1. Cosmological Gamma-ray Opacity and the Extragalactic Background Light Rudy Gilmore University of California, Santa Cruz TeVPA09 - SLAC July 16, 2009 Collaborators:  Joel Primack - UCSC  Rachel Somerville - STScI (Baltimore)  Piero Madau - UCSC  Francesco Haardt - Università dell'Insubria (Como, Italy)

2. The Extra-Galactic Background Light (EBL) Theoretical Definition: The average photon field existing throughout the universe  Roughly 1/20 the energy of the CMB in combined UV, optical and IR Observational Definition: All the light from outside the galaxy. (z=0 background) Motivation: Measurement of the EBL (‘photon archeology’) gives us constraints on structure formation. The EBL has implications for observations of the highest energy (GeV and TeV) extra-galactic gamma-rays. Direct Starlight Thermal Dust Emiss. PAH

3. Absorption of Gamma Rays by EBL Gamma-ray attenuation via e + e - pair production provides a link between galaxy history and high energy astrophysics. Opacity based on integrated EBL flux, tends to increase with energy: Softening and cutoff in gamma ray spectra of distant extragalactic sources (blazars and GRBs) λ ~ 1.24(E γ /TeV) μm (90 o interaction, max σ) σ peaks at 2x threshold energy, falls off as E -1 for higher energies: Threshold condition:

4. Measurement of the Local Background Direct Measurement Photometry measurements must contend with difficult foreground subtraction and calibration issues! Optical - Bernstein (2002, 2007) using Hubble and ground-based data in 3 optical bands IR - DIRBE detections in near-IR (e.g. Wright 2001, Levenson et al. 2007) and far- IR (Hauser et al. 1998, Wright 2004) FIRAS - absolute measurement of CMB and EBL >125 μm (Fixsen et al. 1998)

5. Measurement of the Local Background Direct Measurement Photometry measurements must contend with difficult foreground subtraction and calibration issues! Optical - Bernstein (2002, 2007) using Hubble and ground-based data in 3 optical bands IR - DIRBE detections in near-IR (e.g. Wright 2001, Levenson et al. 2007) and far- IR (Hauser et al. 1998, Wright 2004) FIRAS - absolute measurement of CMB and EBL >125 μm (Fixsen et al. 1998) Galaxy Number Counts Can provide robust lower limits, but degree of convergence often controversial Available in many bands, including UV (GALEX), optical/NIR (HST, various ground-based), mid and far IR (Spitzer, ISO), and submillimeter (SCUBA, BLAST) Limits in optical and near-IR generally below direct photometry estimates

6. Measurement of the Local Background Direct Measurement Photometry measurements must contend with difficult foreground subtraction and calibration issues! Optical - Bernstein (2002, 2007) using Hubble and ground-based data in 3 optical bands IR - DIRBE detections in near-IR (e.g. Wright 2001, Levenson et al. 2007) and far- IR (Hauser et al. 1998, Wright 2004) FIRAS - absolute measurement of CMB and EBL >125 μm (Fixsen et al. 1998) Galaxy Number Counts Can provide robust lower limits, but degree of convergence often controversial Available in many bands, including UV (GALEX), optical/NIR (HST, various ground-based), mid and far IR (Spitzer, ISO), and submillimeter (SCUBA, BLAST) Limits in optical and near-IR generally below direct photometry estimates Extragalactic Gamma-ray Observations Assumption that intrinsic spectra is softer than -Γ = 1.5 (e.g. Aharonian et al. 2006; Albert et al. 2008; also Costamante et al. 2004; Mazin & Raue 2007)

7. Modeling of the galaxy population  Evolution inferred from observations Kneiske et al. 2004; Finke et al. 2009 - models based on star formation rate density, stellar synthesis models, dust reradiation Franceschini et al. 2008 - sophisticated model based on measured LFs, separate treatment of optical and IR, and different galaxy population.  Backwards evolution of the existing galaxy population Stecker et al. 2006 - based on power law evolution of existing galaxy pop.  Forward evolution, begin from cosmological initial conditions Primack et al. 2001, 2005, and this work

8. The Semi-Analytic Model Treats evolution of AGN, black holes, and galaxies in ΛCDM framework Described in detail in Somerville et al. (2008); - based on code described in Somerville & Primack (1999) and Somerville, Primack & Faber (2002) Galaxy formation based on hierarchical buildup of cold dark matter halos. 2 Models (‘Fiducial’ and ‘Low’) created, based on WMAP1 and WMAP3 values for σ 8 (WMAP5 is intermediate to these) Primack, Gilmore, Somerville, arXiv: 0811.3230

9. Stars, dust and AGN in our model: Star formation can occur in quiescent and burst modes. - Quiescent mode uses Schmidt-Kennicutt prescription - Burst occurs during merger, dependent upon available gas and efficiency parameter, SFR falls exponentially. - Universal Chabrier IMF assumed AGN operate in ‘bright’ and ‘radio’ modes - Bright switched on by high accretion rate. - Long-term radio mode accretion prevents gas inflow for large galaxies, model does good job reproducing color bimodality. Optical and UV starlight absorbed using 2-component dust model of Charlot & Fall (2000). Re-emission of starlight by dust using IR templates: - Devriendt & Guiderdoni (2000) - Rieke et al. (2009), which use Spitzer data (preliminary)

10. Gilmore et al., arXiv: 0905.1144 The High Redshift UV Background Affects gamma-rays from distant sources, observed in 10-100 GeV energy range. Fermi LAT is studying the little-understood energy decade of 10-100 GeV. Next generation of ground based experiments (MAGIC-II, H.E.S.S.-II) will observe gamma-rays down to < 50 GeV. New models attempt to accurately compute this background component  Quasar contribution based on observational estimates (Hopkins et al. 2007)  Transfer of ionizing radiation through IGM calculated with CUBA code (Haardt & Madau 2001)  Reasonable estimates of ionizing escape fraction from star-forming galaxies

11. Number Counts - Comparison with Data Optical (SDSS + GOODS + ACS)Infrared (IRAC + MIPS) Low Model Fiducial Model, Rieke Templates

12. Local EBL Flux HST+ Ground Based DIRBE (1.25, 2.2, 3.5) DIRBE (100, 140, 240) Upward-pointing triangles: number counts (lower limits) Other symbols: direct photometry GALEX HST IRAC MIPS

13. Reconstructed intrinsic spectra for observed blazars (dN/dE ~ E -Γ ) Optical depth vs energy at several redshifts Dust model impacts results above ~3 TeV

14. Blazars seen by EGRET (Dermer 2007) Gamma-ray ‘attenuation edge’

15. Conclusions Using our latest SAMs, we have created 2 models of the background from the UV to far-IR, varying primarily in uncertain CDM normalization parameter σ 8 Both match well with number counts, local luminosity functions, and luminosity density Both models are below most direct detection claims, except in far-IR, and are near level of resolved light (number counts) over wide range of wavelengths. Good agreement with limits from gamma-ray experiments (-Γ > 1.5 criterion) Convergence between our results and recent observationally-based models (Finke et al. 2009 (best fit), Franceschini et al. 2008) Low threshold IACTs should be able to view sources out to z > 1 without significant EBL attenuation Future observations by Fermi and IACTs will hopefully tell us more about the non-local background

16. ***Bonus Slides***

17. z = 0 Luminosity Density Low Model Fiducial Model, Rieke Templates GALEX SDSS 6dF, 2MASS IRAS SCUBA

18. Hauser & Dwek (2001) Foregrounds in the Lockman Hole Observed sky brightness Interplanetary dust X Bright and faint stars Interstellar medium Foreground Subtraction Two ‘windows’ in the near- and far- IR are conducive to finding extragalactic contribution, no DIRBE detections in mid-IR. Published values have depended on model for zodiacal light used.

19. Fiducial Model Low Starbursts Integrated Stellar Mass Data: Bell et al. 2004, Fontana et al. 2006, Borch et al. 2006 Star Formation Rate Density Data: Hopkins 2004, Hopkins & Beacom 2006 (grey line) Star-formation History

20. Our models with gamma-ray upper limits Our models are within new low bounds set by blazar observation (Mazin & Raue 2007) H 2356-309 and 1ES 1101-232 (z=0.165 and 0.186) (Aharonian 2006) 3C279 (z=0.536) (Albert 2008)

21. Star Formation Rate Density Semi-analytic model: UV emission Emissivity from Star-Forming Galaxies Fiducial ‘High Peaked’ model considered as well No Dust

22. Low Model, Hopkins QSO LF, f esc = 0.1, 0.2, 0.5 Ionization Rates, as inferred from Lyman opacities Fiducial Model, Hopkins QSO LF, f esc = 0.02, 0.1, 0.2 High-Peaked Fiducial, Hopkins QSO LF, f esc = 0.1 Fiducial w/ S&B Model ‘C’ f esc = 0.02

23. UV ‘softness’, HeII/HI Low Model, Hopkins LF, f esc = 0.1, 0.2, 0.5 Fiducial Model, Hopkins LF, f esc = 0.02, 0.1, 0.2 High-Peaked Fiducial, Hopkins LF, f esc = 0.1 Fiducial w/ S&B Model ‘C’ f esc = 0.02

24. UV EBL Flux History UV background at several slices in redshift Low Model, Hopkins LF, f esc = 0.2 Fiducial Model, Hopkins LF, f esc = 0.1 High-Peaked Fiducial, Hopkins LF, f esc = 0.1 Fiducial w/ S&B Model ‘C’ f esc = 0.02

25. Optical Depths to Gamma-rays Low Model, Hopkins LF, f esc = 0.2 Fiducial Model, Hopkins LF, f esc = 0.1 High-Peaked Fiducial, Hopkins LF, f esc = 0.1 Fiducial w/ S&B Model ‘C’ f esc = 0.02

26. Gamma-ray `Attenuation Edge’( τ = 1) Low Fiducial High-Peaked High QSO

27. Turnover in Optical Counts Results from the Hubble Deep Field suggest convergence in counts Madau & Pozzetti (2000) Faint-end slope < 0.4

29. History of the EBL

31. Changing IMF? Tension between stellar mass -- SFR density (and high EBL) Argued that discrepancy can be solved with changing IMF, more high mass stars at early times or in starbursts. Also: Chen et al. (2008) - star formation rates lower than previously thought for 0 < z < 1 Fardal et al. (2006) Total light in our models: 56.6 (Fiducial) 45.7 (Low)