Very high-energy constraints on the infrared extragalactic background light

¹ Department of Physics and Astronomy (DFA), University of Padova, Vicolo Osservatorio 3, 35122 Padova, Italy
e-mail: alberto.franceschini@unipd.it, luca.foffano@phd.unipd.it
² INFN, Istituto Nazionale di Fisica Nucleare (INFN), Via Marzolo 8, 35131 Padova, Italy
³ Istituto Nazionale di Astrofisica (INAF), Vicolo dell’osservatorio 3, 35131 Padova, Italy

Received: 4 April 2019
Accepted: 8 July 2019

Abstract

Context. Measurements of the extragalactic background light (EBL) are a fundamental source of information on the collective emission of cosmic sources.

Aims. At infrared wavelengths, however, these measurements are precluded by the overwhelming dominance from interplanetary dust emission and the Galactic infrared foreground. Only at λ >  300 μm, where the foregrounds are minimal, has the infrared EBL (IR EBL) been inferred from analysis of the COBE maps. The present paper aims to assess the possibility of evaluating the IR EBL from a few micrometers up to the peak of the emission at > 100 μm using an indirect method that avoids the foreground problem.

Methods. To this purpose we exploit the effect of pair-production from gamma-gamma interaction by considering the highest-energy photons emitted by extragalactic sources and their interaction with the IR EBL photons. We simulate observations of a variety of low-redshift emitters with the forthcoming Imaging Atmospheric Cherenkov Telescope (IACT) arrays (CTA in particular) and water Cherenkov observatories (LHAASO, HAWC, SWGO) to assess their suitability to constrain the EBL at such long wavelengths.

Results. We find that even under the most extremely favorable conditions of huge emission flares, extremely high-energy emitting blazars are not very useful for our purpose because they are much too distant (> 100 Mpc the nearest ones, MKN 501 and MKN 421). Observations of more local AGNs displaying very high-energy emission, like low-redshift radio galaxies (M 87, IC 310, Centaurus A), are better suited and will potentially allow us to constrain the EBL up to λ ≃ 100 μm.