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Register a new userGamma-ray bursts in normal and extreme star-forming galaxies
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We discuss how gamma-ray burst (GRB) optical afterglows and multiwavelength observations of their host galaxies can be used to obtain information about the relative amounts of star formation happening in optical and submillimetre galaxies. That such an analysis will be possible follows from the currently-favoured idea that GRBs are closely linked with high-mass star formation. Studying GRB host galaxies offers a method of finding low-luminosity submillimetre galaxies, which cannot be identified either in optical Lyman break surveys, because so much of their star formation is hidden by dust, or in submillimetre surveys, because their submillimetre fluxes are close to or below the confusion limit. Much of the star formation in the Universe could have occurred in such objects, so searching for them is an important exercise. From current observations, GRB host galaxies appear to be neither optically-luminous Class-2 SCUBA galaxies like SMM J02399$-$0136 or SMM J14011+0252, nor galaxies containing dense molecular cores like local ultraluminous infrared galaxies (ULIGs), but rather some intermediate kind of galaxy. The host galaxy of GRB 980703 is a prototype of this kind of galaxy.
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The data on the location of gamma-ray bursts (GRBs) relative to their host galaxies are used to derive the distribution of surface density of GRBs along the galaxy radius. It is shown that the gradient of GRB surface density changes abruptly near the half-light radius. In the central parts of galaxies the distribution of GRBs resembles closely the luminosity distribution, while in the outer parts the galactic surface brightness falls much steeper than the GRBs density. The radial distribution of type Ib/c supernovae is investigated on the basis of enlarged statistics. It is shown that SNe Ib/c do not differ significantly from other types of supernovae and their distribution is more similar to the one for recent star formation sites than that of GRBs. In spite of the poor statistics of GRBs, the difference in the distributions of active star formation regions and GRBs appears to be significant. We get the Kolmogorov-Smirnov probability P_ks of only 4% that GRBs and star-forming sites belong to the same distribution. The correlation of GRBs with the distribution of dark matter in the outer parts of galaxies is not excluded.
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.
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.
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]
We present the results of the 16-cm-waveband continuum observations of four host galaxies of gamma-ray bursts (GRBs) 990705, 021211, 041006, and 051022 using the Australia Telescope Compact Array. Radio emission was not detected in any of the host galaxies. The 2sigma upper limits on star-formation rates derived from the radio observations of the host galaxies are 23, 45, 27, and 26 Msun/yr, respectively, which are less than about 10 times those derived from UV/optical observations, suggesting that they have no significant dust-obscured star formation. GRBs 021211 and 051022 are known as the so-called dark GRBs and our results imply that dark GRBs do not always occur in galaxies enshrouded by dust. Because large dust extinction was not observed in the afterglow of GRB021211, our result {bf suggests the possibility} that the cause of the dark GRB is the intrinsic faintness of the optical afterglow. On the other hand, by considering the high column density observed in the afterglow of GRB051022, the likely cause of the dark GRB is the dust extinction in the line of sight of the GRB.
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