1. The Kavli FoundationThe National Science Foundation Constraining Populations of Extragalactic Gamma-ray Sources with the Extragalactic Gamma-ray Background Vasiliki Pavlidou University of Chicago Tonia Venters University of Chicago and APC (Paris 7) Jennifer Siegal-Gaskins University of Chicago Brian Fields University of Illinois Angela Olinto University of Chicago and APC (Paris 7) Carolyn Brown University of Chicago

2. The Kavli FoundationThe National Science Foundation Extragalactic Gamma-ray Emitters: 1. Classes with already detected members Blazars: –AGNs with jet aligned with line-of-sight –Most populated class of EGRET identified sources –Only class with identified members at high z Normal galaxies: –EGRET detected only 2: Milky Way, LMC –GLAST will detect only 3 more: SMC, M31, M33? Unidentified EGRET sources: –Likely to include many extragalactic sources –More numerous than blazars! Credit: J. Buckley 1998 (Science), illustration: K. Sutliff Credit: S. Digel

3. The Kavli FoundationThe National Science Foundation Starburst galaxies –GLAST will likely detect ~10 (Torres et a 2004) Merging / accreting clusters of galaxies and large-scale filaments Dark matter density peaks Insert your own here Extragalactic Gamma-ray Emitters: 2. Theorized but yet-undetected classes Credit: B. Whitmore (HST)

4. The Kavli FoundationThe National Science Foundation Theorize, then theorize some more. Use observations of the isotropic diffuse background to constraint properties of yet-undetected classes / exotic physics However: –unresolved members of established classes (blazars, normal galaxies, extragalactic unidetified sources) also have some guaranteed contribution to background –These contributions need to be modeled and subtracted –Task involves formidable uncertainties! How do we make progress on extragalactic source classes with no identified members?

5. The Kavli FoundationThe National Science Foundation Use information on background spectral shape How do we make progress on extragalactic source classes with no identified members? Observational EGRB (Sreekumar et al 1998) Observational EGRB (Strong et al 2004) Strong et al 2004 systematics Unresolved blazars (VP & Venters 2007) Unresolved normal galaxies (VP & Fields 2002) Unresolved extragalactic unidentified sources (VP, Siegal-Gaskins, Fields, Olinto, & Brown 2007)

6. The Kavli FoundationThe National Science Foundation How confident are we in the spectra of unresolved emission from established source classes? Unresolved emission spectra: –calculated from spectral index distributions of detected sources –implicit assumption: spectra of unresolved sources similar to spectra of resolved sources –but also: uncertainty involved in spectral index distribution of resolved sources (low number statistics, large measurement error in individual spectral indices) EGRET constraints on the unresolved blazar emission spectral shape (VP & Venters 2007)

7. The Kavli FoundationThe National Science Foundation GLAST can help! GLAST will detect between 1,000 - 10,000 blazars (EGRET detected ~ 60) v Unresolved FSRQ emission using EGRET data Unresolved FSRQ emission using GLAST data (simulated)

8. The Kavli FoundationThe National Science Foundation Extragalactic diffuse emission encodes valuable information on (a) unresolved members of established classes (b) theorized, yet-undetected classes of extragalactic gamma-ray sources Spectral shapes of different components / observed emission powerful tool: * complementary to studies of relative emission intensity * involves different, independent uncertainties GLAST will allow us to make much more confident predictions for spectral shape of known classes, and place much stricter constraints on properties of undetected classes Conclusions