropic IC
The Radiation FieldR=5 kpc, z=0 kpc
R=8.5 kpc, z=1.5 kpc
R=5 kpc, z=1.5 kpc
The Radial Scale Length of the ISRF Energy Density is ~3 kpcThe Scale Height is ~1.5 kpcIn contrast, gas Scale Height is ~100 pc
A Visual Approach
Although the average intensity is the same, anisotropy is x10-100 greater on the right
Uncertainty II: The Cosmic Ray Spectrum and Intensity - Strong Dependence on
Distance from SNR
R=5 kpc, z=0 kpc
R=5 kpc, z=1.5 kpcR=8.5 kpc, z=1.5 kpc
The Problem ListFor our galaxy:• CR propagation model (Can it be done ?)• Azimuthal angle dependent ISRF using more
recent Sloan, Spitzer and 2MASS dataTo Pinpoint Dark Matter Consider:• Deep GLAST pointings at edge on galaxies –
still need to remove the IC component• Of course, dwarf galaxies and clusters might
remain the best place to identify dark matter• Angular power spectrum of CGRB is likely to be
dominated by unresolved blazars