Challenges in Revealing Dark Matter from the High Energy Gamma-Ray

Background (Continuum)

Ranga-Ram Chary

Spitzer Science Center, Caltech

rchary@caltech.edu

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 data

To 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