Measuring the Diffuse Extragalactic Background Light as a function of redshift Cosmic History of Star Formation: Holy Grail of Observational Cosmology (a “geological record” of all energy generated throughout history of Universe) GLAST will be a BLAST! Matt Malkan, UCLA

Malkan and Stecker 1998; 2001 DIRB Predictions of “Backwards Evolution” Models Start with local IR Luminosity functions, and local Spectral Energy Distributions, Extended into optical/UV Then add the tricky part…

Pure luminosity evolution still not bad approximation, with all L’s evolving with cosmic star formation history Eg: MRR 1999, with extinction corrections included MM +Stecker 98, “Fast” Dust-corrected Global SFR History

Redshifts z=0,1,2,3 Fancy surface plot, also SMS ’05 Simple extension down to Lyman limit Model predictions of Diffuse Photon Spectra

Models for z=0* D O/IRB are getting Well-constrained by COBE and deep counts Either “baseline” or “fast” evolution is currently consistent with observations SMS ‘05 * Too late to get Direct measurements At any higher z’s

Stecker,MM,Scully 2005 Predicted total optical depths as fcn of received energy (at least 2 wiggles in LAT spectra expected) Z=3, with CMWB separated / Summed optical depth as fcn of Z *O/NIR bump: starlight cool dust reradiation

LAT well designed to do most redshifts from 0.1 up to peak of cosmic energy generation Differences between some models (eg fast vs. baseline) are subtle  Good SNR at highest energies becomes crucial Energy (received) at which total  = 1

A few test cases show that this “works” on nearby TeV sources, such as PKS (z=0.17) With simultaneous wide energy coverage, fit absorbed (intrinsically straight) power law; This could also work with GRB spectra, with z known SMS ‘05

What are potential ambiguities? DIRB(z) is a convolution of: LF, SFR history over redshift, stellar population and dust properties:  Expect an increasingly focused search through model parameter space ; Role of Air Shower Experiments is Crucial in filling in lower z’s We assume that Blazar AGN spectra are featureless power laws, but there could be intrinsic (variable) curvature, Or internal absorption (Elliptical galaxy, hot AGN dust?)  Study statistical samples of Blazar spectra at each z

What if the observed Gamma-ray absorption Is shown to disagree with Independent DIRB Estimates? Don’t Worry! An explanation has already been published. It has something to do with broken symmetries. If that turns out, they’ll be looking for cheap airfares to Stockholm (Floyd hasn’t been yet)

IR Luminosity functions of galaxies at z up to 2.5: Great improvement from Spitzer; Herschel will soon do the same For far-IR =

MS98 models More luminous spirals have FIR peaks which are stronger and hotter (IRAS) Trend continues into sub-mm: ISOPHOT (Spinoglio, Malkan, Andreani 2002) Starlight peak Dust reradiation peak Huge SFR: ultraluminous starburst, at 25-60um Very low SFR, hardly any young stars