ABSTRACT
Gaia EDR3 data were used to identify potential members in the outskirts of three ultra-faint dwarf (UFD) galaxies: Coma Berenices (>2Rh), Ursa Major I (∼4Rh), and Boötes I (∼4Rh), as well as ...a new member in the central region of Ursa Major I. These targets were observed with the Gemini GRACES spectrograph, which was used to determine precision radial velocities and metallicities that confirm their associations with the UFD galaxies. The spectra were also used to measure absorption lines for 10 elements (Na, Mg, K, Ca, Sc, Ti, Cr, Fe, Ni, and Ba), which confirm that the chemical abundances of the outermost stars are in good agreement with stars in the central regions. The abundance ratios and chemical patterns of the stars in Coma Berenices are consistent with contributions from SN Ia, which is unusual for its star formation history and in conflict with previous suggestions that this system evolved chemically from a single core collapse supernova event. The chemistries for all three galaxies are consistent with the outermost stars forming in the central regions, then moving to their current locations through tidal stripping and/or supernova feedback. In Boötes I, however, the lower metallicity and lack of strong carbon enrichment of its outermost stars could also be evidence of a dwarf galaxy merger.
ABSTRACT
The investigation of the metal-poor tail in the Galactic bulge provides unique information on the early Milky Way assembly and evolution. A chemo-dynamical analysis of 17 very metal-poor ...stars (VMP, Fe/H<−2.0) selected from the Pristine Inner Galaxy Survey was carried out based on Gemini/GRACES spectra. The chemistry suggests that the majority of our stars are very similar to metal-poor stars in the Galactic halo. Orbits calculated from Gaia EDR3 imply these stars are brought into the bulge during the earliest Galactic assembly. Most of our stars have large Na,Ca/Mg abundances, and thus show little evidence of enrichment by pair-instability supernovae. Two of our stars (P171457 and P184700) have chemical abundances compatible with second-generation globular cluster stars, suggestive of the presence of ancient and now dissolved globular clusters in the inner Galaxy. One of them (P171457) is extremely metal-poor (Fe/H<−3.0) and well below the metallicity floor of globular clusters, which supports the growing evidence for the existence of lower-metallicity globular clusters in the early Universe. A third star (P180956, Fe/H∼−2) has low Na,Ca/Mg and very low Ba/Fe for its metallicity, which are consistent with formation in a system polluted by only one or a few low-mass supernovae. Interestingly, its orbit is confined to the Galactic plane, like other very metal-poor stars found in the literature, which have been associated with the earliest building blocks of the Milky Way.
ABSTRACT
Dwarf galaxies are valuable laboratories for dynamical studies related to dark matter and galaxy evolution, yet it is currently unknown just how physically extended their stellar components ...are. Satellites orbiting the Galaxy’s potential may undergo tidal stripping by the host, or alternatively, may themselves have accreted smaller systems whose debris populates the dwarf’s own stellar halo. Evidence of these past interactions, if present, is best searched for in the outskirts of the satellite. However, foreground contamination dominates the signal at these large radial distances, making observation of stars in these regions difficult. In this work, we introduce an updated algorithm for application to Gaia data that identifies candidate member stars of dwarf galaxies, based on spatial, colour–magnitude and proper motion information, and which allows for an outer component to the stellar distribution. Our method shows excellent consistency with spectroscopically confirmed members from the literature despite having no requirement for radial velocity information. We apply the algorithm to all ∼60 Milky Way dwarf galaxy satellites, and we find nine dwarfs (Boötes 1, Boötes 3, Draco 2, Grus 2, Segue 1, Sculptor, Tucana 2, Tucana 3, and Ursa Minor) that exhibit evidence for a secondary, low-density outer profile. We identify many member stars which are located beyond 5 half-light radii (and in some cases, beyond 10). We argue these distant stars are likely tracers of dwarf stellar haloes or tidal streams, though ongoing spectroscopic follow-up will be required to determine the origin of these extended stellar populations.
ABSTRACT
Pristine_183.6849 + 04.8619 (P1836849) is an extremely metal-poor (Fe/H = −3.3 ± 0.1) star on a prograde orbit confined to the Galactic disc. Such stars are rare and may have their origins ...in protogalactic fragments that formed the early Milky Way, in low-mass satellites accreted later, or forming in situ in the Galactic plane. Here, we present a chemo-dynamical analysis of the spectral features between 3700−11 000 Å from a high-resolution spectrum taken during Science Verification of the new Gemini High-resolution Optical SpecTrograph. Spectral features for many chemical elements are analysed (Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Ni), and valuable upper limits are determined for others (C, Na, Sr, Ba). This main sequence star exhibits several rare chemical signatures, including (i) extremely low metallicity for a star in the Galactic disc, (ii) very low abundances of the light α-elements (Na, Mg, Si) compared to other metal-poor stars, and (iii) unusually large abundances of Cr and Mn, where Cr, Mn/FeNLTE > +0.5. A comparison to theoretical yields from supernova models suggests that two low-mass Population III objects (one 10 M⊙ supernova and one 17 M⊙ hypernova) can reproduce the abundance pattern well (reduced χ2 < 1). When this star is compared to other extremely metal-poor stars on quasi-circular, prograde planar orbits, differences in both chemistry and kinematics imply there is little evidence for a common origin. The unique chemistry of P1836849 is discussed in terms of the earliest stages in the formation of the Milky Way.
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.
Abstract The Gemini High Resolution Optical Spectrograph (GHOST) is a fiber-fed spectrograph system on the Gemini South telescope that provides simultaneous wavelength coverage from 348 to 1061 nm, ...and is designed for optimal performance between 363 and 950 nm. It can observe up to two objects simultaneously in a 7.′5 diameter field of regard at R ≃ 56,000 or a single object at R ≃ 75,000. The spectral resolution modes are obtained by using integral field units to image slice a 1.″2 aperture by a factor of five in width using 19 fibers in the high resolution mode and by a factor of three in width using 7 fibers in the standard resolution mode. GHOST is equipped with hardware to allow for precision radial velocity measurements, expected to approach meters per second precision. Here, we describe the basic design and operational capabilities of GHOST, and proceed to derive and quantify the key aspects of its on-sky performance that are of most relevance to its science users.
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.
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.
Dwarf galaxies are valuable laboratories for dynamical studies related to dark matter and galaxy evolution, yet it is currently unknown just how physically extended their stellar components are. ...Satellites orbiting the Galaxy's potential may undergo tidal stripping by the host, or alternatively, may themselves have accreted smaller systems whose debris populates the dwarf's own stellar halo. Evidence of these past interactions, if present, is best searched for in the outskirts of the satellite. However, foreground contamination dominates the signal at these large radial distances, making observation of stars in these regions difficult. In this work, we introduce an updated algorithm for application to Gaia data that identifies candidate member stars of dwarf galaxies, based on spatial, color-magnitude and proper motion information, and which allows for an outer component to the stellar distribution. Our method shows excellent consistency with spectroscopically confirmed members from the literature despite having no requirement for radial velocity information. We apply the algorithm to all \(\sim\)60 Milky Way dwarf galaxy satellites, and we find 9 dwarfs (Bo\"otes 1, Bo\"otes 3, Draco 2, Grus 2, Segue 1, Sculptor, Tucana 2, Tucana 3, and Ursa Minor) that exhibit evidence for a secondary, low-density outer profile. We identify many member stars which are located beyond 5 half-light radii (and in some cases, beyond 10). We argue these distant stars are likely tracers of dwarf stellar haloes or tidal streams, though ongoing spectroscopic follow-up will be required to determine the origin of these extended stellar populations.
Gaia EDR3 data was used to identify potential members in the outskirts of
three ultra faint dwarf (UFD) galaxies; Coma Berenices (> 2Rh), Ursa Major I
($\sim$ 4Rh), and Bo\"otes I ($\sim$ 4Rh), as ...well as a new member in the
central region of Ursa Major I. These targets were observed with the Gemini
GRACES spectrograph, which was used to determine precision radial velocities
and metallicities that confirm their associations with the UFD galaxies. The
spectra were also used to measure absorption lines for 10 elements (Na, Mg, K,
Ca, Sc, Ti, Cr, Fe, Ni, and Ba), which confirm that the chemical abundances of
the outermost stars are in good agreement with stars in the central regions.
The abundance ratios and chemical patterns of the stars in Coma Berenices are
consistent with contributions from SN Ia, which is unusual for its star
formation history and in conflict with previous suggestions that this system
evolved chemically from a single core collapse supernova event. The chemistries
for all three galaxies are consistent with the outermost stars forming in the
central regions, then moving to their current locations through tidal stripping
and/or supernova feedback. In Bo\"otes I, however, the lower metallicity and
lack of strong carbon enrichment of its outermost stars could also be evidence
of a dwarf galaxy merger.
