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Register a new userThe Cosmological Impact of Luminous TeV Blazars I: Implications of Plasma Instabilities for the Intergalactic Magnetic Field and Extragalactic Gamma-Ray Background
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Avery E. Broderick
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Inverse-Compton cascades initiated by energetic gamma rays (E>100 GeV) enhance the GeV emission from bright, extragalactic TeV sources. The absence of this emission from bright TeV blazars has been used to constrain the intergalactic magnetic field (IGMF), and the stringent limits placed upon the unresolved extragalactic gamma-ray background (EGRB) by Fermi has been used to argue against a large number of such objects at high redshifts. However, these are predicated upon the assumption that inverse-Compton scattering is the primary energy-loss mechanism for the ultra-relativistic pairs produced by the annihilation of the energetic gamma rays on extragalactic background light photons. Here we show that for sufficiently bright TeV sources (isotropic-equivalent luminosities >10^{42} erg/s) plasma beam instabilities, specifically the oblique instability, present a plausible mechanism by which the energy of these pairs can be dissipated locally, heating the intergalactic medium. Since these instabilities typically grow on timescales short in comparison to the inverse-Compton cooling rate, they necessarily suppress the inverse-Compton cascades. As a consequence, this places a severe constraint upon efforts to limit the IGMF from the lack of a discernible GeV bump in TeV sources. Similarly, it considerably weakens the Fermi limits upon the evolution of blazar populations. Specifically, we construct a TeV-blazar luminosity function from those objects presently observed and find that it is very well described by the quasar luminosity function at z~0.1, shifted to lower luminosities and number densities, suggesting that both classes of sources are regulated by similar processes. Extending this relationship to higher redshifts, we show that the magnitude and shape of the EGRB above ~10 GeV is naturally reproduced with this particular example of a rapidly evolving TeV-blazar luminosity function.
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Fermi has been instrumental in constraining the luminosity function and redshift evolution of gamma-ray bright blazars. This includes limits upon the spectrum and anisotropy of the extragalactic gamma-ray background (EGRB), redshift distribution of nearby Fermi active galactic nuclei (AGN), and the construction of a log(N)-log(S) relation. Based upon these, it has been argued that the evolution of the gamma-ray bright blazar population must be much less dramatic than that of other AGN. However, critical to such claims is the assumption that inverse Compton cascades reprocess emission above a TeV into the Fermi energy range, substantially enhancing the strength of the observed limits. Here we demonstrate that in the absence of such a process, due, e.g., to the presence of virulent plasma beam instabilities that preempt the cascade, a population of TeV-bright blazars that evolve similarly to quasars is consistent with the population of hard gamma-ray blazars observed by Fermi. Specifically, we show that a simple model for the properties and luminosity function is simultaneously able to reproduce their log(N)-log(S) relation, local redshift distribution, and contribution to the EGRB and its anisotropy without any free parameters. Insofar the naturalness of a picture in which the hard gamma-ray blazar population exhibits the strong redshift evolution observed in other tracers of the cosmological history of accretion onto halos is desirable, this lends support for the absence of the inverse Compton cascades and the existence of the beam plasma instabilities.
The Universe is opaque to extragalactic very high-energy gamma rays (VHEGRs, E>100 GeV) because they annihilate and pair produce on the extragalactic background light. The resulting ultra-relativistic pairs are assumed to lose energy through inverse Compton scattering of CMB photons. In Broderick et al. (2011, Paper I of this three paper series), we argued that instead powerful plasma instabilities in the ultra-relativistic pair beam dissipate the kinetic energy of the TeV-generated pairs locally, heating the intergalactic medium (IGM). Here, we explore the effect of this heating upon the thermal history of the IGM. We collate the observed extragalactic VHEGR sources to determine a local VHEGR heating rate and correct for the pointed nature of VHEGR observations using Fermi observations of high and intermediate peaked BL Lacs. Because the local extragalactic VHEGR flux is dominated by TeV blazars, we tie the TeV blazar luminosity density to the quasar luminosity density, and produce a VHEGR heating rate as a function of redshift. This heating is relatively homogeneous for z<~4 with increasing spatial variation at higher redshift (order unity at z~6). This new heating process dominates photoheating at low redshift and the inclusion of TeV blazar heating qualitatively and quantitatively changes the structure and history of the IGM. TeV blazars produce a uniform volumetric heating rate that is sufficient to increase the temperature of the mean density IGM by nearly an order of magnitude, and at low densities by substantially more, naturally producing an inverted equation of state inferred by observations of the Ly-alpha forest, a feature that is difficult to reconcile with standard reionization models. Finally, we close with a discussion on the possibility of detecting this hot low-density IGM, but find that such measurements are currently not feasible. (abridged)
A subset of blazars emit TeV gamma rays which annihilate and pair produce on the extragalactic background light. We have argued in Broderick et al. (2011, Paper I) that plasma beam instabilities can dissipate the pairs energy locally. This heats the intergalactic medium and dramatically increases its entropy after redshift z~2, with important implications for structure formation: (1) This suggests a scenario for the origin of the cool core (CC)/non-cool core (NCC) bimodality in galaxy clusters and groups. Early forming galaxy groups are unaffected because they can efficiently radiate the additional entropy, developing a CC. However, late forming groups do not have sufficient time to cool before the entropy is gravitationally reprocessed through successive mergers - counteracting cooling and raising the core entropy further. Hence blazar heating works different than feedback by active galactic nuclei, which balances radiative cooling but is unable to transform CC into NCC clusters due to the weak coupling to the cluster gas. (2) We predict a suppression of the Sunyaev-Zeldovich power spectrum on angular scales smaller than 5 due to the globally reduced central pressure of groups and clusters forming after z~1. (3) Our redshift dependent entropy floor increases the characteristic halo mass below which dwarf galaxies cannot form by a factor of ~10 (50) at mean density (in voids) over that found in models that include photoionization alone. This prevents the formation of late forming dwarf galaxies (z<2) with masses ranging from 10^{10} to 10^{11} M_sun for redshifts z~2 to 0, respectively. This may help resolve the missing satellite problem in the Milky Way and the void phenomenon of the low observed abundances of dwarf satellites compared to cold dark matter simulations and may bring the observed early star formation histories into agreement with galaxy formation models. (abridged)
Data from (non-) attenuation of gamma rays from active galactic nuclei (AGN) and gamma ray bursts (GRBs) give upper limits on the extragalactic background light (EBL) from the UV to the mid-IR that are only a little above the lower limits from observed galaxies. These upper limits now rule out some EBL models and purported observations, with improved data likely to provide even stronger constraints. We present EBL calculations both based on multiwavelength observations of thousands of galaxies and also based on semi-analytic models, and show that they are consistent with these lower limits from observed galaxies and with the gamma-ray upper limit constraints. Such comparisons close the loop on cosmological galaxy formation models, since they account for all the light, including that from galaxies too faint to see. We compare our results with those of other recent works, and discuss the implications of these new EBL calculations for gamma ray attenuation. Catching a few GRBs with groundbased atmospheric Cherenkov Telescope (ACT) arrays or water Cherenkov detectors could provide important new constraints on the high-redshift star formation history of the universe.
Recent claims that the strength B_IGMF of the intergalactic magnetic field (IGMF) is >~ 1e-15 G are based on upper limits to the expected cascade flux in the GeV band produced by blazar TeV photons absorbed by the extragalactic background light. This limit depends on an assumption that the mean blazar TeV flux remains constant on timescales >~2 (B_ IGMF/1e-18 G)^2 / (E/{10 GeV})^2 yr for an IGMF coherence length ~ 1 Mpc, where E is the measured photon energy. Restricting TeV activity of 1ES 0229+200 to ~3 -- 4 years during which the source has been observed leads to a more robust lower limit of B_IGMF >~ 1e-18 G, which can be larger by an order of magnitude if the intrinsic source flux above ~5 -- 10 TeV from 1ES 0229+200 is strong.
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