Constraining the IMF using TeV gamma ray absorption

Abstract Gamma rays from distant sources suffer attenuation due to pair production off of ∼ 1 μm extragalactic background light (EBL) photons via γγ → e + e − (R.J. Gould, G.P. Schreder, Phys. Rev. 155 (1967) 1404). The cross section for the pair production is maximized just above threshold, when λ EBL ∼ ( E γ TeV ) μ m . We may exploit this process in order to indirectly measure the EBL, and constrain models of galaxy formation. Here, using semi-analytic models of galaxy formation, we examine how gamma ray absorption may be used as an indirect probe of the stellar initial mass function (IMF), although there is a degeneracy with dust modeling. We point out that with the new generation of gamma ray telescopes including STACEE, MAGIC, HESS, VERITAS, and Milagro, we should soon possess a wealth of new data and a new method for probing the nature of the IMF.