A novel method for measuring the extragalactic background light: Fermi application to the lobes of Fornax A

TL;DR

This paper introduces a new, parameter-free method to measure the extragalactic background light by analyzing gamma-ray inverse Compton emission from radio galaxy lobes, demonstrated on Fornax A.

Contribution

The paper presents a novel approach leveraging high-energy electron spectral features to detect the EBL via gamma-ray observations, applicable with Fermi data.

Findings

Fermi can set upper limits on optical EBL.

Fermi can measure infrared EBL.

Method successfully applied to Fornax A.

Abstract

We describe a new method for measuring the extragalactic background light (EBL) through the detection of $\gamma$-ray inverse Compton (IC) emission due to scattering of the EBL photons off relativistic electrons in the lobes of radio galaxies. Our method has no free physical parameters and is a powerful tool when the lobes are characterized by a high energy sharp break or cutoff in their electron energy distribution (EED). We show that such a feature will produce a high energy IC `imprint' of the EBL spectrum in which the radio lobes are embedded, and show how this imprint can be used to derive the EBL. We apply our method to the bright nearby radio galaxy Fornax A, for which we demonstrate, using WMAP and EGRET observations, that the EED of its lobes is characterized by a conveniently located cutoff, bringing the IC EBL emission into the {\sl Fermi} energy range. We show that {\sl Fermi} will set upper limits to the optical EBL and measure the more elusive infrared EBL.