The extragalactic background light: lower vs upper limits

TL;DR

This paper discusses the constraints on the extragalactic background light (EBL) derived from gamma-ray observations and resolves apparent conflicts with recent higher estimates by demonstrating consistent intrinsic spectra.

Contribution

It shows that the canonical gamma-ray spectral index can be recovered even with higher EBL densities, reconciling upper and lower EBL limits.

Findings

Higher EBL density is compatible with observed gamma-ray spectra.

Canonical gamma-ray spectra (~1.5) can be recovered with increased EBL.

Reconciliation of EBL limits from different methods is possible.

Abstract

The discovery of distant sources of very high energy (VHE) gamma-rays with hard energy spectra enabled to derive strong upper limits on the density of the extragalactic background light (EBL). These limits are close to the lower limits derived from deep source counts. A recent re-determination of the EBL contribution from resolved sources at 3.6 micrometer finds a higher EBL density, which is claimed to be in conflict with the assumptions utilized to derive the EBL upper limits from VHE spectra. Here, it is shown that is possible to recover the canonical Gamma~1.5 intrinsic spectra for such a higher EBL density.