Is There an Imprint of Primordial Stars in the TeV γ‐Ray Spectrum of Blazars?

Abstract

The 1-5 μm diffuse sky emission from which local foreground emission from the solar system and the Galaxy have been subtracted exceeds the brightness that can be attributed to normal star-forming galaxies. The nature of this excess near-infrared background light (NIRBL) is controversial. On the one hand, its sharp rise at ~1.25 μm has been interpreted as a distinct spectral feature created by the redshifted emission from primordial (Population III) stars that have formed at redshifts 8. On the other hand, the measured NIRBL spectrum is almost identical to that of the zodiacal cloud, raising the possibility that it is of local origin. Blazars can, in principle, offer a simple test for the nature and origin of the NIRBL. Very high energy γ-ray photons emitted by these objects are attenuated en route to Earth by γ-γ interactions with the extragalactic background light (EBL). Assuming that the NIRBL is of extragalactic origin, its distinct spectral feature should give rise to a corresponding absorption feature in the observed γ-ray spectra of these sources. This paper examines whether the extragalactic nature of the NIRBL can be determined from the analysis of the γ-ray spectra of blazars. We calculate the γ-ray opacity toward two blazars H1426+428 and PKS 2155-304 located at redshifts of z ≈ 0.13 for several EBL scenarios with and without the alleged spectral signature of the Population III stars. We show that if the NIRBL is extragalactic, it may be very difficult to reproduce the absorption-corrected spectrum of PKS 2155-304 with a synchrotron self-Compton (SSC) model that is also consistent with the X-ray and EGRET γ-ray data. This, and other arguments presented in this paper, cast serious doubts on the possibility that the NIRBL represents the spectral imprint of the first generation of stellar objects on the EBL.