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Register a new userWhy Are Some Gamma-Ray Bursts Hosted by Oxygen-rich Galaxies?
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Theoretically long gamma-ray bursts (GRBs) are expected to happen in low-metallicity environments, because in a single massive star scenario, low iron abundance prevents loss of angular momentum through stellar wind, resulting in ultra-relativistic jets and the burst. In this sense, not just a simple metallicity measurement but also low iron abundance ([Fe/H]<-1.0) is essentially important. Observationally, however, oxygen abundance has been measured more often due to stronger emission. In terms of oxygen abundance, some GRBs have been reported to be hosted by high-metallicity star-forming galaxies, in tension with theoretical predictions. Here we compare iron and oxygen abundances for the first time for GRB host galaxies (GRB 980425 and 080517) based on the emission-line diagnostics. The estimated total iron abundances, including iron in both gas and dust, are well below the solar value. The total iron abundances can be explained by the typical value of theoretical predictions ([Fe/H]<-1.0), despite high oxygen abundance in one of them. According to our iron abundance measurements, the single massive star scenario still survives even if the oxygen abundance of the host is very high, such as the solar value. Relying only on oxygen abundance could mislead us on the origin of the GRBs. The measured oxygen-to-iron ratios, [O/Fe], can be comparable to the highest values among the iron-measured galaxies in the Sloan Digital Sky Survey. Possible theoretical explanations of such high [O/Fe] include the young age of the hosts, top-heavy initial mass function, and fallback mechanism of the iron element in supernova explosions.
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The confirmation of a globular cluster (GC) in the recently discovered ultrafaint galaxy Eridanus II (Eri II) motivated us to examine the question posed in the title. After estimating the halo mass of Eri II using a published stellar mass - halo mass relation, the one GC in this galaxy supports extending the relationship between the number of GCs hosted by a galaxy and the galaxys total mass about two orders of magnitude in stellar mass below the previous limit. For this empirically determined specific frequency of between 0.06 and 0.39 globular clusters per 10$^9$ $M_odot$ of total mass, the surviving Milky Way (MW) subhalos with masses smaller than $10^{10} M_odot$ could host as many as 5 to 31 GCs, broadly consistent with the actual population of outer halo MW GCs, although matching the radial distribution in detail remains a challenge. Using a subhalo mass function from published high resolution numerical simulations and a Poissonian model for populating those halos with the aforementioned empirically constrained frequency, we find that about 90$%$ of these GCs lie in lower-mass subhalos than that of Eri II. From what we know about the stellar mass-halo mass function, the subhalo mass function, and the mass-normalized GC specific frequency, we conclude that some of the MWs outer halo GCs are likely to be hosted by undetected subhalos with extremely modest stellar populations.
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]
Since the launch of the highly successful and ongoing Swift mission, the field of gamma-ray bursts (GRBs) has undergone a revolution. The arcsecond GRB localizations available within just a few minutes of the GRB alert has signified the continual sampling of the GRB evolution through the prompt to afterglow phases revealing unexpected flaring and plateau phases, the first detection of a kilonova coincident with a short GRB, and the identification of samples of low-luminosity, ultra-long, and highly dust extinguished GRBs. The increased numbers of GRB afterglows, GRB-supernova detections, redshifts, and host galaxy associations has greatly improved our understanding of what produces and powers these immense, cosmological explosions. Nevertheless, more high quality data often also reveal greater complexity. In this review, I summarize some of the milestones made in GRB research during the Swift era, and how previous widely accepted theoretical models have had to adapt to accommodate the new wealth of observational data.
The discovery of a number of gamma-ray bursts with duration exceeding 1,000 seconds, in particular the exceptional case of GRB 111209A with a duration of about 25,000 seconds, has opened the question on whether these bursts form a new class of sources, the so called {em ultra-long} GRBs, or if they are rather the tail of the distribution of the standard long GRB duration. In this Letter, using the long GRB sample detected by {em Swift}, we investigate on the statistical properties of ultra-long GRBs and compare them with the overall long burst population. We discuss also on the differences observed in their spectral properties. We find that ultra-long GRBs are statistically different from the standard long GRBs with typical burst duration less than 100-500 seconds, for which a Wolf Rayet star progenitor is usually invoked. We interpret this result as an indication that an alternative scenario has to be found in order to explain the ultra-long GRB extreme energetics, as well as the mass reservoir and its size that can feed the central engine for such a long time.
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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