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Session 70 - Cosmic Backgrounds and the Distant Universe.
Oral session, Thursday, June 11
Presidio,
Although unresolved AGNs (predominantly Seyfert galaxies) are believed to be the dominant contributors to the Cosmic \gamma-ray Background (CGB) below E_\gamma \sim 300 keV, and blazars explain the bulk of the CGB flux above 10 MeV, the origin of the CGB in the MeV region is perhaps less certain. The most promising candidates in this energy regime are cosmological supernovae (Clayton amp; Silk,ApJ,158,L43,1969, The et al.,ApJ,403,32,1993). The \gamma-ray emission resulting from the decay chain \(^56Ni\rightarrow \) \(^56Co \rightarrow\) \(^56Fe \) (847,1238,1770,2030,2599,3250 keV) produces a line spectrum plus a Compton scattering continuum. Although iron synthesis occurs in all types of supernovae, SNIa contribute predominantly to the CGB, due to larger Ni yields and higher photon escape probabilities. Estimates of the star formation history in the local universe yield a present-day rate density of \simeq 3.7 \times 10^-2h^3\ M_ødot\ Mpc^-3\ yr^-1 (Cole et al.,MNRAS,271,781,1994) , but the star formation rate was about ten times higher at a redshift of \sim 1.5-2. The rate then decreases, reaching the present-day value near redshifts \simeq 5 (Lilly et al., ApJ,460,L1,1996). In addition to \gamma-rays from SNIa we also consider contributions to the CGB due to lines from SNII (such as ^26Al at 1.8 MeV, ^44Ti at 1.157 MeV, and ^60Co at 1.17 amp; 1.33 MeV). The \gamma-ray spectrum of Model W10HMM (Pinto amp; Woosley, ApJ,329,820,1988) was time integrated to derive a template spectrum for Type II supernovae. For SNIa we also included the \gamma-ray continuum, using the W7 model of Nomoto et al. (ApJ,286,644,1984), integrated over 600 days, as template. Lifetimes of SNIa progenitors (\sim 1-2 Gyrs; Yoshii et al. 1996,ApJ,462,266) are taken into account. We discuss the resultant SNIa contribution to the CGB and compare the models to observations of COMPTEL, SMM, and earlier instruments. We discuss the use of the CGB as a powerful tool to constrain cosmological and galactic chemical evolution parameters.
