[Paper Review] The Cosmological Evolution of Blazars and the Extragalactic Gamma-Ray Background in the Fermi Era

Yoshiyuki Inoue, Tomonori Totani+4 more|arXiv (Cornell University)|Dec 31, 2009

Astrophysics and Cosmic Phenomena1 references3 citations

TL;DR

This paper proposes a non-trivial model of the extragalactic gamma-ray background (EGRB) using the blazar spectral energy distribution (SED) sequence and luminosity-dependent density evolution (LDDE), showing excellent agreement with Fermi's EGRB spectrum. The key result is that blazars dominate the EGRB, accounting for ~45% of the flux above 100 MeV, with non-blazar AGNs contributing significantly at lower energies, and Fermi is predicted to detect ~1,200 blazars over five years, including some up to z ~ 6.

ABSTRACT

The latest determination of the extragalactic gamma-ray background (EGRB) radiation by Fermi is compared with the theoretical prediction of the blazar component by Inoue & Totani (2009; hereafter IT09). The Fermi EGRB spectrum is in excellent agreement with IT09, indicating that blazars are the dominant component of the EGRB, and contributions from any other sources (e.g., dark matter annihilations) are minor. It also indicates that the blazar SED (spectral energy distribution) sequence taken into account in IT09 is a valid description of mean blazar SEDs. The possible contribution of MeV blazars to the EGRB in the MeV band is also discussed. In five total years of observations, we predict that Fermi will detect ~1200 blazars all sky down to the corresponding sensitivity limit. We also address the detectability of the highest-redshift blazars. Updating our model with regard to high-redshift evolution based on SDSS quasar data, we show that Fermi may find some blazars up to z~6 during the five-year survey. Such blazars could provide a new probe of early star and galaxy formation through GeV spectral attenuation signatures induced by high-redshift UV background radiation.

Motivation & Objective

• To test the validity of the blazar SED sequence in predicting the extragalactic gamma-ray background (EGRB) using Fermi-era data.
• To determine the relative contributions of blazars and non-blazar AGNs to the EGRB across MeV to GeV energies.
• To predict the number of blazars detectable by Fermi over a five-year survey, including high-redshift sources.
• To assess the detectability of high-redshift blazars (z ~ 6) and their potential as probes of the high-redshift UV background and early galaxy formation.

Proposed method

• Construct a blazar gamma-ray luminosity function (GLF) model incorporating the blazar SED sequence and luminosity-dependent density evolution (LDDE) based on X-ray AGN luminosity functions.
• Use the SED sequence to model the spectral energy distributions of blazars, with peak frequencies and luminosities correlated to bolometric luminosity.
• Predict the EGRB spectrum by integrating the GLF over redshift and luminosity, accounting for cosmological evolution and photon propagation effects.
• Compare the predicted EGRB spectrum with Fermi's observed EGRB data, including the 100 GeV extension of the spectrum.
• Estimate detectable blazar counts by Fermi using sensitivity limits of 3×10⁻⁹ and 1×10⁻⁹ photons cm⁻² s⁻¹ above 100 MeV for one- and five-year surveys.
• Update the GLF model with high-redshift evolution constraints from SDSS quasar data to predict detectability of blazars up to z ~ 6.

Experimental results

Research questions

• RQ1 Does the blazar SED sequence provide a valid and non-trivial prediction for the EGRB spectrum that matches Fermi observations?
• RQ2 What fraction of the EGRB above 100 MeV is contributed by blazars versus non-blazar AGNs?
• RQ3 How many blazars will Fermi detect over a five-year mission, and what is the expected redshift distribution?
• RQ4 Can high-redshift blazars (z ~ 6) be detected by Fermi, and can their spectra reveal signatures of the high-redshift UV background?

Key findings

• The predicted EGRB spectrum from blazars using the SED sequence and LDDE model shows excellent agreement with Fermi's observed EGRB spectrum, confirming the model's validity.
• Blazars account for approximately 45% of the observed EGRB flux above 100 MeV, with non-blazar AGNs contributing significantly at lower energies.
• Fermi is predicted to detect approximately 1,200 blazars over a five-year survey, assuming a sensitivity limit of 1×10⁻⁹ photons cm⁻² s⁻¹ above 100 MeV.
• The model predicts that Fermi will resolve over 98% of the EGRB flux from blazars into discrete sources at energies >1 GeV, while less than 0.1% of the non-blazar AGN contribution will be resolved.
• High-redshift blazars up to z ~ 6 are predicted to be detectable by Fermi, offering a new probe of the high-redshift extragalactic UV background via GeV spectral attenuation.
• The dominant component of the MeV EGRB is non-blazar AGNs, not 'MeV blazars' as previously suggested, based on the nonthermal coronal electron model.