Normalization of the extragalactic background light from high-energy gamma-ray observations

Abstract

Extragalactic background light (EBL) plays an important role in cosmology since it traces the history of galaxy formation and evolution. Such diffuse radiation from near-UV to far-infrared wavelengths can interact with $\gamma$-rays from distant sources such as active galactic nuclei (AGNs), and is responsible for the high-energy absorption observed in their spectra. However, probing the EBL from $\gamma$-ray spectra of AGNs is not trivial due to internal processes that can mimic its effect. Such processes are usually taken into account in terms of curvature of the intrinsic spectrum. Hence, an improper choice of parametrization for the latter can seriously affect EBL reconstruction. In this paper, we propose a statistical approach that avoids a priori assumptions on the intrinsic spectral curvature and that, for each source, selects the best-fit model on a solid statistical basis. By combining the ${\it Fermi}$-LAT observations of 490 blazars, we determine the $\gamma$-ray-inferred level of EBL for various state-of-the-art EBL models. We discuss the EBL level obtained from the spectra of both BL Lacs and flat spectrum radio quasars (FSRQ) in order to investigate the impact of internal absorption in different classes of objects. We further scrutinize constraints on the EBL evolution from $\gamma$-ray observations by reconstructing the EBL level in four redshift ranges, up to $z\sim2.5$. The approach implemented in this paper, carefully addressing the question of the modeling of the intrinsic emission at the source, can serve as a solid stepping stone for studies of hundreds of high-quality spectra acquired by next-generation $\gamma$-ray instruments.