Extragalactic Background Light and Gamma-Ray Attenuation

TL;DR

This paper reviews gamma-ray attenuation data from AGN and GRBs to constrain the extragalactic background light (EBL), compares observational and model-based EBL calculations, and discusses implications for galaxy formation and high-redshift star formation.

Contribution

It provides a comprehensive comparison of EBL models with observational constraints, integrating galaxy observations and semi-analytic models to refine understanding of cosmic light and gamma-ray attenuation.

Findings

EBL upper limits are close to galaxy observed lower limits.

Some EBL models and observations are ruled out by gamma-ray constraints.

Future gamma-ray observations could improve high-redshift star formation estimates.

Abstract

Data from (non-) attenuation of gamma rays from active galactic nuclei (AGN) and gamma ray bursts (GRBs) give upper limits on the extragalactic background light (EBL) from the UV to the mid-IR that are only a little above the lower limits from observed galaxies. These upper limits now rule out some EBL models and purported observations, with improved data likely to provide even stronger constraints. We present EBL calculations both based on multiwavelength observations of thousands of galaxies and also based on semi-analytic models, and show that they are consistent with these lower limits from observed galaxies and with the gamma-ray upper limit constraints. Such comparisons "close the loop" on cosmological galaxy formation models, since they account for all the light, including that from galaxies too faint to see. We compare our results with those of other recent works, and discuss the implications of these new EBL calculations for gamma ray attenuation. Catching a few GRBs with groundbased atmospheric Cherenkov Telescope (ACT) arrays or water Cherenkov detectors could provide important new constraints on the high-redshift star formation history of the universe.