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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

Cosmology with the extragalactic gamma-ray background

Vasiliki PavlidouUniversity of Chicago

outline
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

egrb vs cmb
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

egrb how is it measured what does it look like

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

what makes up the egrb
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

and now cosmology
And now… cosmology

Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007

the cosmic star formation rate

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

the sf gamma ray connection

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

how do we utilize this connection

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

conclusions
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