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Cosmology with the extragalactic gamma-ray background

Cosmology with the extragalactic gamma-ray background. Vasiliki Pavlidou University of Chicago. Outline. EGRB vs CMB: information content Cosmology with the EGRB: constraining the Cosmic Star Formation Rate The future. EGRB vs CMB. CMB EGRB

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Cosmology with the extragalactic gamma-ray background

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  1. Cosmology with the extragalactic gamma-ray background Vasiliki PavlidouUniversity of Chicago

  2. Outline • EGRB vs CMB: information content • Cosmology with the EGRB: constraining the Cosmic Star Formation Rate • The future Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007

  3. EGRB vs CMB CMB EGRB • Origin: cosmological cosmological truly diffuse unresolved point sources • Foregrounds: Milky Way Milky Way • Energies: ~ 2.7 K ~ 109eV (1GeV) • Spectrum: blackbody power law Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007

  4. Credit: Strong et al 2004 EGRB: how is it measured? what does it look like? • Space-born gamma-ray telescopes: • EGRET aboard CGRO (1990’s) • LAT aboard GLAST (about to be launched) • Take all -sky map, subtract: • Emission from the Milky Way • Point sources Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007

  5. What makes up the EGRB? • Guaranteed contributions: established classes of gamma-ray emitters • Normal galaxies • Active galaxies • Extragalactic unidentified sources • Truly diffuse emission? • Exotic physics? Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007

  6. And now… cosmology Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007

  7. compilation by Hopkins & Beacom 2006 The Cosmic Star Formation Rate • The Cosmic Star Formation Rate: how much gas mass is converted to stars per unit time per unit cosmic volume • An essential measure of: • baryonic energy production • feedback processes in galaxy formation • stellar contribution to reionization • metal production • Traditional measures: SF makes stars - young stars emit in UV, IR Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007

  8. Stecker 1999 Starforming galaxies (VP & Fields 2002)Unidentified sources (VP, Siegal-Gaskins, Fields, Olinto & Brown 2007)Blazars (VP & Venters 2007)EGRET gamma-ray background, conservative (Strong et al 2004) Maximal EGRET gamma-ray background (Sreekumar et al 1998) The SF - gamma-ray connection • Star Formation -> Supernovae -> Cosmic ray acceleration -> interaction with ISM -> gamma rays • Characteristic normal galaxy spectral feature imprinted on spectral shape of gamma-ray background • Star Formation -> Background starlight (EBL) -> interaction with gamma rays • EBL imprinted on spectra of: individual -ray sources, -ray background Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007

  9. Prodanovic, VP & Fields preliminary Strigari, Beacom, Walker & Zhang 2005 How do we utilize this connection? • Until now: use knowledge of CSFR to predict signal/effects for gamma-ray telescopes • The future: GLAST observations will allow inversion of the problem: use observations of gamma-ray signal/effects to constrain CSFR • Uncertainties: significant, BUT largely uncorrelated with uncertainties of low-E methods Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007

  10. Conclusions • Cosmic star formation history imprinted on extragalactic gamma-ray background: • Normal galaxy spectral feature @ ≈ 1GeV • EBL absorption pileup/suppression @ ≥20GeV • GLAST will: • resolve thousands of bright point sources (e.g. AGNs) but at most 3 normal galaxies -> normal galaxy feature expected to become visible • Probe the >20GeV regime, map the shape of high-E absorption feature • A new era: observations of the EGRB can strongly constrain the cosmic history of star formation Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007

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