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Cosmology with the extragalactic gamma-ray background
Vasiliki PavlidouUniversity of Chicago
Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007
Outline
o EGRB vs CMB: information content
o Cosmology with the EGRB: constraining the Cosmic Star Formation Rate
o The future
Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007
EGRB vs CMB
CMB EGRBo Origin: cosmological
cosmologicaltruly diffuse unresolved
point sources
o Foregrounds: Milky Way Milky Wayo Energies: ~ 2.7 K ~ 109eV
(1GeV)o 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?
o Space-born gamma-ray telescopes: EGRET aboard CGRO (1990’s) LAT aboard GLAST
(about to be launched)
o Take all -sky map, subtract: Emission from the Milky Way Point sources
Credit: Strong et al 2004
Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007
What makes up the EGRB?
o Guaranteed contributions: established classes of gamma-ray emitters Normal galaxies Active galaxies Extragalactic unidentified sources
o Truly diffuse emission? o Exotic physics?
Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007
And now… cosmology
Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007
The Cosmic Star Formation Rate
o The Cosmic Star Formation Rate: how much gas mass is converted to stars per unit time per unit cosmic volume
o An essential measure of: baryonic energy production feedback processes in galaxy formation stellar contribution to reionization metal production
o Traditional measures: SF makes stars - young stars emit in UV, IR
compilation by Hopkins & Beacom 2006
Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007
The SF - gamma-ray connection
o Star Formation -> Supernovae -> Cosmic ray acceleration -> interaction with ISM -> gamma rays Characteristic normal galaxy spectral feature imprinted on
spectral shape of gamma-ray background
o Star Formation -> Background starlight (EBL) -> interaction with gamma rays EBL imprinted on spectra of: individual -ray sources, -ray
backgroundStarforming 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)
Stecker 1999
Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007
How do we utilize this connection?
o Until now: use knowledge of CSFR to predict signal/effects for gamma-ray telescopes
o The future: GLAST observations will allow inversion of the problem: use observations of gamma-ray signal/effects to constrain CSFR
o Uncertainties: significant, BUT largely uncorrelated with uncertainties of low-E methods
Strigari, Beacom, Walker & Zhang 2005
Prodanovic, VP & Fields preliminary
Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007
Conclusions
o Cosmic star formation history imprinted on extragalactic gamma-ray background: Normal galaxy spectral feature @ ≈ 1GeV EBL absorption pileup/suppression @ ≥20GeV
o 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
o A new era: observations of the EGRB can strongly constrain the cosmic history of star formation
