The Cosmological Evolution of Blazars and the Extragalactic Gamma-Ray Background in the Fermi Era

arXiv:1001.0103v1 [astro-ph.HE] 31 Dec 2009 2009 Fermi Symposium, Washington, D.C., Nov. 2-5 1 The Cosmological Evolution of Blazars and the Extragalactic Gamma-Ray Background in the Fermi Era Yoshiyuki Inoue, Tomonori Totani Department of Astronomy, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan Susumu Inoue Department of Physics, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan Masakazu A. R. Kobayashi Optical and Infrared Astronomy Division, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan Jun Kataoka Waseda University, 1-104 Totsukamachi, Shinjuku-ku, Tokyo 169-8050, Japan Rie Sato Department of High Energy Astrophysics, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai, Sagamihara, 229-8510, Japan 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; here- after 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 distribu- tion) 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 correspond- ing 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. I. INTRODUCTION The origin of the extragalactic diffuse gamma-ray background (EGRB) is a long-standing puzzle. First discovered by the SAS 2 satellite [1, 2], its exis- tence was subsequently confirmed up to ∼50 GeV by EGRET (Energetic Gamma-Ray Experiment Tele- scope) on board the Compton Gamma Ray Obser- vatory. EGRET measured a flux of about 1× 10−5 photons cm−2 s−1 sr−1 above 100 MeV and an ap- proximately power-law spectrum with photon index of ∼−2 over the wide range of 30 MeV – 50 GeV [3, 4]. Very recently, Ref. [5] reported Fermi observations of the EGRB spectrum up to 100 GeV, which connects smoothly to the EGRET results below ∼200 MeV [4]. However, above this energy, the spectrum is dis- crepant from EGRET, being a single power-law with index ∼−2.4, and showing no signs of a GeV excess. Although different types of gamma-ray sources (e.g., clusters of galaxies or dark matter annihila- tion) have been proposed to be significant contrib- utors to the EGRB [see, e.g., Ref. [6] and refer- ences therein], active galactic nuclei (AGNs) of the blazar class are considered the primary candidates, since almost all of the extragalactic sources detected by EGRET were blazars. The blazar contribution to the EGRB has been estimated by numerous authors, who have reached different conclusions using different approaches, ranging from 20% to 100 % of the ob- served flux [6–18]. In most past studies, the spectral energy distri- butions (SEDs) of the blazars were assumed to be power laws for all objects. In such models, it is ob- vious that the predicted EGRB spectrum is mainly determined by the assumed power-law indices. How- ever, multi-wavelength observational studies indicate the existence of a non-trivial trend among blazar SEDs: the energies of the two characteristic spectral peaks in blazar SEDs (each likely reflecting the syn- chrotron and inverse Compton emission) systemati- cally decrease as the bolometric luminosity increases [19–25]. This is often referred to as the blazar SED sequence. Although its validity is currently still a mat- ter of debate (e.g., Ref. [24, 25, 27]), one can make a non-trivial prediction of the EGRB spectrum if this blazar sequence is assumed. Here we compare the latest Fermi EGRB results with our model predictions, previously published in eConf C091122 2 2009 Fermi Symposium, Washington, D.C., Nov. 2-5 Ref. [26] (IT09) before the former was announced. IT09 calculated the EGRB from blazars by construct- ing a blazar gamma-ray luminosity function (GLF) model that is consistent with the flux and redshift dis- tributions of the EGRET blazars, accounting for the blazar sequence as well as the luminosity-dependent density evolution (LDDE) scheme that describes well the evolution of the X-ray luminosity function (XLF) of AGNs. By introducing the blazar SED sequence, we were able to make a reasonable and non-trivial pre- diction of the EGRB spectrum that can be compared with observations including the new Ferm

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