Search

We present data and initial results from VLT/X-Shooter emission-line spectroscopy of 96 galaxies selected by long γ-ray bursts (GRBs) at 0.1 med ~ 1.6. Based on Balmer and/or forbidden lines of oxygen, nitrogen, and neon, we measure systemic redshifts, star formation rates (SFR), visual attenuations (AV), oxygen abundances (12 + log (O/H)), and emission-line widths (σ). We study GRB hosts up to z ~ 3.5 and find a strong change in their typical physical properties with redshift. The median SFR of our GRB hosts increases from SFRmed ~ 0.6 M⊙ yr-1 at z ~ 0.6 up to SFRmed ~ 15 M⊙ yr-1 at z ~ 2. A higher ratio of [OIII]/[OII] at higher redshifts leads to an increasing distance of GRB-selected galaxies to the locus of local galaxies in the Baldwin-Phillips-Terlevich diagram. There is weak evidence for a redshift evolution in AV and σ, with the highest values seen at z ~ 1.5(AV) or z ~ 2 (σ). Oxygen abundances of the galaxies are distributed between 12 + log (O/H) = 7.9 and 12 + log (O/H) = 9.0 with a median 12 + log (O/H)med ~ 8.5. The fraction of GRB-selected galaxies with super-solar metallicities is ~20% at z< 1 in the adopted metallicity scale. This is significantly less than the fraction of total star formation in similar galaxies, illustrating that GRBs are scarce in high metallicity environments. At z ~ 3, sensitivity limits us to probing only the most luminous GRB hosts for which we derive metallicities of Z ≲ 0.5 Z⊙. Together with a high incidence of Z ~ 0.5 Z⊙ galaxies at z ~ 1.5, this indicates that a metallicity dependence at low redshift will not be dominant at z ~ 3. Significant correlations exist between the hosts' physical properties. Oxygen abundance, for example, relates to AV (12 + log (O/H) ∝ 0.17⋅AV), line width (12 + log(O/H) ∝ σ0.6), and SFR (12 + log(O/H) ∝ SFR0.2). In the last two cases, the normalization of the relations shift to lower metallicities at z> 2 by~0.4 dex. These properties of GRB hosts and their evolution with redshift can be understood in a cosmological context of star-forming galaxies and a picture in which the hosts' properties at low redshift are influenced by the tendency of GRBs to avoid the most metal-rich environments. [ABSTRACT FROM AUTHOR]

Context. The reionisation of the Universe is a process that is thought to have ended around z ~ 6, as inferred from spectroscopy of distant bright background sources, such as quasars (QSO) and gamma-ray burst (GRB) afterglows. Furthermore, spectroscopy of a GRB afterglow provides insight in its host galaxy, which is often too dim and distant to study otherwise. Aims. For the Swift GRB130606A at z = 5.913 we have obtained a high S/N spectrum covering the full optical and near-IR wavelength region at intermediate spectral resolution with VLT/X-Shooter. We aim to measure the degree of ionisation of the intergalactic medium (IGM) between z = 5.02-5.84 and to study the chemical abundance pattern and dust content of its host galaxy. Methods. We estimated the UV continuum of the GRB afterglow using a power-law extrapolation, then measured the flux decrement due to absorption at Lyα, β, and γ wavelength regions. Furthermore, we fitted the shape of the red damping wing of Lyα. The hydrogen and metal absorption lines formed in the host galaxy were fitted with Voigt profiles to obtain column densities. We investigated whether ionisation corrections needed to be applied. Results. Our measurements of the Lyα-forest optical depth are consistent with previous measurements of QSOs, but have a much smaller uncertainty. The analysis of the red damping wing yields a neutral fraction xHi < 0.05 (3σ). We obtain column density measurements of H, Al, Si, and Fe; for C, O, S and Ni we obtain limits. The ionisation due to the GRB is estimated to be negligible (corrections <0.03 dex), but larger corrections may apply due to the pre-existing radiation field (up to 0.4 dex based on sub-DLA studies). Assuming that [Si/Fe] = +0.79 ± 0.13 is due to dust depletion, the dust-to-metal ratio is similar to the Galactic value. Conclusions. Our measurements confirm that the Universe is already predominantly ionised over the redshift range probed in this work, but was slightly more neutral at z > 5.6. GRBs are useful probes of the ionisation state of the IGM in the early Universe, but because of internal scatter we need a larger statistical sample to draw robust conclusions. The high [Si/Fe] in the host can be due to dust depletion, α-element enhancement, or a combination of both. The very high value of [Al/Fe] = 2.40 ± 0.78 might be due to a proton capture process and is probably connected to the stellar population history. We estimate the host metallicity to be -1.7 < [M/H] < -0.9 (2%-13% of solar). [ABSTRACT FROM AUTHOR]

We present observations of three new sources in the European Southern Observatory VLT/X-shooter survey dedicated to the detection of the emitting counterparts of damped Lyα (DLA) systems towards bright quasars (QSOs). The aim is to bridge the observational gap between absorption (i.e. DLAs) and emission-selected galaxies at z ∼ 2.2–2.5, in order to get a more complete picture of (proto)galaxies around this epoch. The hypothesis is that because DLA galaxies fulfil metallicity–velocity width and luminosity–metallicity relations, high-metallicity DLAs are more likely to be detected in emission. The region around each QSO is covered with slits (1.3 arcsec × 11 arcsec) at three different position angles. In the DLA towards QSO J205922.4−052842 (zDLA = 2.210, [S/H] = −0.91 ± 0.06), Lyα emission is detected at 3σ confidence limit at an impact parameter of <6.3 kpc, and indicates a star formation rate >0.40 M⊙ yr−1 for the associated DLA galaxy. We do not detect the associated emission of two other DLAs in the spectra of QSOs J003034.4−512946 (zDLA = 2.452, [Zn/H] = −1.48 ± 0.34) and J105744.5+062914 (zDLA = 2.499, [Zn/H] = −0.24 ± 0.11, [S/H] = −0.15 ± 0.06). We conclude that focusing on metal-rich DLAs is a good way to find counterparts, but the non-detections at high metallicity (e.g. that of the DLA in J105744.5+062914) show that there is not a one-to-one relationship, and cautions us to not naively apply the properties of the DLA counterparts to all metal-rich DLAs. [ABSTRACT FROM AUTHOR]

Observations of the afterglows of long gamma-ray bursts (GRBs) allow the study of star-forming galaxies across most of cosmic history. Here we present observations of GRB 111008A, from which we can measure metallicity, chemical abundance patterns, dust-to-metals ratio (DTM), and extinction of the GRB host galaxy at z = 5.0. The host absorption system is a damped Lyα absorber with a very large neutral hydrogen column density of and a metallicity of [S/H] = –1.70 ± 0.10. It is the highest-redshift GRB with such a precise metallicity measurement. The presence of fine-structure lines confirms the z = 5.0 system as the GRB host galaxy and makes this the highest redshift where Fe II fine-structure lines have been detected. The afterglow is mildly reddened with AV = 0.11 ± 0.04 mag, and the host galaxy has a DTM that is consistent with being equal to or lower than typical values in the Local Group. [ABSTRACT FROM AUTHOR]

Aims. We present a study of the environment of the Swift long gamma-ray burst GRB 120327 A at z ≈ 2.8 through optical spectroscopy of its afterglow. Methods. We analyzed medium-resolution, multi-epoch spectroscopic observations (R ∼ 7000-12 000, corresponding to ∼15-23 km s-1, S/N = 15-30 and wavelength range 3000-25 000 Å) of the optical afterglow of GRB 120327 A, taken with X-shooter at the VLT 2.13 and 27.65 hr after the GRB trigger. Results. The first epoch spectrum shows that the ISM in the GRB host galaxy at z = 2.8145 is extremely rich in absorption features, with three components contributing to the line profiles. The hydrogen column density associated with GRB 120327 A has log NH/cm-2 = 22.01 ± 0.09, and the metallicity of the host galaxy is in the range [X/H] = -1.3 to -1.1. In addition to the ground state lines, we detect absorption features associated with excited states of C II, O I, Si II, Fe II, and Ni II, which we used to derive information on the distance between the host absorbing gas and the site of the GRB explosion. The variability of the Fe II λ2396 excited line between the two epochs proves that these features are excited by the GRB UV flux. Moreover, the distance of component I is found to be dI = 200+100 -60 pc, while component II is located closer to the GRB, at dII = 100+40 -30 pc. These values are among the lowest found in GRBs. Component III does not show excited transitions, so it should be located farther away from the GRB. The presence of H2 molecules is firmly established, with a molecular fraction f in the range f = 4 × 10-7-10-4. This particularly low value can be attributed to the small dust content. This represents the third positive detection of molecules in a GRB environment. [ABSTRACT FROM AUTHOR]

Context. Oxygen sequence Wolf-Rayet (WO) stars are thought to represent the final evolutionary stage of the most massive stars. The characteristic strong Ovi emission possibly originates from an enhanced oxygen abundance in the stellar wind. Alternatively, the Ovi emission can be caused by the high temperature of these stars, in which case the WO stars are the high-temperature extension of the more common carbon sequence Wolf-Rayet (WC) stars. Aims. By constraining the physical properties and evolutionary status of DR1, a WO star in the low-metallicity Local Group dwarf galaxy IC 1613 and one of only two objects of its class known in a SMC-like metallicity environment, we aim to investigate the nature of WO stars and their evolutionary connection with WC stars. Methods. We use the non-local thermodynamic equilibrium atmosphere code cmfgen to model the observed spectrum of DR1 and to derive its stellar and wind parameters. We compare our values with other studies of WC and WO stars, as well as with the predictions of evolutionary models. We also model the surrounding nebula using the photo-ionization code cloudy. Results. The oxygen and carbon abundances that we obtain are comparable to values found for WC stars. The temperature and luminosity are, however, higher than those of WC stars. DR1 is embedded in the hottest known Hii region in the Local Group. The nebular properties can be consistently reproduced by cloudy models adopting DR1 as central ionizing source. Conclusions. Comparison of the abundances and temperature of DR1 with core helium-burning models show that DR1 is currently well into the second half of helium burning. If the properties of DR1 are representative for theWO class, it would imply that WO stars are the high-temperature and high-luminosity extension of the WC stars, and do not necessarily represent a later evolutionary stage. [ABSTRACT FROM AUTHOR]

ABSTRACT As part of the Very Large Telescope (VLT)/X-shooter science verification, we obtained the first optical medium-resolution spectrum of a previously identified bright O-type object in NGC 55, a Large Magellanic Cloud (LMC)-like galaxy at a distance of ∼2.0 Mpc. Based on the stellar and nebular spectra, we investigate the nature and evolutionary status of the central object(s) and its influence on the surrounding interstellar medium. We conclude that the source, NGC 55 C1_31, is a composite object, likely a stellar cluster, which contains one or several hot ( Teff≃ 50 000 K) WN stars with a high mass-loss rate (∼3 × 10−5 M⊙ yr−1) and a helium-rich composition ( NHe/ NH= 0.8). The visual flux is dominated by OB-type (super)giant stars with Teff≲ 35 000 K, solar helium abundance ( NHe/ NH= 0.1) and mass-loss rate ∼2 × 10−6 M⊙ yr−1. The surrounding H ii region has an electron density of ne≤ 102 cm−3 and an electron temperature of T(O iii) ≃ 11 500 ± 600 K. The oxygen abundance of this region is [O/H] = 8.18 ± 0.03, which corresponds to Z= 0.31 ± 0.04 Z⊙. We observed no significant gradients in T(O iii), ne or [O/H] on a scale of 73 pc extending in four directions from the ionizing source. The properties of the H ii region can be reproduced by a cloudy model which uses the central cluster as ionizing source, thus providing a self-consistent interpretation of the data. We also report on the serendipitous discovery of He ii nebular emission associated with the nearby source NGC 55 C2_35, a feature usually associated with strong X-ray sources. [ABSTRACT FROM AUTHOR]

ABSTRACT As part of the Very Large Telescope (VLT)/X-shooter science verification, we obtained the first optical medium-resolution spectrum of a previously identified bright O-type object in NGC 55, a Large Magellanic Cloud (LMC)-like galaxy at a distance of ∼2.0 Mpc. Based on the stellar and nebular spectra, we investigate the nature and evolutionary status of the central object(s) and its influence on the surrounding interstellar medium. We conclude that the source, NGC 55 C1_31, is a composite object, likely a stellar cluster, which contains one or several hot ( Teff≃ 50 000 K) WN stars with a high mass-loss rate (∼3 × 10−5 M⊙ yr−1) and a helium-rich composition ( NHe/ NH= 0.8). The visual flux is dominated by OB-type (super)giant stars with Teff≲ 35 000 K, solar helium abundance ( NHe/ NH= 0.1) and mass-loss rate ∼2 × 10−6 M⊙ yr−1. The surrounding H ii region has an electron density of ne≤ 102 cm−3 and an electron temperature of T(O iii) ≃ 11 500 ± 600 K. The oxygen abundance of this region is [O/H] = 8.18 ± 0.03, which corresponds to Z= 0.31 ± 0.04 Z⊙. We observed no significant gradients in T(O iii), ne or [O/H] on a scale of 73 pc extending in four directions from the ionizing source. The properties of the H ii region can be reproduced by a cloudy model which uses the central cluster as ionizing source, thus providing a self-consistent interpretation of the data. We also report on the serendipitous discovery of He ii nebular emission associated with the nearby source NGC 55 C2_35, a feature usually associated with strong X-ray sources. [ABSTRACT FROM AUTHOR]

For Gamma-Ray Burst 100901A, we have obtained Gemini-North and Very Large Telescope optical afterglow spectra at four epochs: one hour, one day, three days and one week after the burst, thanks to the afterglow remaining unusually bright at late times. Apart from a wealth of metal resonance lines, we also detect lines arising from fine-structure levels of the ground state of Fe ii, and from metastable levels of Fe ii and Ni ii at the host redshift (z = 1.4084). These lines are found to vary significantly in time. The combination of the data and modelling results shows that we detect the fall of the Ni ii 4F9/2 metastable level population, which to date has not been observed. Assuming that the population of the excited states is due to the UV-radiation of the afterglow, we estimate an absorber distance of a few hundred pc. This appears to be a typical value when compared to similar studies. We detect two intervening absorbers (z = 1.3147, 1.3179). Despite the wide temporal range of the data, we do not see significant variation in the absorption lines of these two intervening systems., Astronomy