First stars and the extragalactic background light: How recent gamma-ray observations constrain the early universe

TL;DR

This paper uses recent gamma-ray observations to set constraints on the properties and star formation rate of the first stars (PopIII) by modeling their contribution to the extragalactic background light.

Contribution

It presents a new model linking PopIII/LM PopII star properties to EBL density limits, providing constraints on early universe star formation rates.

Findings

Derived upper limits on PopIII star formation rate between 0.3 and 3 M_solar Mpc^-3 yr^-1.

Showed how EBL density limits constrain early star formation scenarios.

Discussed potential detection of spectral cut-offs related to early stars.

Abstract

The formation of the first stars (Population III; PopIII) marks the end of the dark ages of the universe, a subject of lively scientific debate. Not (yet) accessible to direct observations, this early stage of the universe is mostly studied via theoretical calculations and numerical simulations. An indirect window is provided by integrated present day observables such as the metal abundance or the diffuse extragalactic photon fields. We aim to derive constraints on the properties of the PopIII and low metallicity Population II (LM PopII) stars utilizing limits on the density of the extragalactic background light (EBL), recently derived from very-high-energy (E > 100 GeV; VHE) observations. A model calculation for the evolving EBL density produced by PopIII/LM PopII stars is presented. The model utilizes stellar population spectra (SPS) for zero and low metallicity stars and accounts for the changing emission of an aging stellar population. Emission from the dense HII regions surrounding the stars (nebula) is included. The resulting EBL density for different scenarios (metallicity, star formation rate, initial mass function) is compared to the limit on the EBL density. The potential for detecting a cut-off in HE/VHE spectra is discussed. Assuming a maximum contribution from PopIII/LM PopII stars to the EBL density of 5 nW m^-2 s-1 at 2 micrometer a limit on the star formation rate (SFR) of the first stars of 0.3 to 3 M_solar Mpc^-3 yr^-1 in the redshift range 7 - 14 is derived. The limit depends on the assumed shape of the SFR and metallicity.