On the determination of the cosmic infrared background radiation from the high-energy spectrum of extragalactic gamma-ray sources

In a recent paper Stecker, De Jager, & Salamon have suggested using the observed ~MeV to TeV spectra of extragalactic γ-ray sources as probes of the local density of the cosmic infrared background radiation (CIBR) and have subsequently claimed a first possible measurement of the CIBR from the analysis of the γ-ray spectrum of Mrk 421 (De Jager, Stecker, & Salamon). The CIBR from normal galaxies consists of two components: a stellar emission component (CIBRs), and a thermal dust emission component (CIBRd). Photons with energies in the ~0.1-2 TeV range interact primarily with the CIBRs, whereas interactions with CIBRd dominate the absorption of photons in the ~2-100 TeV energy range. SDS92 and DSS94 considered only the interaction of the γ-rays with the dust emission component of the CIBR. We present here an improved analysis of the absorption of extragalactic TeV gamma rays by the CIBR, taking the dual nature of its origin into account. Applying the analysis to the observed γ-ray spectrum of Mrk 421, a BL Lac object at z = 0.031, we find in agreement with DSS94 tentative evidence for absorption by the CIBRd. However, we find no evidence for absorption by the CIBRs. Our analysis therefore limits the detection of the CIBR to the ~15-40 micron wavelength regime which, considering the uncertainties in the highest energy (>4 TeV) data and in the possibility of absorption inside the source, many turn out to be an upper limit on its energy density. At shorter wavelengths (λ ~1-15 microns), where the γ-ray interactions are dominated by the CIBRs, our analysis definitely yields only an upper limit on the energy density of the CIBR. In contrast, DSS94 have claimed a possible first measurement of the CIBR over the entire 1-120 micron wavelength region. The upper limit on the CIBRs and tentative detection of the CIBRd are consistent with normal galaxies contributing most of the energy to the CIBR, and constrain the contribution of some exotic sources. With careful modeling of infrared foreground emissions, these constraints on the CIBR are above the values measurable by the DIRBE experiment on board the COBE satellite.