Probing the evolution of the EBL photon density out to $z~\sim 1$ via $\gamma$-ray propagation measurements with Fermi

TL;DR

This study uses Fermi-LAT gamma-ray data to investigate how the density of the Extragalactic Background Light (EBL) evolves up to redshift 1, testing models of cosmic photon density evolution through gamma-ray absorption measurements.

Contribution

It introduces a method to constrain EBL evolution models by fitting gamma-ray horizon data from Fermi-LAT observations, providing new insights into the redshift dependence of EBL photon density.

Findings

EBL photon density evolution is consistent with certain models up to z~1.

Fermi-LAT gamma-ray horizon measurements effectively constrain EBL models.

Comparison shows some models overestimate EBL density at higher redshifts.

Abstract

The redshift ($z$) evolution of the Extragalactic Background Light (EBL) photon density is very important to understand the history of cosmological structure formation of galaxies and stars since the epoch of recombination. The EBL photons with the characteristic spectral energy distribution ranging from ultraviolet/optical to far-infrared provide a major source of opacity of the Universe to the GeV-TeV $\gamma$-rays travelling over cosmological distances. The effect of the EBL is very significant through $\gamma \gamma \rightarrow e^- e^+$ absorption process on the propagation of the $\gamma$-ray photons with energy $E >$ 50 GeV emitted from the sources at $z \sim 1$. This effect is characterized by the optical depth ($\tau$) which strongly depends on $E$, $z$ and density of the EBL photons. The proper density of the EBL photons increases with $z$ due to expansion of the Universe whereas evolution of radiation sources contributing to the EBL leads to a decrease in the density with increasing $z$. Therefore, the resultant volumetric evolution of the EBL photon density is approximated by a modified redshift dependence. In this work, we probe evolution of the EBL photon density predicted by two prominent models using cosmic gamma-ray horizon ($\tau (E,z)=$ 1) determined by the measurements from the \emph{Fermi}-Large Area Telescope (LAT) observations. The modified redshift dependence of the EBL photon density is optimized for a given EBL model by estimating the same gamma-ray horizon as predicted by the \emph{Fermi}-LAT observations. We further compare the optical depth estimates in the energy range $E =$ 4 GeV-1 TeV and redshift range $z =0.01-1$ from the \emph{Fermi}-LAT observations with the values derived from the two EBL models to further constrain the evolution of the EBL photon density in the $z~\sim 1$ Universe.