Gamma-Ray Absorption by the Cosmic Lyman Continuum from Star-forming Galaxies (original) (raw)
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Gamma-Ray Absorption from The Cosmic Lyman Continuum Background
2020
Until recently, it was assumed that the diffuse extragalactic background light (EBL) in the FUV has a sharp and complete cut-off at energies above the Lyman limit. However, newly discovered extreme emission line starburst galaxies (EELGs) and other galaxies with substantial escape fractions of EUV photons in the Lyman continuum (LyC) region are increasingly common at high redshifts. The recent observations indicate that many more LyC photons at high redshifts escape from such galaxies into intergalactic space than was previously suspected. They are probably the major cause of reionization of the intergalactic medium. In this paper we estimate the contribution of these hitherto unconsidered EUV photons from EELGs at all redshifts to the (EBL), and their subsequent effect on the absorption of extragalactic high energy gamma\gammagamma-rays. For the more distant gamma\gammagamma-ray sources, particularly at zge3z \ge 3zge3, the intergalactic opacity above a few GeV is significantly higher than previously es...
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.
Lyman Continuum Emission Escaping from Green Pea Galaxies at z=0.5
2019
Compact starburst galaxies are thought to include many or most of the galaxies from which substantial Lyman continuum emission can escape into the intergalactic medium. Li and Malkan (2018) used SDSS photometry to find a population of such starburst galaxies at z~0.5. They were discovered by their extremely strong [OIII]4959+5007 emission lines, which produce a clearly detectable excess brightness in the i bandpass, compared with surrounding filters. We therefore used the HST/COS spectrograph to observe two of the newly discovered i-band excess galaxies around their Lyman limits. One has very strongly detected continuum below its Lyman limit, corresponding to an escape fraction of ionizing photons of 20+/-2%. The other, which is less compact in UV imaging, has a 3-sigma upper limit to its Lyman escape fraction of <5%. Before the UV spectroscopy, the existing data could hardly distinguish these two galaxies. This suggests that roughly ~half of the strong [OIII] emitters as a class...
Lyman Continuum Emission Escaping from Luminous Green Pea Galaxies at z = 0.5
The Astrophysical Journal, 2021
Compact starburst galaxies are thought to include many or most of the galaxies from which substantial Lyman continuum emission can escape into the intergalactic medium. Li & Malkan used Sloan Digital Sky Survey photometry to find a population of such starburst galaxies at z ∼ 0.5. They were discovered by their extremely strong [O iii] λλ4959+5007 emission lines, which produce a clearly detectable excess brightness in the i bandpass, compared with surrounding filters. We therefore used the Hubble Space Telescope (HST)/COS spectrograph to observe two of the newly discovered i-band excess galaxies around their Lyman limits. One has strongly detected continuum below its Lyman limit, corresponding to a relative escape fraction of ionizing photons of 20% ± 2%. The other, which is less compact in UV imaging, has a 2σ upper limit to its Lyman escape fraction of <5%. Before the UV spectroscopy, the existing data could not distinguish these two galaxies. Although a sample of two is hardly ...
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.
Astronomy & Astrophysics, 2015
Aims. We aim to investigate the effect of the escaping ionizing radiation on the color selection of high redshift galaxies and identify candidate Lyman continuum (LyC) emitters. Methods. The intergalactic medium prescription of and galaxy synthesis models of have been used to properly treat the ultraviolet stellar emission and the stochasticity of the intergalactic transmission and mean free path in the ionizing regime. Color tracks are computed by turning on/off the escape fraction fesc of ionizing radiation. Results. At variance with recent studies, a careful treatment of IGM transmission leads to no significant effects on the high-redshift broad-band color selection, even adopting the most extreme ionizing emission model (with age of 1Myr, zero dust and metallicity Z/Z =0.02). The decreasing mean free path of ionizing photons with increasing redshift further diminishes the contribution of the LyC to broad-band colors. We demonstrate that prominent LyC sources can be selected under suitable conditions by calculating the probability of a null escaping ionizing radiation. It is performed by running ad hoc Monte Carlo simulations anchored to the observed photometry, exploring the stochasticity of the IGM and comparing the simulated and observed colors encompassing the Lyman edge. The method is applied to a sample of galaxies extracted from the GOODS-S field. A known LyC source at z = 3.795 is successfully recovered as a LyC emitter candidate and another convincing candidate at z = 3.212 is reported. A detailed analysis of the two sources (including their variability and morphology) suggests a possible mixture of stellar and non-stellar (AGN) contribution in the ultraviolet.
Astrophysical Journal, 1997
We have reevaluated the constraints on the Lyman continuum emission from four starburst galaxies observed with the Hopkins Ultraviolet Telescope (HUT) during the Astro-2 mission. Applying a detailed model of the absorption by interstellar gases in our Galaxy, and using the latest HUT data products, we find upper limits to the redshifted Lyman continuum that are less restrictive than those reported previously (Leitherer et al. 1995 ApJ, 454, L19). Well determined astrophysical and instrumental effects permit 2 σ upper limits no tighter than 5.2%, 11%, 57%, and 3.2% to be set on the escape fraction of Lyman continuum photons from IRAS 08339+6517, Mrk 1267, Mrk 66, and Mrk 496, respectively. Absorption from undetected interstellar components (including H 2 ) or modulation of the emergent spectrum by gas or dust in the parent galaxy could allow the true escape fractions to exceed these revised upper limits.
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.