First stars and the extragalactic background light: how recent γ-ray observations constrain the early universe

¹ Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany e-mail: martin.raue@mpi-hd.mpg.de
² Institut für Experimentalphysik, Universität Hamburg, Luruper Chaussee 149, 22765 Hamburg, Germany
³ Institut de Fisica d'Altes Energies (IFAE), Edifici Cn. Universitat Autonoma de Barcelona, 08193 Bellaterra (Barcelona), Spain

Received: 16 June 2008
Accepted: 28 January 2009

Abstract

Context. 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.

Aims. 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 ( GeV; VHE) observations.

Methods. 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.

Results. Assuming a maximum contribution from PopIII/LM PopII stars to the EBL density of 5 nW m^-2 s^-1 at 2 μm, a limit on the star formation rate (SFR) of the first stars of 0.3 to 3  Mpc^-3 yr^-1 in the redshift range is derived. The limit depends on the assumed shape of the SFR and metallicity.

Conclusions. The EBL can be used as a probe to investigate the properties of PopIII/LM PopII stars. Limits on the EBL density derived from VHE observations can provide constraints on the parameters of the these stars, in particular the star formation rate.