Constraints on the first stars from the meta-galactic radiation field

The end of the dark-ages of the universe - the epoch of reionization - is a one of the big open questions in cosmology. The first stars are commonly believed to be responsible for the reionization of the universe. They form in a pristine environment in clouds with zero (primordial) metallicity, where the cooling is dominated by H_2 or H cooling. The first stars are hotter and (probably) more massive then the second generation stars, providing copious amounts of ionizing photons. They also produce the first metals and effectively enrich the universe, paving the way for the second generation of stars, and thereby sealing their own fate. Their formation has been extensively studied via numerical simulations, but a direct detection is still pending. An indirect way to derive constraints on the parameters of the first stars is to connect them with present day observables. All photons produced in the universe, which are not absorbed, are accumulated in the meta-galactic radiation field (MRF). Emission from the first stars is expected to contribute to the MRF density in the optical to near- nfrared wavelength region, possibly creating a unique spectral signature in the MRF. While difficult to access via direct measurements, lower limits on the optical to near-infrared MRF are derived from deep source counts. Furthermore, the MRF in the optical to infrared is an opacity source for high energy gamma-rays from distant sources, creating an attenuation signature in the measured high-energy spectra. Recently, strong constraints on the MRF in near-infrared have been derived utilizing measured very high energy spectra together with assumptions about the source physics. These limits are compared with model calculations for the MRF resulting from the first stars. The model traces the evolving emissivity of an aging stellar population and takes into account the nebula emission from the dense clouds surrounding the sites of the first star formation. Different parameters of the first stars (star formation rate, stellar initial mass function, metallicity) are probed and limits on these parameters will be presented