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

摘要

The end of the dark-ages of the universe - the epoch of reionization - is a one of the big open questions incosmology. The first stars are commonly believed to be responsible for the reionization of the universe. They form in a pristineenvironment in clouds with zero (primordial) metallicity, where the cooling is dominated by H_2or H cooling. The first starsare hotter and (probably) more massive than the second generation stars, providing copious amounts of ionizing photons.Their formation has been extensively studied via numerical simulations, but a direct detection is still pending. Emissionfrom the first stars is expected to contribute to the extragalactic background light (EBL) density in the optical to near-infraredwavelength region, possibly creating a unique spectral signature in the EBL. While difficult to access via direct measurements,lower limits on the optical to near-infrared EBL are derived from deep source counts. Recently, strong constraints on the EBLin near-infrared have been derived utilizing measured very high energy spectra together with assumptions about the sourcephysics. These limits are compared with model calculations for the EBL resulting from the first stars. The model tracesthe evolving emissivity of an aging stellar population and takes into account the nebula emission from the dense cloudssurrounding the sites of the first star formation. Different parameters of the first stars (star formation rate, stellar initial massfunction, metallicity) are probed and limits on these parameters are presented.