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Register a new userGamma-Ray Absorption By The Cosmic Lyman Continuum From Star-forming Galaxies
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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 $gamma$-rays. For the more distant $gamma$-ray sources, particularly at $z ge 3$, 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 $gamma$-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.
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We present the observations of Lyman continuum (LyC) emission in the afterglow spectra of GRB 191004B at $z=3.5055$, together with those of the other two previously known LyC-emitting long gamma-ray bursts (LGRB) (GRB 050908 at $z=3.3467$, and GRB 060607A at $z=3.0749$), to determine their LyC escape fraction and compare their properties. From the afterglow spectrum of GRB 191004B we determine a neutral hydrogen column density at the LGRB redshift of $log(N_{rm HI}/cm^{-2})= 17.2 pm 0.15$, and negligible extinction ($A_{rm V}=0.03 pm 0.02$ mag). The only metal absorption lines detected are CIV and SiIV. In contrast to GRB 050908 and GRB 060607A, the host galaxy of GRB 191004B displays significant Ly$alpha$ emission. From its Ly$alpha$ emission and the non-detection of Balmer emission lines we constrain its star-formation rate (SFR) to $1 leq$ SFR $leq 4.7$ M$_{odot} yr^{-1}$. We fit the Ly$alpha$ emission with a shell model and find parameters values consistent with the observed ones. The absolute LyC escape fractions we find for GRB 191004B, GRB 050908 and GRB 060607A are of $0.35^{+0.10}_{-0.11}$, $0.08^{+0.05}_{-0.04}$ and $0.20^{+0.05}_{-0.05}$, respectively. We compare the LyC escape fraction of LGRBs to the values of other LyC emitters found from the literature, showing that LGRB afterglows can be powerful tools to study LyC escape for faint high-redshift star-forming galaxies. Indeed we could push LyC leakage studies to much higher absolute magnitudes. The host galaxies of the three LGRB presented here have all $M_{rm 1600} > -19.5$ mag, with the GRB 060607A host at $M_{rm 1600} > -16$ mag. LGRB hosts may therefore be particularly suitable for exploring the ionizing escape fraction in galaxies that are too faint or distant for conventional techniques. Furthermore the time investment is very small compared to galaxy studies. [Abridged]
Following our first detection reported in Izotov et al. (2016), we present the detection of Lyman continuum (LyC) radiation of four other compact star-forming galaxies observed with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST). These galaxies, at redshifts of z~0.3, are characterized by high emission-line flux ratios [OIII]5007/[OII]3727 > 5. The escape fractions of the LyC radiation fesc(LyC) in these galaxies are in the range of ~6%-13%, the highest values found so far in low-redshift star-forming galaxies. Narrow double-peaked Lyalpha emission lines are detected in the spectra of all four galaxies, compatible with predictions for Lyman continuum leakers. We find escape fractions of Lyalpha, fesc(Lyalpha) ~20%-40%, among the highest known for Lyalpha emitters (LAEs). Surface brightness profiles produced from the COS acquisition images reveal bright star-forming regions in the center and exponential discs in the outskirts with disc scale lengths alpha in the range ~0.6-1.4 kpc. Our galaxies are characterized by low metallicity, ~1/8-1/5 solar, low stellar mass ~(0.2 - 4)e9 Msun, high star formation rates SFR~14-36 Msun/yr, and high SFR densities Sigma~2-35 Msun/yr/kpc^2. These properties are comparable to those of high-redshift star-forming galaxies. Finally, our observations, combined with our first detection reported in Izotov et al. (2016), reveal that a selection for compact star-forming galaxies showing high [OIII]5007/[OII]3727 ratios appears to pick up very efficiently sources with escaping Lyman continuum radiation: all five of our selected galaxies are LyC leakers.
A majority of the $gamma$-ray emission from star-forming galaxies is generated by the interaction of high-energy cosmic rays with the interstellar gas and radiation fields. Star-forming galaxies are expected to contribute to both the extragalactic $gamma$-ray background and the IceCube astrophysical neutrino flux. Using roughly 10,years of $gamma$-ray data taken by the {it Fermi} Large Area Telescope, in this study we constrain the $gamma$-ray properties of star-forming galaxies. We report the detection of 11 bona-fide $gamma$-ray emitting galaxies and 2 candidates. Moreover, we show that the cumulative $gamma$-ray emission of below-threshold galaxies is also significantly detected at $sim$5,$sigma$ confidence. The $gamma$-ray luminosity of resolved and unresolved galaxies is found to correlate with the total (8-1000,$mu$m) infrared luminosity as previously determined. Above 1,GeV, the spectral energy distribution of resolved and unresolved galaxies is found to be compatible with a power law with a photon index of $approx2.2-2.3$. Finally, we find that star-forming galaxies account for roughly 5,% and 3,% of the extragalactic $gamma$-ray background and the IceCube neutrino flux, respectively.
In recent years, $gamma$-ray emission has been detected from star-forming galaxies (SFGs) in the local universe, including M82, NGC 253, Arp 220 and M33. The bulk of this emission is thought to be of hadronic origin, arising from the interactions of cosmic rays (CRs) with the interstellar medium of their host galaxy. Distant SFGs are presumably also bright in $gamma$-rays. Although they would not be resolvable as point sources, distant unresolved SFG populations contribute $gamma$-rays to the extra-galactic $gamma$-ray background (EGB). Despite the wealth of high-quality all-sky EGB data collected over more than a decade of operation with the textit{Fermi}-LAT $gamma$-ray space telescope, the exact contribution of SFGs to the EGB remains unsettled. In this study, we model the $gamma$-ray emission from SFG populations and demonstrate that such emission can be characterized by just a small number of physically-motivated parameters. We further show that source populations would leave anisotropic signatures in the EGB, which could be used to yield information about the underlying properties, dynamics and evolution of CR-rich SFGs.
Star forming galaxies emit GeV- and TeV-gamma rays that are thought to originate from hadronic interactions of cosmic-ray (CR) nuclei with the interstellar medium. To understand the emission, we have used the moving mesh code Arepo to perform magneto-hydrodynamical galaxy formation simulations with self-consistent CR physics. Our galaxy models exhibit a first burst of star formation that injects CRs at supernovae. Once CRs have sufficiently accumulated in our Milky-Way like galaxy, their buoyancy force overcomes the magnetic tension of the toroidal disk field. As field lines open up, they enable anisotropically diffusing CRs to escape into the halo and to accelerate a bubble-like, CR-dominated outflow. However, these bubbles are invisible in our simulated gamma-ray maps of hadronic pion-decay and secondary inverse-Compton emission because of low gas density in the outflows. By adopting a phenomenological relation between star formation rate (SFR) and far-infrared emission and assuming that gamma rays mainly originate from decaying pions, our simulated galaxies can reproduce the observed tight relation between far-infrared and gamma-ray emission, independent of whether we account for anisotropic CR diffusion. This demonstrates that uncertainties in modeling active CR transport processes only play a minor role in predicting gamma-ray emission from galaxies. We find that in starbursts, most of the CR energy is calorimetrically lost to hadronic interactions. In contrast, the gamma-ray emission deviates from this calorimetric property at low SFRs due to adiabatic losses, which cannot be identified in traditional one-zone models.
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