Background Infrared Radiation

In traversing intergalactic distances, gamma-rays can be absorbed by photon-photon pair production on background photon fields ([Gould & Schréder 1967]). For VHE photons, the most important interaction is with infrared (IR) fields. Intergalactic IR photons arise from normal galaxy formation, with the spectrum and density of the IR background yielding information about the epoch and evolution of the formation process. Exotic mechanisms, such as Very Massive Objects (VMOs) or the decay of massive neutrinos, may also contribute to the IR background. Measurements of VHE spectra from extragalactic sources permit indirect investigation into the nature of the IR background field because distortions in the gamma-ray spectrum due to pair-production depend on the density spectrum of the background field. VHE spectra from Mrk 421 and Mrk 501 have already been used to set upper limits on the IR background from 0.025eV to 0.3eV ([Stecker & de Jager 1998]; [Dwek & Slavin 1992]; [Biller et al. 1998]). At some wavelengths, these limits are as much as an order of magnitude below the upper limits set by the DIRBE/COBE satellite (see Figure 8). These limits place substantial restrictions on several proposed particle physics and cosmological models which would contribute to the IR background (cf., [Biller et al. 1998]). In particular, the contribution of VMOs to the dark matter in galactic halos must be <5% of the critical density unless dust grains substantially reprocess their radiation. In addition, the VHE results improve limits on the lifetime/branching ratio for radiative decay of heavy neutrinos in the mass range $\sim$0.05eV by up to two orders of magnitude.

  

Figure 8: The diffuse intergalactic infrared background. $E_\gamma$ is the energy at which the pair-production cross-section peaks for head on collisions with photons of wavelength $\lambda$. Upper limits derived from gamma-ray spectra are shown as horizontal bars with arrows, marked as B98 ([Biller et al. 1998]). The points and the solid curve represent experimental limits or detections. The dashed line on the left is the 2.7K cosmic microwave background and the three dashed curves are models of the IR background. Figure from Vassiliev (1999).

The current limits on the IR density are $\sim$5 to 10 times higher than predicted from galaxy evolution (e.g., [Primack et al. 1999]; [Malkan & Stecker 1998]). However, with better spectral measurements of existing sources and detections of more objects, particularly at higher redshift, VERITAS can substantially improve these limits, and may eventually detect the effects of the IR field.