Transcript Multi-channel Astrophysics and Cosmology at the Highest

Cosmology with the
extragalactic gamma-ray background
Vasiliki Pavlidou
University of Chicago
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
o
o
o
o
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?
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
background
Starforming galaxies (VP & Fields 2002)
Unidentified sources (VP, Siegal-Gaskins, Fields, Olinto & Brown 2007)
Blazars (VP & Venters 2007)
Stecker 1999
EGRET gamma-ray background, conservative (Strong et al 2004)
Maximal EGRET gamma-ray background (Sreekumar et al 1998)
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
Prodanovic, VP & Fields preliminary
Strigari, Beacom, Walker & Zhang 2005
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
Vasiliki Pavlidou
Great Lakes Cosmology Workshop 8
Ohio State University, 01June 2007