Gamma-Ray Absorption from The Cosmic Lyman Continuum Background (original) (raw)
Related papers
Gamma-Ray Absorption by the Cosmic Lyman Continuum from Star-forming Galaxies
The Astrophysical Journal
Motivated by the discovery of the ultra-strong emission line starburst galaxies (EELGs) known as "green pea galaxies", we consider here their contribution to the intergalactic flux of ionizing UV at high redshifts. Most galaxies that have been observed show a precipitous drop in their flux blueward of the Lyman limit. However, recent observations of EELGs have discovered that many more Lyman continuum photons escape from them into intergalactic space than was previously suspected. We calculate their contribution to the extragalactic background light (EBL). We also calculate the effect of these photons on the absorption of high energy γ-rays. For the more distant γ-ray sources, particularly at z ≥ 3, the intergalactic opacity above a few GeV is significantly higher than previous estimates which ignored the Lyman continuum photons. We calculate the results of this increased opacity on observed γ-ray spectra, which produces a high-energy turnover starting at lower energies than previously thought, and a gradual spectral steepening that may also be observable.
The Astrophysical Journal, 2006
We calculate the intergalactic photon density as a function of both energy and redshift for 0 < z < 6 for photon energies from .003 eV to the Lyman limit cutoff at 13.6 eV in a ΛCDM universe with Ω Λ = 0.7 and Ω m = 0.3. The basic features of our backwards evolution model for galaxies were developed in . With a few improvements, we find that this evolutionary model gives predictions of new deep number counts from Spitzer as well as a calculation spectral energy distribution of the diffuse infrared background which are in good agreement with the data. We then use our calculated intergalactic photon densities to extend previous work on the absorption of high energy γ-rays in intergalactic space owing to interactions with low energy photons and the 2.7 K cosmic microwave background radiation. We calculate the optical depth of the universe, τ , for γ-rays having energies from 4 GeV to 100 TeV emitted by sources at redshifts from 0 to 5. We also give an analytic fit with numerical coefficients for approximating τ (E γ , z). As an example of the application of our results, we calculate the absorbed spectrum of the blazar PKS 2155-304 at z = 0.117 and compare it with the spectrum observed by the H.E.S.S. air Cherenkov γ-ray telescope array.
Absorption of high energy gamma-rays by low energy intergalactic photons
Space Science Reviews, 1996
Following our previously proposed technique, we have used the recent -ray observations of Mkr421 to place theoretically signi cant constraints on and possible estimates of the intergalactic infrared radiation eld (IIRF). Our 2 upper limits and estimates are consistent with normal galactic IR production by stars and dust. They rule out exotic mechanisms proposed to produce a larger IIRF. We nd possible evidence for intergalactic absorption of -rays above 3 TeV energy in the observed spectrum of Mkr421. The implied IIRF is of the magnitude expected to be produced by stellar emission and reprocessing in galaxies. The sharpness of the spectral turnover is not consistent with the interpretation of this feature as a cuto in the emission from the source. However, should the cuto primarily be a result of absorption within the source itself, our possible extragalactic ux estimate becomes the strongest upper limit extant on the IIRF. Using models for the low energy intergalactic photon spectrum from microwave to ultraviolet energies, we calculate the opacity of intergalactic space to -rays as a function of energy and redshift. These calculations indicate that the GeV -ray burst recently observed by the CGRO EGRET detector originates at a redshift less than 1:5.
Intergalactic extinction of high energy gamma-rays
Astroparticle Physics, 1999
We discuss the determination of the intergalactic pair-production absorption coefficient as derived by Stecker and De Jager by making use of a new empirically based calculation of the spectral energy distribution of the intergalactic infrared radiation field as given by Malkan and Stecker. We show that the results of the Malkan and Stecker calculation agree well with recent data on the infrared background. We then show that previous spectral data from observations of Mrk 421 and Mrk 501 are consistent with the amount of intergalactic absorption predicted by Stecker and De Jager and that the new HEGRA observations of the flaring spectrum of Mrk 501 presented at this conference actually appear to show the amount of intergalactic absorption which we predict. As a further test for intergalactic absorption, we give a predicted spectrum, with absorption included, for PKS 2155-304. This XBL lies at a redshift of 0.12, the highest redshift source yet observed at an energy above 0.3 TeV. This source should have its spectrum steepened by ∼ 1 in its spectral index between ∼ 0.3 and ∼ 3 TeV and should show an absorption cutoff above ∼ 6 TeV. We also discuss the determination of the γ-ray opacity at higher redshifts (out to z = 3), following the treatment of Salamon and Stecker.
The extragalactic background light and the gamma-ray opacity of the universe
Astroparticle Physics, 2013
The extragalactic background light (EBL) is one of the fundamental observational quantities in cosmology. All energy releases from resolved and unresolved extragalactic sources, and the light from any truly diffuse background, excluding the cosmic microwave background (CMB), contribute to its intensity and spectral energy distribution. It therefore plays a crucial role in cosmological tests for the formation and evolution of stellar objects and galaxies, and for setting limits on exotic energy releases in the universe. The EBL also plays an important role in the propagation of very high energy γ−rays which are attenuated en route to Earth by pair producing γ − γ interactions with the EBL and CMB. The EBL affects the spectrum of the sources, predominantly blazars, in the ∼ 10 GeV to 10 TeV energy regime. Knowledge of the EBL intensity and spectrum will allow the determination of the intrinsic blazar spectrum in a crucial energy regime that can be used to test particle acceleration mechanisms and VHE γ−ray production models. Conversely, knowledge of the intrinsic γ−ray spectrum and the detection of blazars at increasingly higher redshifts will set strong limits on the EBL and its evolution. This paper reviews the latest developments in the determination of the EBL and its impact on the current understanding of the origin and production mechanisms of γ−rays in blazars, and on energy releases in the universe. The review concludes with a summary and future directions in Cherenkov Telescope Array techniques and in infrared ground-based and space observatories that will greatly improve our knowledge of the EBL and Preprint submitted to Astroparticle Physics the origin and production of very high energy γ−rays.
The extragalactic diffuse gamma-ray emission
1997
The all-sky surveys in γ-rays by the imaging Compton telescope (COMPTEL) and the Energetic Gamma Ray Experiment Telescope (EGRET) on board the Compton Gamma Ray Observatory for the first time allows detailed studies of the extragalactic diffuse emission at γ-ray energies greater 1 MeV. A preliminary analysis of COMPTEL data indicates a significant decrease in the level of the derived cosmic diffuse emission from previous estimates in the 1-30 MeV range, with no evidence for an MeV-excess, at least not at the levels claimed previously. The 1-30 MeV flux measurements are compatible with power-law extrapolation from lower and higher energies. These new results indicate that the possible contributions to the extragalactic emission from processes that could explain the MeV-excess, such as matter-antimatter annihilation, is significantly reduced. At high energies (> 30 MeV), the extragalactic emission is well described by a power law photon spectrum with an index of -(2.10±0.03) in the 30 MeV to 100 GeV energy range. No large scale spatial anisotropy or changes in the energy spectrum are observed in the deduced extragalactic emission. The most likely explanation for the origin of this extragalactic γ-ray emission above 10 MeV, is that it arises primarily from unresolved γ-ray-emitting blazars. The consistency of the average γ-ray blazar spectrum with the derived extragalactic diffuse spectrum strongly argues in favor of such an origin. The extension of the power law spectrum to 100 GeV implies the average emission from γ-ray blazars extends to 100 GeV.
Components of the Extragalactic Gamma-Ray Background
The Astrophysical Journal, 2011
We present new theoretical estimates of the relative contributions of unresolved blazars and starforming galaxies to the extragalactic γ-ray background (EGB) and discuss constraints on the contributions from alternative mechanisms such as dark matter annihilation and truly diffuse γ-ray production. We find that the Fermi source count data do not rule out a scenario in which the EGB is dominated by emission from unresolved blazars, though unresolved star-forming galaxies may also contribute significantly to the background, within order-of-magnitude uncertainties. In addition, we find that the spectrum of the unresolved star-forming galaxy contribution cannot explain the EGB spectrum found by EGRET at energies between 50 and 200 MeV, whereas the spectrum of unresolved FSRQs, when accounting for the energy-dependent effects of source confusion, could be consistent with the combined spectrum of the low-energy EGRET EGB measurements and the Fermi-LAT EGB measurements.
Absorption of Intergalactic TeV Gamma-Rays
We discuss the problem of the absorption of very high-energy gamma-rays by pair production interactions with extragalactic photons which originate from stellar emission in the near IR-UV and reradiation of starlight in the mid- and far-IR. The absorption of gamma-rays above 1 TeV is dominated by interactions with infrared photons. We make a new determination of the optical depth of the universe to multi-TeV photons as a function of energy and redshift and use the results to compare with recent spectral data of Mrk 421 and Mrk 501, sources that have been observed in the flaring state up to apx. 10 TeV energy. For the optical depth calculations, we have made use of a new, empirically based calculation of the intergalactic radiation field by Malkan & Stecker which we consider to be more accurate than that based on previous theoretical modeling. We also discuss the absorption of sub-TeV gamma-rays by starlight photons at high redshifts.
The case for a low extragalactic gamma-ray background
Journal of Cosmology and Astroparticle Physics, 2004
Measurements of the diffuse extragalactic γ-ray background (EGRB) are complicated by a strong Galactic foreground. Estimates of the EGRB flux and spectrum, obtained by modeling the Galactic emission, have produced a variety of (sometimes conflicting) results. The latest analysis of the EGRET data found an isotropic flux I x = 1.45 ± 0.05 above 100 MeV, in units of 10 −5 ph s −1 cm −2 sr −1 . We analyze the EGRET data in search for robust constraints on the EGRB flux, finding the γ-ray sky strongly dominated by Galactic foreground even at high latitudes, with no conclusive evidence for an additional isotropic component. The γ-ray intensity measured towards the Galactic poles is similar to or lower than previous estimates of I x , even before Galactic foreground subtraction. The high latitude profile of the γ-ray data is disk-like for 40 • |b| 70 • , and even steeper for |b| 70 • ; overall it exhibits strong Galactic features and is well fit by a simple Galactic model. Based on the |b| > 40 • data we find that I x < 0.5 at a 99% confidence level, with evidence for a much lower flux. We show that correlations with Galactic tracers, previously used to identify the Galactic foreground and estimate I x , are not satisfactory; the results depend on the tracers used and on the part of the sky examined, because the Galactic emission is not linear in the Galactic tracers, and exhibits spectral variations across the sky. The low EGRB flux favored by our analysis places stringent limits on extragalactic scenarios involving γ-ray emission, such as radiation from blazars, intergalactic shocks and production of ultra-high energy cosmic rays and neutrinos. We suggest methods by which future γ-ray missions such as GLAST and AGILE could indirectly identify the EGRB.
Escape of about five per cent of Lyman-α photons from high-redshift star-forming galaxies
Nature, 2010
The Lyman-α (Lyα) emission line is the primary observational signature of starforming galaxies at the highest redshifts 1 , and has enabled the compilation of large samples of galaxies with which to study cosmic evolution 2-5 . The resonant nature of the line, however, means that Lyα photons scatter in the neutral interstellar medium of their host galaxies, and their sensitivity to absorption by interstellar dust may therefore be enhanced greatly. This implies that the Lyα luminosity may be significantly reduced, or even completely suppressed. Hitherto, no unbiased empirical test of the escaping fraction (f esc ) of Lyα photons has been performed at high redshifts. Here we report that the average f esc from star-forming galaxies at redshift z = 2.2 is just 5 per cent by performing a blind narrowband survey in Lyα and Hα. This implies that numerous conclusions based on Lyα-selected samples will require upwards revision by an order of magnitude and we provide a benchmark for this revision. We demonstrate that almost 90 per cent of starforming galaxies emit insufficient Lyα to be detected by standard selection criteria 2-5 . Both samples show an anti-correlation of f esc with dust content, and we show that Lyα-and Hα-selection recovers populations that differ substantially in dust content and f esc .