2002/01/09 by Joel R. Primack, Primack, Joel R.
Physics and Astronomy · #Astrophysics (astro-ph) #Astrophysics and Cosmic Phenomena #Dark Matter and Cosmic Phenomena #FOS: Physical sciences #Particle Detector Development and Performance #astro-ph
paper · pdf · doi:10.48550/arxiv.astro-ph/0201119
22 pages, 7 figures -- This paper is an updated version of astro-ph/001147
arxiv created 2002/01/09 · openalex publication_date 2002/01/09 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
I discuss how measurements of the absorption of γ-rays from GeV to TeV energies via pair production on the extragalactic background light (EBL) can probe important issues in galaxy formation. We use semi-analytic models (SAMs) of galaxy formation, based on the flat LCDM hierarchical structure formation scenario with Ωm=0.3 and Hubble parameter h=0.65, to obtain predictions of the EBL from 0.1 to 1000μm. SAMs incorporate simplified physical treatments of the key processes of galaxy formation - including gravitational collapse and merging of dark matter halos, gas cooling and dissipation, star formation, supernova feedback and metal production -- and have been shown to reproduce key observations at low and high redshift. We investigate the consequences of variations in input assumptions such as the stellar initial mass function (IMF) and the efficiency of converting cold gas into stars. We also discuss recent attempts to determine the emitted spectrum of high energy gamma rays from blazars such as Mrk 501 from the observed X-rays using the synchrotron self-Compton model, and note that our favorite SAM EBL plus the observed spectrum of Mrk 501 do not imply unphysical upturns in the high energy emitted spectrum - thus undermining recent claims of a crisis with drastic possible consequences such as breaking of Lorentz invariance. We conclude that observational studies of the absorption of γ-rays will help to determine the EBL, and also help to explain its origin by constraining some of the most uncertain features of galaxy formation theory, including the IMF, the history of star formation, and the reprocessing of light by dust.