Cosmology with the extragalactic gamma-ray background

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Cosmology with the extragalactic gamma-ray
background

• Vasiliki PavlidouUniversity of Chicago

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Outline

• EGRB vs CMB information content
• Cosmology with the EGRB constraining the Cosmic
  Star Formation Rate
• The future

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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

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EGRB how is it measured? what does it look like?

• Space-born gamma-ray telescopes
• EGRET aboard CGRO (1990s)
• LAT aboard GLAST (about to be launched)
• Take all -sky map, subtract
• Emission from the Milky Way
• Point sources

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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?

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And now cosmology
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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

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The SF - gamma-ray connection

• Star Formation -gt Supernovae -gt Cosmic ray
  acceleration -gt interaction with ISM -gt gamma
  rays
• Characteristic normal galaxy spectral feature
  imprinted on spectral shape of gamma-ray
  background
• Star Formation -gt Background starlight (EBL) -gt
  interaction with gamma rays
• EBL imprinted on spectra of individual ?-ray
  sources, ?-ray background

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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

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Conclusions

• Cosmic star formation history imprinted on
  extragalactic gamma-ray background
• Normal galaxy spectral feature _at_ 1GeV
• EBL absorption pileup/suppression _at_ 20GeV
• GLAST will
• resolve thousands of bright point sources (e.g.
  AGNs) but at most 3 normal galaxies -gt normal
  galaxy feature expected to become visible
• Probe the gt20GeV 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