Abstract
The first stars were born from chemically pristine gas. They were likely massive, and thus they rapidly exploded as supernovae, enriching the surrounding gas with the first heavy elements. ...In the Local Group, the chemical signatures of the first stellar population were identified among low-mass, long-lived, very metal-poor (Fe/H < −2) stars, characterized by high abundances of carbon over iron (C/Fe > +0.7): the so-called carbon-enhanced metal-poor stars. Conversely, a similar carbon excess caused by first-star pollution was not found in dense neutral gas traced by absorption systems at different cosmic time. Here we present the detection of 14 very metal-poor, optically thick absorbers at redshift
z
∼ 3–4. Among these, 3 are carbon-enhanced and reveal an overabundance with respect to Fe of all the analyzed chemical elements (O, Mg, Al, and Si). Their relative abundances show a distribution with respect to Fe/H that is in very good agreement with those observed in nearby very metal-poor stars. All the tests we performed support the idea that these C-rich absorbers preserve the chemical yields of the first stars. Our new findings suggest that the first-star signatures can survive in optically thick but relatively diffuse absorbers, which are not sufficiently dense to sustain star formation and hence are not dominated by the chemical products of normal stars.
Abstract
We report on a chemo-dynamical analysis of SPLUS J142445.34−254247.1 (SPLUS J1424−2542), an extremely metal-poor halo star enhanced in elements formed by the rapid neutron-capture process (
...r
-process). This star was first selected as a metal-poor candidate from its narrowband S-PLUS photometry and followed up spectroscopically in medium resolution with Gemini-South/GMOS, which confirmed its low-metallicity status. High-resolution spectroscopy was gathered with GHOST at Gemini-South, allowing for the determination of the chemical abundances for 36 elements, from carbon to thorium. At Fe/H = −3.39, SPLUS J1424−2542 is one of the lowest-metallicity stars with measured Th and has the highest
log
ϵ
(
Th
/
Eu
)
observed to date, making it part of the “actinide-boost” category of
r
-process–enhanced stars. The analysis presented here suggests that the gas cloud from which SPLUS J1424−2542 formed must have been enriched by at least two progenitor populations. The light-element (
Z
≤ 30) abundance pattern is consistent with the yields from a supernova explosion of metal-free stars with 11.3–13.4
M
⊙
, and the heavy-element (
Z
≥ 38) abundance pattern can be reproduced by the yields from a neutron star merger (1.66
M
⊙
and 1.27
M
⊙
) event. A kinematical analysis also reveals that SPLUS J1424−2542 is a low-mass, old halo star with a likely in situ origin, not associated with any known early merger events in the Milky Way.
ABSTRACT
We present the measured gas-phase metal column densities in 155 sub-damped Ly α systems (subDLAs) with the aim to investigate the contribution of subDLAs to the chemical evolution of the ...Universe. The sample was identified within the absorber-blind XQ-100 quasar spectroscopic survey over the redshift range 2.4 ≤ zabs ≤ 4.3. Using all available column densities of the ionic species investigated (mainly C iv, Si ii, Mg ii, Si iv, Al ii, Fe ii, C ii, and O i; in order of decreasing detection frequency), we estimate the ionization-corrected gas-phase metallicity of each system using Markov chain Monte Carlo techniques to explore a large grid of cloudy ionization models. Without accounting for ionization and dust depletion effects, we find that the H i-weighted gas-phase metallicity evolution of subDLAs is consistent with damped Ly α systems (DLAs). When ionization corrections are included, subDLAs are systematically more metal poor than DLAs (between ≈0.5σ and ≈3σ significance) by up to ≈1.0 dex over the redshift range 3 ≤ zabs ≤ 4.3. The correlation of gas phase Si/Fe with metallicity in subDLAs appears to be consistent with that of DLAs, suggesting that the two classes of absorbers have a similar relative dust depletion pattern. As previously seen for Lyman limit systems, the gas phase C/O in subDLAs remains constantly solar for all metallicities indicating that both subDLAs and Lyman limit systems could trace carbon-rich ejecta, potentially in circumgalactic environments.
Abstract
Sub-damped Lyman α systems (subDLAs; H i column densities of 19.0 ≤ logN(H i) < 20.3) are rarely included in the cosmic H i census performed at redshifts zabs ≳ 1.5, yet are expected to ...contribute significantly to the overall H i mass budget of the Universe. In this paper, we present a blindly selected sample of 155 subDLAs found along 100 quasar sightlines (with a redshift path-length ΔX = 475) in the XQ-100 legacy survey to investigate the contribution of subDLAs to the H i mass density of the Universe. The impact of X-Shooter’s spectral resolution on Ly α absorber identification is evaluated, and found to be sufficient for reliably finding absorbers down to a column density of logN(H i) ≥ 18.9. We compared the implications of searching for subDLAs solely using H i absorption versus the use of metal lines to confirm the identification, and found that metal-selection techniques would have missed 75 subDLAs. Using a bootstrap Monte Carlo simulation, we computed the column density distribution function (f(N, X)) and the cosmological H i mass density ($\Omega _{\rm H\,{\small I}}$) of subDLAs and compared with our previous work based on the XQ-100 damped Lyman α systems. We do not find any significant redshift evolution in f(N, X) or $\Omega _{\rm H\,{\small I}}$ for subDLAs. However, subDLAs contribute 10–20 per cent of the total $\Omega _{\rm H\,{\small I}}$ measured at redshifts 2 < z < 5, and thus have a small but significant contribution to the H i budget of the Universe.
ABSTRACT
This study focuses on Pristine$\_180956.78$−294759.8 (hereafter P180956, Fe/H = −1.95 ± 0.02), a star selected from the Pristine Inner Galaxy Survey (PIGS), and followed-up with the recently ...commissioned Gemini High-resolution Optical SpecTrograph (GHOST) at the Gemini South telescope. The GHOST spectrograph’s high efficiency in the blue spectral region (3700−4800 Å) enables the detection of elemental tracers of early supernovae (e.g. Al, Mn, Sr, and Eu). The star exhibits chemical signatures resembling those found in ultrafaint dwarf (UFD) systems, characterized by very low abundances of neutron-capture elements (Sr, Ba, and Eu), which are uncommon among stars in the Milky Way halo. Our analysis suggests that P180956 bears the chemical imprints of a small number (2 or 4) of low-mass hypernovae ($\sim 10{-}15{\rm \, M_\odot }$), which are needed to mostly reproduce the abundance pattern of the light-elements (e.g. Si, Ti/Mg, Ca ∼0.6), and one fast-rotating intermediate-mass supernova ($\sim 300{\rm \, km \ s^{-1}}$, $\sim 80{-}120{\rm \, M_\odot }$), which is the main channel contributing to the high Sr/Ba (∼+1.2). The small pericentric ($\sim 0.7{\rm \, kpc}$) and apocentric ($\sim 13{\rm \, kpc}$) distances and its orbit confined to the plane ($\lesssim 2{\rm \, kpc}$) indicate that this star was likely accreted during the early Galactic assembly phase. Its chemo-dynamical properties suggest that P180956 formed in a system similar to a UFD galaxy accreted either alone, as one of the low-mass building blocks of the proto-Galaxy, or as a satellite of Gaia–Sausage–Enceladus. The combination of Gemini’s large aperture with GHOST’s high efficiency and broad spectral coverage makes this new spectrograph one of the leading instruments for near-field cosmology investigations.
Using our sample of the most metal-rich damped Lyman α systems (DLAs) at z
abs ∼ 2, and two literature compilations of chemical abundances in 341 DLAs and 2818 stars, we present an analysis of the ...chemical composition of DLAs in the context of the Local Group. The metal-rich sample of DLAs at z
abs ∼ 2 probes metallicities as high as the Galactic disc and the most metal-rich dwarf spheroidals (dSphs), permitting an analysis of many elements typically observed in DLAs (Fe, Zn, Cr, Mn, Si, and S) in comparison to stellar abundances observed in the Galaxy and its satellites (in particular dSphs). Our main conclusions are: (1) non-solar Zn/Fe abundances in metal-poor Galactic stars and in dSphs over the full metallicity range probed by DLAs, suggest that Zn is not a simple proxy for Fe in DLAs and therefore not a suitable indicator of dust depletion. After correcting for dust depletion, the majority of DLAs have subsolar Zn/Fe similar to dSphs; (2) at Fe/H ∼ −0.5, a constant Mn/Fe∼−0.5 and near-solar α/Fe (requiring an assumption about dust depletion) are in better agreement with dwarf galaxies than Galactic disc stars; (3) α/Zn is usually solar or subsolar in DLAs. However, although low ratios of α/Fe are usually considered more ‘dwarf-like’ than ‘Milky Way-like’, subsolar Zn/Fe in Local Group dwarfs leads to supersolar α/Zn in the dSphs, in contrast with the DLAs. Therefore, whilst DLAs exhibit some similarities with the Local Group dwarf population, there are also notable differences.
As of October 2021 (Period 108), the European Southern Observatory (ESO) offers a new mode of the ESPRESSO spectrograph designed to use the high-resolution grating with 4 × 2 binning (spatial by ...spectral; HR42 mode), with the specific objective of observing faint targets with a single Unit Telescope at Paranal. We validated the new HR42 mode using four hours of on-target observations of the quasar J0003-2603, known to host an intervening metal-poor absorber along the line of sight. The capabilities of the ESPRESSO HR42 mode (resolving power
R
≈ 137 000) were evaluated by comparing them to a UVES spectrum of the same target with a similar integration time but lower resolving power (
R
≈ 48 000). For both data sets, we tested the ability to decompose the velocity profile of the intervening absorber using Voigt profile fitting and extracted the total column densities of C
IV
, N
I
, Si
II
, Al
II
, Fe
II
, and Ni
II
. With ≈3× the resolving power and ≈2× lower signal-noise ratio (S/N) for a nearly equivalent exposure time, the ESPRESSO data is able to just as accurately characterise the individual components of the absorption lines as the comparison UVES data, but it has the added bonus of identifying narrower components not detected by UVES. For UVES to provide similar spectral resolution (
R
> 100 000; 0.3″ slit) and the broad wavelength coverage of ESPRESSO, the Exposure Time Calculator (ETC) supplied by ESO estimates 8 h of exposure time spread over two settings, requiring double the time investment compared to that of ESPRESSO’s HR42 mode whilst not properly sampling the UVES spectral resolution element. Thus, ESPRESSO’s HR42 mode offers nearly triple the resolving power of UVES (0.8″ slit to match typical ambient conditions at Paranal) and provides more accurate characterisation of quasar absorption features for an equivalent exposure time.
The relation between the stellar mass and size of a galaxy's structural subcomponents, such as discs and spheroids, is a powerful way to understand the processes involved in their formation. Using ...very large catalogues of photometric bulge+disc structural decompositions and stellar masses from the Sloan Digital Sky Survey Data Release Seven, we carefully define two large subsamples of spheroids in a quantitative manner such that both samples share similar characteristics with one important exception: the 'bulges' are embedded in a disc and the 'pure spheroids' are galaxies with a single structural component. Our bulge and pure spheroid subsample sizes are 76 012 and 171 243, respectively. Above a stellar mass of ∼1010 M, the mass-size relations of both subsamples are parallel to one another and are close to lines of constant surface mass density. However, the relations are offset by a factor of 1.4, which may be explained by the dominance of dissipation in their formation processes. Whereas the size-mass relation of bulges in discs is consistent with gas-rich mergers, pure spheroids appear to have been formed via a combination of 'dry' and 'wet' mergers.
ABSTRACT The Gemini High-resolution Optical SpecTrograph (GHOST) is a new Echelle spectrograph available on the Gemini-South telescope as of Semester 2024A. We present the first high-resolution ...spectrum of the quasar J1449−1227 (redshift zem = 3.27) using data taken during the commissioning of GHOST. The observed quasar hosts an intervening iron-poor {Fe/H = −2.5} damped Lyman α system (DLA) at redshift z = 2.904. Taking advantage of the high spectral resolving power of GHOST (R ≈ 55 000), we are able to accurately model the metal absorption lines of the metal-poor DLA and find a supersolar Si/Fe, suggesting that the DLA gas is in an early stage of chemical enrichment. Using simple ionization models, we find that the large range in the C iv/Si iv column density ratio of individual components within the DLA’s high-ionization absorption profile can be reproduced by several metal-poor Lyman limit systems surrounding the low-ionization gas of the DLA. It is possible that this metal-poor DLA resides within a complex system of metal-poor galaxies or filaments with inflowing gas. The high spectral resolution, wavelength coverage, and sensitivity of GHOST make it an ideal spectrograph for characterizing the chemistry and kinematics of quasar absorption lines.
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