Constraining the star formation history with Fermi-LAT observations of the gamma-ray opacity of the universe
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Date
2021
Authors
Le Ray, Ashlynn Merial
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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.
Description
A dissertation submitted in fulfilment of the requirements for the degree of Master of Science to the Faculty of Science, School of Physics, University of the Witwatersrand, Johannesburg, 2021