From ESO VLT/FLAMES/GIRAFFE spectra, abundance measurements of Zn have been made in ≈100 individual red giant branch (RGB) stars in the Sculptor dwarf spheroidal galaxy. This is the largest sample of ...individual Zn abundance measurements within a stellar system beyond the Milky Way. In the observed metallicity range, −2.7 ≤ Fe/H ≤ −0.9, the general trend of Zn abundances in Sculptor is similar to that of α-elements. That is, super-solar abundance ratios of Zn/Fe at low metallicities, which decrease with increasing Fe/H, eventually reaching subsolar values. However, at the higher metallicities in Sculptor, Fe/H ≳ −1.8, we find a significant scatter, −0.8 ≲ Zn/Fe ≲ +0.4, which is not seen in any α-element. Our results are consistent with previous observations of a limited number of stars in Sculptor and in other dwarf galaxies. These results suggest that zinc has a complex nucleosynthetic origin, behaving neither completely like an α- nor an iron-peak element.
The origin of carbon-enhanced metal-poor (CEMP) stars and their possible connection with the chemical elements produced by the first stellar generation is still highly debated. In contrast to the ...Galactic halo, not many CEMP stars have been found in the dwarf spheroidal galaxies around the Milky Way. Here we present detailed abundances from ESO VLT/UVES high-resolution spectroscopy for ET0097, the first CEMP star found in the Sculptor dwarf spheroidal, which is one of the best studied dwarf galaxies in the Local Group. This star has Fe/H = −2.03 ± 0.10, C/Fe = 0.51 ± 0.10 and N/Fe = 1.18 ± 0.20, which is the first nitrogen measurement in this galaxy. The traditional definition of CEMP stars is C/Fe ≥ 0.70, but taking into account that this luminous red giant branch star has undergone mixing, it was intrinsically less nitrogen enhanced and more carbon-rich when it was formed, and so it falls under the definition of CEMP stars, as proposed by Aoki et al. (2007, ApJ, 655, 492) to account for this effect. By making corrections for this mixing, we conclude that the star had C/Fe ≈ 0.8 during its earlier evolutionary stages. Apart from the enhanced C and N abundances, ET0097 shows no peculiarities in other elements lighter than Zn, and no enhancement of the heavier neutron-capture elements (Ba, La, Ce, Nd, Sm, Eu, Dy), making this a CEMP-no star. However, the star does show signs of the weak r-process, with an overabundance of the lighter neutron-capture elements (Sr, Y, Zr). To explain the abundance pattern observed in ET0097, we explore the possibility that this star was enriched by primordial stars. In addition to the detailed abundances for ET0097, we present estimates and upper limits for C abundances in 85 other stars in Sculptor derived from CN molecular lines, including 11 stars with Fe/H ≤ −2. Combining these limits with observations from the literature, the fraction of CEMP-no stars in Sculptor seems to be significantly lower than in the Galactic halo.
Aims. We present new measurements of the abundances of carbon and oxygen derived from high-excitation C i and O i absorption lines in metal-poor halo stars, with the aim of clarifying the main ...sources of these two elements in the early stages of the chemical enrichment of the Galaxy. Methods. We target 15 new stars compared to our previous study, with an emphasis on additional C/O determinations in the crucial metallicity range -3 $\la$ Fe/H$\la$ -2. The stellar effective temperatures were estimated from the profile of the Hβ line. Departures from local thermodynamic equilibrium were accounted for in the line formation for both carbon and oxygen. The non-LTE effects are very strong at the lowest metallicities but, contrary to what has sometimes been assumed in the past due to a simplified assessment, of different degrees for the two elements. In addition, for the 28 stars with Fe/H < -1 previously analysed, stellar parameters were re-derived and non-LTE corrections applied in the same fashion as for the rest of our sample, giving consistent abundances for 43 halo stars in total. Results. The new observations and non-LTE calculations strengthen previous suggestions of an upturn in C/O towards lower metallicity (particularly for O/H $\la$ -2). The C/O values derived for these very metal-poor stars are, however, sensitive to excitation via the still poorly quantified inelastic H collisions. While these do not significantly affect the non-LTE results for C i, they greatly modify the O i outcome. Adopting the H collisional cross-sections estimated from the classical Drawin formula leads to C/O ≈ 0 at O/H ≈ -3. To remove the upturn in C/O, near-LTE formation for O i lines would be required, which could only happen if the H collisional efficiency with the Drawin recipe is underestimated by factors of up to several tens of times, a possibility which we consider unlikely. Conclusions. The high C/O values derived at the lowest metallicities may be revealing the fingerprints of Population III stars or may signal rotationally-aided nucleosynthesis in more normal Population II stars.
Abundances of the volatile elements S and Zn have now been measured in around 80 individual stars in the Sculptor dwarf spheroidal galaxy, covering the metallicity range − 2.4 ≤ Fe/H ≤−0.9. These two ...elements are of particular interest as they are not depleted onto dust in gas, and their ratio, S/Zn, has thus commonly been used as a proxy for α/Fe in Damped Lyman-α systems (DLAs). The S abundances in Sculptor are similar to other α-elements in this galaxy, consistent with S being mainly created in core-collapse supernovae, but also having some contribution from type Ia supernovae. However, our results show that Zn and Fe do not trace all the same nucleosynthetic production channels. In particular, (contrary to Fe) Zn is not significantly produced by type Ia supernovae. Thus, S/Zn cannot be reliably used as a proxy for α/Fe. We propose O/S as a function of S/H as a possible alternative. At higher metallicities, the values of S/Zn measured in DLAs are inconsistent with those in local dwarf galaxies, and are more compatible with the Milky Way disk. Low-metallicity DLAs are, however, consistent with the most metal-poor stars in Local Group dwarf spheroidal galaxies. Assuming that the dust depletions of S and Zn are negligible, our comparison indicates that the star formation histories of DLAs are on average different from both the Milky Way and the Sculptor dwarf spheroidal galaxy.
ABSTRACT
We present early science results from the First Large Absorption Survey in H i (FLASH), a spectroscopically blind survey for 21-cm absorption lines in cold hydrogen (H i) gas at cosmological ...distances using the Australian Square Kilometre Array Pathfinder (ASKAP). We have searched for H i absorption towards 1253 radio sources in the GAMA 23 field, covering redshifts between z = 0.34 and 0.79 over a sky area of approximately 50 deg2. In a purely blind search, we did not obtain any detections of 21-cm absorbers above our reliability threshold. Assuming a fiducial value for the H i spin temperature of Tspin = 100 K and source covering fraction cf = 1, the total comoving absorption path-length sensitive to all Damped Lyman α Absorbers (DLAs; NH i ≥ 2 × 1020 cm−2) is ΔX = 6.6 ± 0.3 (Δz = 3.7 ± 0.2) and super-DLAs (NH i ≥ 2 × 1021 cm−2) is ΔX = 111 ± 6 (Δz= 63 ± 3). We estimate upper limits on the H i column density frequency distribution function that are consistent with measurements from prior surveys for redshifted optical DLAs, and nearby 21-cm emission and absorption. By cross-matching our sample of radio sources with optical spectroscopic identifications of galaxies in the GAMA 23 field, we were able to detect 21-cm absorption at z = 0.3562 towards NVSS J224500−343030, with a column density of $N_{\rm H\,\small{I}} = (1.2 \pm 0.1) \times 10^{20}\, (T_{\rm spin}/100\, \mathrm{K})$ cm−2. The absorber is associated with GAMA J22450.05−343031.7, a massive early-type galaxy at an impact parameter of 17 kpc with respect to the radio source and which may contain a massive (MH i ≳ 3 × 109 M⊙) gas disc. Such gas-rich early types are rare, but have been detected in the nearby Universe.
ABSTRACT
The distribution of gas and metals in the circumgalactic medium (CGM) plays a critical role in how galaxies evolve. The MUSE-ALMA Haloes survey combines MUSE, ALMA, and HST observations to ...constrain the properties of the multiphase gas in the CGM and the galaxies associated with the gas probed in absorption. In this paper, we analyse the properties of galaxies associated with 32 strong ${\rm H\, {\small I}}$ Ly-α absorbers at redshift 0.2 ≲ z ≲ 1.4. We detect 79 galaxies within ±500 kms−1 of the absorbers in our 19 MUSE fields. These associated galaxies are found at physical distances from 5.7 kpc and reach star formation rates as low as 0.1 M⊙ yr−1. The significant number of associated galaxies allows us to map their physical distribution on the Δv and b plane. Building on previous studies, we examine the physical and nebular properties of these associated galaxies and find the following: (i) 27/32 absorbers have galaxy counterparts and more than 50 per cent of the absorbers have two or more associated galaxies, (ii) the ${\rm H\, {\small I}}$ column density of absorbers is anticorrelated with the impact parameter (scaled by virial radius) of the nearest galaxy as expected from simulations, (iii) the metallicity of associated galaxies is typically larger than the absorber metallicity, which decreases at larger impact parameters. It becomes clear that while strong ${\rm H\, {\small I}}$ absorbers are typically associated with more than a single galaxy, we can use them to statistically map the gas and metal distribution in the CGM.
A spectroscopic study of atomic oxygen (O i) is performed using the Fourier-transform vacuum ultraviolet (VUV) spectrometer at the Dichroïsme Et Spectroscopie par Interaction avec le Rayonnement ...Synchrotron beamline of the SOLEIL synchrotron. Transition frequencies of O i lines in the range between 76,000 and 151,000 cm−1, corresponding to wavelengths between 665 and 1306 Å, are presented. In the wavelength window a data set of 208 lines is obtained for which line centers were determined at an accuracy of ≤0.035 cm−1, verified through a comparison with previous VUV-laser spectroscopy at an accuracy of 0.008 cm−1. Transitions to upper states belonging to several ns and nd Rydberg series that converge to the 4S3/2, 2D, and 2P ionization limits are presented. Besides studies on the main 16O isotope, measurements of the 18O isotope are also performed, for which an enriched sample of 18O2 gas is used. A least-squares optimization analysis is performed after merging the present new data set with some relevant accurate literature data to extract a list of level energies at high accuracy. When comparing with the database of the National Institute of Standards and Technology, good agreement is found for levels with low principal quantum numbers n ≤ 7 or below excitation energies of 108,000 cm−1; however, significant deviations were found for levels with n > 7. The main result of the present study is the realization of an improved data set of transition frequencies and level energies for O i, with its usefulness demonstrated in a comparison with high-resolution astronomical spectra showing absorption in the line of sight to distant quasars.
ABSTRACT
The gas cycling in the circumgalactic regions of galaxies is known to be multi-phase. The MUSE–ALMA Haloes survey gathers a large multi-wavelength observational sample of absorption and ...emission data with the goal to significantly advance our understanding of the physical properties of such CGM gas. A key component of the MUSE–ALMA Haloes survey is the multi-facility observational campaign conducted with VLT/MUSE, ALMA, and HST. MUSE–ALMA Haloes targets comprise 19 VLT/MUSE IFS quasar fields, including 32 zabs <0.85 strong absorbers with measured N(H i) ≥1018 cm−2 from UV-spectroscopy. We additionally use a new complementary HST medium program to characterize the stellar content of the galaxies through a 40-orbit three-band UVIS and IR WFC3 imaging. Beyond the absorber-selected targets, we detect 3658 sources all fields combined, including 703 objects with spectroscopic redshifts. This galaxy-selected sample constitutes the main focus of the current paper. We have secured millimeter ALMA observations of some of the fields to probe the molecular gas properties of these objects. Here, we present the overall survey science goals, target selection, observational strategy, data processing and source identification of the full sample. Furthermore, we provide catalogues of magnitude measurements for all objects detected in VLT/MUSE, ALMA, and HST broad-band images and associated spectroscopic redshifts derived from VLT/MUSE observations. Together, this data set provides robust characterization of the neutral atomic gas, molecular gas and stars in the same objects resulting in the baryon census of condensed matter in complex galaxy structures.
We present broadband (radio, optical, and X-ray) light curves and spectra of the afterglows of four long-duration gamma-ray bursts (GRBs; GRBs 09032,09032,090902B, and 090926A) detected by the ...Gamma-Ray Burst Monitor and Large Area Telescope (LAT) instruments on the Fermi satellite. With its wide spectral bandpass, extending to GeV energies, Fermi is sensitive to GRBs with very large isotropic energy releases (1054 erg). Although rare, these events are particularly important for testing GRB central-engine models. When combined with spectroscopic redshifts, our afterglow data for these four events are able to constrain jet collimation angles, the density structure of the circumburst medium, and both the true radiated energy release and the kinetic energy of the outflows. In agreement with our earlier work, we find that the relativistic energy budget of at least one of these events (GRB 090926A) exceeds the canonical value of 1051 erg by an order of magnitude. Such energies pose a severe challenge for models in which the GRB is powered by a magnetar or a neutrino-driven collapsar, but remain compatible with theoretical expectations for magnetohydrodynamical collapsar models (e.g., the Blandford-Znajek mechanism). Our jet opening angles ( Delta *c) are similar to those found for pre-Fermi GRBs, but the large initial Lorentz factors ( Delta *G0) inferred from the detection of GeV photons imply Delta *c Delta *G0 70-90, values which are above those predicted in magnetohydrodynamic models of jet acceleration. Finally, we find that these Fermi-LAT events preferentially occur in a low-density circumburst environment, and we speculate that this might result from the lower mass-loss rates of their lower-metallicity progenitor stars. Future studies of Fermi-LAT afterglows at radio wavelengths with the order-of-magnitude improvement in sensitivity offered by the Extended Very Large Array should definitively establish the relativistic energy budgets of these events.
In this work, we present the first results of our imaging campaign at Keck Observatory to identify the host galaxies of 'dark' gamma-ray bursts (GRBs), events with no detected optical afterglow or ...with detected optical flux significantly fainter than expected from the observed X-ray afterglow. We find that out of a uniform sample of 29 Swift bursts rapidly observed by the Palomar 60 inch telescope through 2008 March (14 of which we classify as dark), all events have either a detected optical afterglow, a probable optical host-galaxy detection, or both. Our results constrain the fraction of Swift GRBs coming from very high redshift (z>7), such as the recent GRB 090423, to between 0.2% and 7% at 80% confidence. In contrast, a significant fraction of the sample requires large extinction columns (host-frame AV 1 mag, with several events showing AV > 2-6 mag), identifying dust extinction as the dominant cause of the dark GRB phenomenon. We infer that a significant fraction of GRBs (and, by association, of high-mass star formation) occurs in highly obscured regions. However, the host galaxies of dark GRBs seem to have normal optical colors, suggesting that the source of obscuring dust is local to the vicinity of the GRB progenitor or highly unevenly distributed within the host galaxy.
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