Constraining the star formation history with Fermi-LAT observations of the gamma-ray opacity of the universe

Abstract

This work is based on a two-part paper series developed by Kneiske et al. (2002) and Kneiske et al. (2004). Our project investigates the effects of changes in the parameters of star forma tion history on the predicted extragalactic background light and the gamma-ray opacity of the Universe. We use observations from Fermi-LAT to constrain our model parameters. We introduce a model of stellar emission and dust re-emission as a function of redshift proposed by Kneiske et al. (2002) in our calculations, since the extragalactic background light (EBL) consists of the contribution from dust and stellar emission. The components that contribute to the stellar emission involved the use of simple stellar population (SSP) models that were computed using the Padova (1994) evolutionary tracks Bruzual and Charlot (2003). The SED of an simple stellar population (SSP) is a function of time, since stars progress over various cosmological times, depending on their mass. The infrared emission associated with each SSP-spectrum is computed as the addition of three modified Blackbody spectra. The total spectra of starlight and re-emission are used to calculate the luminosity density, which is then used to calculate the spectrum of the EBL at different redshifts. We use the Best-fit model developed by Kneiske et al. (2004) and updated stellar populations (Bruzual and Charlot, 2003) pertaining to the star formation history (SFH) by constraining the parameters of the SFH which lead to the comoving EBL spectrum. This model leads to EBL predictions to be compared with EBL measurements from Fermi data. We then calculate the optical depth and use Fermi observations to constrain our models. The opacity produced in this work deviates slightly from the observations published by Fermi-LAT.