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)

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

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:

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)

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

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)

Modeling of the galaxy population  Evolution inferred from observations Kneiske et al. 2004; Finke et al models based on star formation rate density, stellar synthesis models, dust reradiation Franceschini et al 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 based on power law evolution of existing galaxy pop.  Forward evolution, begin from cosmological initial conditions Primack et al. 2001, 2005, and this work

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:

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)

Gilmore et al., arXiv: The High Redshift UV Background Affects gamma-rays from distant sources, observed in GeV energy range. Fermi LAT is studying the little-understood energy decade of 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

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

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

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

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

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 (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

***Bonus Slides***

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

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.

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

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

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

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

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

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

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

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

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

History of the EBL

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)