First stars and the extragalactic background light How recent gamma-ray observations constr.pdf

Astronomy Astrophysics manuscript no. raue˙kneiske˙mazin˙2008 c? ESO 2008 June 16, 2008 First stars and the extragalactic background light: How recent γ-ray observations constrain the early universe. Martin Raue1, Tanja Kneiske2, and Daniel Mazin3 1 Max-Planck-Institut fu?r Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany 2 Institut fu?r Experimentalphysik, Universita?t 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 / Accepted ABSTRACT Context. The formation of the first stars 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 simula- tions. 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 first stars using limits on the extragalactic background light (EBL) recently derived from very-high-energy (E100 GeV; VHE) observations. Methods. A model calculation for the evolving extragalactic background light (EBL) produced by the first 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 EBL density contribution from the first stars 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 MMpc?3 yr?1 in the redshift