arXiv subjects

Vanessa Hill

Publications and source records attributed to Vanessa Hill.

At least 37 records · Page 2Linked to original sources

arXiv2023

A 3D view of dwarf galaxies with Gaia and VLT/FLAMES I. The Sculptor dwarf spheroidal

We present a new homogeneous survey of VLT/FLAMES LR8 line-of-sight radial velocities (vlos) for 1604 resolved red giant branch stars in the Sculptor dwarf spheroidal galaxy. In addition, we provide reliable Ca II triplet metallicities, [Fe/H], for 1339 of these stars. From this combination of new observations (2257 individual spectra) with ESO archival data (2389 spectra), we obtain the largest and most complete sample of vlos and [Fe/H] measurements for individual stars in any dwarf galaxy. Our sample includes VLT/FLAMES LR8 spectra for 55% of the red giant branch stars at G $<20$ from Gaia DR3, and $>70$% of the brightest stars, G $<18.75$. Our spectroscopic velocities are combined with Gaia DR3 proper motions and parallax measurements for a new and more precise membership analysis. We look again at the global characteristics of Sculptor, deriving a mean metallicity of $\langle$[Fe/H]$\rangle = -1.82 \pm 0.45$ and a mean line-of-sight velocity of $\langle$vlos$\rangle = +111.2 \pm 0.25$km/s. There is a clear metallicity gradient in Sculptor, -0.7deg/dex, with the most metal-rich population being the most centrally concentrated. Furthermore, the most metal-poor population in Sculptor, [Fe/H]$<-2.5$, appears to show kinematic properties distinct from the rest of the stellar population. Finally, we combine our results with the exquisite Gaia DR3 multi-colour photometry to further investigate the colour-magnitude diagram of the resolved stellar population in Sculptor. Our detailed analysis shows a similar global picture as previous studies, but with much more precise detail, revealing that Sculptor has more complex properties than previously thought. This survey emphasises the role of the stellar spectroscopy technique and this galaxy as a benchmark system for modelling galaxy formation and evolution on small scales.

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arXiv2023

Automatic line selection for abundance determination in large stellar spectroscopic surveys

*Context: The optimisation of new multiplex spectrographs (resolution, wavelength range,...), their associated surveys (choice of setup), or their parameterisation pipelines require methods that estimate which wavelengths contain useful information. *Aim: We propose a method that establishes the usefulness (purity & detectability) of an atomic line. We show two applications: a) optimising an instrument, by comparing the number of useful lines at a given setup, and b) optimising the line-list for a given setup by choosing the least blended lines detectable at different signal-to-noise ratios. *Method: The method compares pre-computed synthetic stellar spectra containing all of the elements and molecules with spectra containing the lines of specific elements alone. Then, the flux ratios between the full spectrum and the element spectrum are computed to estimate the line purities. The method identifies automatically (i) the line's central wavelength, (ii) its detectability based on its depth and a given S/N threshold and (iii) its usefulness based on the purity ratio. *Results: We compare the three WEAVE high-resolution setups (Blue: 404-465nm, Green: 473-545nm, Red: 595-685nm), and find that the Green+Red setup both allows one to measure more elements and contains more useful lines. However, there is a disparity in terms of which elements are detected, which we characterise. We also study the performances of R~20 000 and R~6000 spectra covering the entire optical range. Assuming a purity threshold of 60%, we find that the HR setup contains a much wealthier selection of lines, for any of the considered elements, whereas the LR has a "loss" of 50 to 90% of the lines even for higher S/N. *Conclusions: The method provides a diagnostic of where to focus to get the most out of a spectrograph, and is easy to implement for future instruments, or for pipelines that require line masks.

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arXiv2022

The Pristine Inner Galaxy Survey (PIGS) VI: Different vertical distributions between two DIBs at 442.8 nm and 862.1 nm

Although diffuse interstellar bands (DIBs) were discovered over 100 years ago, for most of them, their origins are still unknown. Investigation on the correlations between different DIBs is an important way to study the behavior and distributions of their carriers. Based on stacking thousands of spectra from the Pristine Inner Galaxy Survey, we study the correlations between two DIBs at 442.8 nm ($λ$442.8) and 862.1 nm ($λ$862.1), as well as the dust grains, in a range of latitude spanning ${\sim}22^{\circ}$ ($4^{\circ}\,{<}\,|b|\,{<}\,15^{\circ}$) toward the Galactic center ($|\ell|\,{<}\,11^{\circ}$). Tight linear intensity correlations can be found between $λ$442.8, $λ$862.1, and dust grains for $|b|\,{<}\,12^{\circ}$ or $E(B-V)\,{>}\,0.3$ mag. For $|b|\,{>}\,12^{\circ}$, $λ$442.8 and $λ$862.1 present larger relative strength with respect to the dust grains. A systematic variation of the relative strength between $λ$442.8 and $λ$862.1 with $|b|$ and $E(B-V)$ concludes that the two DIBs do not share a common carrier. Furthermore, the carrier of $λ$862.1 is more abundant at high latitudes than that of $λ$442.8. This work can be treated as an example showing the significance and potentials to the DIB research covering a large latitude range.

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arXiv2022

The Pristine survey -- XVI. The metallicity of 26 stellar streams around the Milky Way detected with the STREAMFINDER in Gaia EDR3

We use the photometric metallicities provided by the panoramic Pristine survey to study the veracity and derive the metallicities of the numerous stellar streams found by the application of the STREAMFINDER algorithm to the Gaia EDR3 data. All 26 streams present in Pristine show a clear metallicity distribution function, which provides an independent check of the reality of these structures, supporting the reliability of STREAMFINDER in finding streams and the power of Pristine to measure precise metallicities. We further present 6 candidate structures with coherent phase-space and metallicity signals that are very likely streams. The majority of studied streams are very metal-poor (14 structures with [Fe/H]<-2.0) and include 3 systems with [Fe/H]<-2.9 (C-11, C-19, and C-20). These streams could be the closest debris of low-luminosity dwarf galaxies or may have originated from globular clusters of significantly lower metallicity than any known current Milky Way globular cluster. Our study shows that the promise of the Gaia data for Galactic Archeology studies can be substantially strengthened by quality photometric metallicities, allowing us to peer back into the earliest epochs of the formation of our Galaxy and its stellar halo constituents.

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arXiv2022

A stellar stream remnant of a globular cluster below the metallicity floor

Stellar ejecta gradually enrich the gas out of which subsequent stars form, making the least chemically enriched stellar systems direct fossils of structures formed in the early universe. Although a few hundred stars with metal content below one thousandth of the solar iron content are known in the Galaxy, none of them inhabit globular clusters, some of the oldest known stellar structures. These show metal content of at least ~0.2 percent of the solar metallicity ([Fe/H] > -2.7). This metallicity floor appears universal and it has been proposed that proto-galaxies that merge into the galaxies we observe today were simply not massive enough to form clusters that survived to the present day. Here, we report the discovery of a stellar stream, C-19, whose metallicity is less than 0.05 per cent the solar metallicity ([Fe/H]=-3.38 +/- 0.06 (stat.) +/- 0.20 (syst.)). The low metallicity dispersion and the chemical abundances of the C-19 stars show that this stream is the tidal remnant of the most metal-poor globular cluster ever discovered, and significantly below the purported metallicity floor: clusters with significantly lower metallicities than observed today existed in the past and contributed their stars to the Milky Way halo.

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arXiv2021

Ram Pressure Candidates in UNIONS

We present a search for disturbed, candidate ram pressure stripping galaxies across more than 50 spectroscopically selected SDSS groups and clusters. Forty-eight ram pressure candidates are visually identified in these systems using high quality UNIONS imaging from the Canada-France Hawaii Telescope, covering ~6200 and ~2800 square degrees in the u- and r-bands respectively. Ram pressure candidates are found in groups and clusters spanning a wide range in halo mass and include ~30 ram pressure candidates in the group regime ($M_h < 10^{14}$). The observed frequency of ram pressure candidates shows substantial scatter with group/cluster mass, but on average is larger in clusters ($M_h > 10^{14}\,M_\odot$) than groups ($M_h < 10^{14}\,M_\odot$) by a factor of ~2. We find that ram pressure candidates are most commonly low-mass galaxies and have enhanced star formation rates relative to star-forming field galaxies. The enhancement in star formation is largely independent of galaxy mass and strongest for galaxies in clusters. As a result of the large survey footprint and excellent image quality from UNIONS, we are able to identify disturbed galaxies, potentially affected by ram pressure stripping, across a wide range of host environment.

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arXiv2021

The Pristine survey XIII: Uncovering the very metal-poor tail of the thin disc

We evaluate the rotational velocity of stars observed by the Pristine survey towards the Galactic anticentre, spanning a wide range of metallicities from the extremely metal-poor regime ($\mathrm{[Fe/H]}<-3$) to nearly solar metallicity. In the Galactic anticentre direction, the rotational velocity ($V_ϕ$) is similar to the tangential velocity in the galactic longitude direction ($V_{\ell}$). This allows us to estimate $V_ϕ$ from Gaia early data-release 3 (Gaia EDR3) proper motions for stars without radial velocity measurements. This substantially increases the sample of stars in the outer disc with estimated rotational velocities. Our stellar sample towards the anticentre is dominated by a kinematical thin disc with a mean rotation of $\sim -220$ km $\mathrm{s}^{-1}$. However, our analysis reveals the presence of more stellar substructures. The most intriguing is a well populated extension of the kinematical thin disc down to $\mathrm{[Fe/H]} \sim -2$. A scarser fast rotating population reaching the extremely metal-poor regime, down to $\mathrm{[Fe/H]} \sim -3.5$ is also detected, but without statistical significance to unambiguously state whether this is the extremely metal-poor extension of the thin disc or the high rotating tail of hotter structures (like the thick disc or the halo). In addition, a more slowly rotating kinematical thick disc component is also required to explain the observed $V_{\ell}$ distribution at $\mathrm{[Fe/H]} > -1.5$. Furthermore, we detect signatures of a "heated disc", the so-called Splash, at metallicities higher than $\sim-1.5$. Finally, at $\mathrm{[Fe/H]} < -1.5$ our anticentre sample is dominated by a kinematical halo with a net prograde motion.

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arXiv2021

Zero-metallicity hypernova uncovered by an ultra metal-poor star in the Sculptor dwarf spheroidal galaxy

Although true metal-free "Population III" stars have so-far escaped discovery, their nature, and that of their supernovae, is revealed in the chemical products left behind in the next generations of stars. Here we report the detection of an ultra-metal poor star in the Sculptor dwarf spheroidal galaxy, AS0039. With [Fe/H]$_{\rm LTE}=-4.11$, it is the most metal-poor star so far discovered in any external galaxy. Contrary to the majority of Milky Way stars at this metallicity, AS0039 is clearly not enhanced in carbon, with [C/Fe]$_{\rm LTE}=-0.75$ and A(C)=+3.60, making it the lowest detected carbon abundance in any star to date. It furthermore lacks $α$-element uniformity, having extremely low [Mg/Ca]$_{\rm NLTE}=-0.60$ and [Mg/Ti]$_{\rm NLTE}=-0.86$, in stark contrast with the near solar ratios observed in C-normal stars within the Milky Way halo. The unique abundance pattern indicates that AS0039 formed out of material that was predominantly enriched by a $\sim$20$ M_\odot$ progenitor star with an unusually high explosion energy $E=10\times10^{51}$ erg. The star AS0039 is thus one of the first observational evidence for zero-metallicity hypernovae and provides a unique opportunity to investigate the diverse nature of Population III stars.

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arXiv2021

The Pristine survey -- XII: Gemini-GRACES chemo-dynamical study of newly discovered extremely metal-poor stars in the Galaxy

High-resolution optical spectra of 30 metal-poor stars selected from the \Pristine\ survey are presented, based on observations taken with the Gemini Observatory GRACES spectrograph. Stellar parameters \teff and \logg are determined using a Gaia DR2 colour-temperature calibration and surface gravity from the Stefan-Boltzmann equation. GRACES spectra are used to determine chemical abundances (or upper-limits) for 20 elements (Li, O, Na, Mg, K, Ca, Ti, Sc, Cr, Mn, Fe, Ni, Cu, Zn, Y, Zr, Ba, La, Nd, Eu). These stars are confirmed to be metal-poor ([Fe/H]$<-2.5$), with higher precision than from earlier medium-resolution analyses. The chemistry for most targets is similar to other extremely metal-poor stars in the Galactic halo. Three stars near [Fe/H]$=-3.0$ have unusually low Ca and high Mg, suggestive of contributions from few SN~II where alpha-element formation through hydrostatic nucleosynthesis was more efficient. Three new carbon-enhanced metal-poor stars are also identified (two CEMP-s and one potential CEMP-no star) when our chemical abundances are combined with carbon from previous medium-resolution analyses. The GRACES spectra also provide precision radial velocities ($σ_{\rm RV}\le0.2$km\,s$^{-1}$) for dynamical orbit calculations with the Gaia DR2 proper motions. Most of our targets are dynamically associated with the Galactic halo; however, five stars with [Fe/H]$<-3$ have planar-like orbits, including one retrograde star. Another five stars are dynamically consistent with the Gaia-Sequoia accretion event; three have typical halo [$α$/Fe] ratios for their metallicities, whereas two are [Mg/Fe]-deficient, and one is a new CEMP-s candidate. These results are discussed in terms of the formation and early chemical evolution of the Galaxy.

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arXiv2021

The Pristine Inner Galaxy Survey (PIGS) III: carbon-enhanced metal-poor stars in the bulge

The most metal-deficient stars hold important clues about the early build-up and chemical evolution of the Milky Way, and carbon-enhanced metal-poor (CEMP) stars are of special interest. However, little is known about CEMP stars in the Galactic bulge. In this paper, we use the large spectroscopic sample of metal-poor stars from the Pristine Inner Galaxy Survey (PIGS) to identify CEMP stars ([C/Fe] > +0.7) in the bulge region and to derive a CEMP fraction. We identify 96 new CEMP stars in the inner Galaxy, of which 62 are very metal-poor ([Fe/H] < -2.0); this is more than a ten-fold increase compared to the seven previously known bulge CEMP stars. The cumulative fraction of CEMP stars in PIGS is $42^{\,+14\,}_{\,-13} \%$ for stars with [Fe/H] < -3.0, and decreases to $16^{\,+3\,}_{\,-3} \%$ for [Fe/H] < -2.5 and $5.7^{\,+0.6\,}_{\,-0.5} \%$ for [Fe/H] < -2.0. The PIGS inner Galaxy CEMP fraction for [Fe/H] < -3.0 is consistent with the halo fraction found in the literature, but at higher metallicities the PIGS fraction is substantially lower. While this can partly be attributed to a photometric selection bias, such bias is unlikely to fully explain the low CEMP fraction at higher metallicities. Considering the typical carbon excesses and metallicity ranges for halo CEMP-s and CEMP-no stars, our results point to a possible deficiency of both CEMP-s and CEMP-no stars (especially the more metal-rich) in the inner Galaxy. The former is potentially related to a difference in the binary fraction, whereas the latter may be the result of a fast chemical enrichment in the early building blocks of the inner Galaxy.

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arXiv2020

The chemodynamics of prograde and retrograde Milky Way stars

Context: The accretion history of the Milky Way is still unknown, despite the recent discovery of stellar systems that stand out in terms of their energy-angular momentum space, such as Gaia-Enceladus-Sausage. In particular, it is still unclear how these groups are linked and to what extent they are well-mixed. Aims: We investigate the similarities and differences in the properties between the prograde and retrograde (counter-rotating) stars and set those results in context by using the properties of Gaia-Enceladus-Sausage, Thamnos/Sequoia, and other suggested accreted populations. Methods: We used the stellar metallicities of the major large spectroscopic surveys (APOGEE, Gaia-ESO, GALAH, LAMOST, RAVE, SEGUE) in combination with astrometric and photometric data from Gaia's second data-release. We investigated the presence of radial and vertical metallicity gradients as well as the possible correlations between the azimuthal velocity, $v_ϕ,$ and metallicity, [M/H], as qualitative indicators of the presence of mixed populations. Results: We find that a handful of super metal-rich stars exist on retrograde orbits at various distances from the Galactic center and the Galactic plane. We also find that the counter-rotating stars appear to be a well-mixed population, exhibiting radial and vertical metallicity gradients on the order of $\sim$ -0.04 dex/kpc and -0.06 dex/kpc, respectively, with little (if any) variation when different regions of the Galaxy are probed. The prograde stars show a $v_ϕ$-[M/H] relation that flattens -- and, perhaps, even reverses as a function of distance from the plane. Retrograde samples selected to roughly probe Thamnos and Gaia-Enceladus-Sausage appear to be different populations yet they also appear to be quite linked, as they follow the same trend in terms of the eccentricity versus metallicity space.

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arXiv2020

The bar resonances and low angular momentum moving groups in the Galaxy revealed by stellar ages

We use the second Gaia data release in combination with the catalog of Sanders & Das (2018) to dissect the Milky Way disc in phase-space and relative ages. We confirm and report the existence of multiple velocity moving groups at low azimuthal velocities and angular momenta, below Arcturus, regularly separated by $\sim~18-20\,\rm{km s^{-1}}$ in azimuthal velocity. Such features were predicted to exist more than ten years ago from the reaction of the Milky Way to a perturbation in the disc undergoing phase-mixing. These structures appear slightly younger than their phase-space surroundings, arguing against an extra-galactic origin. We also identify in relative age many of the classical ridges in the plane of azimuthal velocity vs. Galactocentric radius, traditionally associated with resonance features. These ridges are also younger than their phase-space surroundings in accordance with predictions from recent state-of-the-art cosmological hydrodynamical simulations of Milky Way-like galaxies. We study the response of dynamically young and old stellar disc populations to resonances from an analytic model of a large bar, which, remarkably, qualitatively reproduces the trends seen in the data. Our results re-inforce the idea that the Galactic disc is currently being shaped by both internal and external perturbations, and that, while absolute isochrone ages have to be taken with great care, exploring the dynamical structure of the disc in stellar ages, especially with future asteroseismic data, will provide much stronger constraints than metallicity/abundance trends alone.

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arXiv2020

The Pristine Inner Galaxy Survey (PIGS) II: Uncovering the most metal-poor populations in the inner Milky Way

Metal-poor stars are important tools for tracing the early history of the Milky Way, and for learning about the first generations of stars. Simulations suggest that the oldest metal-poor stars are to be found in the inner Galaxy. Typical bulge surveys, however, lack low metallicity ([Fe/H] < -1.0) stars because the inner Galaxy is predominantly metal-rich. The aim of the Pristine Inner Galaxy Survey (PIGS) is to study the metal-poor and very metal-poor (VMP, [Fe/H] < -2.0) stars in this region. In PIGS, metal-poor targets for spectroscopic follow-up are selected from metallicity-sensitive CaHK photometry from the CFHT. This work presents the ~250 deg^2 photometric survey as well as intermediate-resolution spectroscopic follow-up observations for ~8000 stars using AAOmega on the AAT. The spectra are analysed using two independent tools: ULySS with an empirical spectral library, and FERRE with a library of synthetic spectra. The comparison between the two methods enables a robust determination of the stellar parameters and their uncertainties. We present a sample of 1300 VMP stars -- the largest sample of VMP stars in the inner Galaxy to date. Additionally, our spectroscopic dataset includes ~1700 horizontal branch stars, which are useful metal-poor standard candles. We furthermore show that PIGS photometry selects VMP stars with unprecedented efficiency: 86%/80% (lower/higher extinction) of the best candidates satisfy [Fe/H] < -2.0, as do 80%/63% of a larger, less strictly selected sample. We discuss future applications of this unique dataset that will further our understanding of the chemical and dynamical evolution of the innermost regions of our Galaxy.

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arXiv2019

The Pristine survey X: a large population of low-metallicity stars permeates the Galactic disk

The orbits of the least chemically enriched stars open a window on the formation of our Galaxy when it was still in its infancy. The common picture is that these low-metallicity stars are distributed as an isotropic, pressure-supported component since these stars were either accreted from the early building blocks of the assembling Milky Way, or were later brought by the accretion of faint dwarf galaxies. Combining the metallicities and radial velocities from the Pristine and LAMOST surveys and Gaia DR2 parallaxes and proper motions for an unprecedented large and unbiased sample of very metal-poor stars at $[Fe/H]\leq-2.5$ we show that this picture is incomplete. This sample shows strong statistical evidence (at the $5.0σ$ level) of asymmetry in their kinematics, favouring prograde motion. Moreover, we find that $31\%$ of the stars that currently reside in the disk do not venture outside of the disk plane throughout their orbit. The discovery of this population implies that a significant fraction of stars with iron abundances $[Fe/H]\leq-2.5$ formed within or concurrently with the Milky Way disk and that the history of the disk was quiet enough to allow them to retain their disk-like orbital properties.

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arXiv2019

Pristine IX: CFHT ESPaDOnS Spectroscopic Analysis of 115 Bright Metal-Poor Candidate Stars

A chemo-dynamical analysis of 115 metal-poor candidate stars selected from the narrow-band Pristine photometric survey is presented based on CFHT high-resolution ESPaDoNS spectroscopy. We have discover 28 new bright (V < 15) stars with [Fe/H]<-2.5 and 5 with [Fe/H]<-3.0 for success rates of 40% (28/70) and 19% (5/27), respectively. A detailed model atmospheres analysis is carried out for the 28 new metal-poor stars. Stellar parameters were determined from SDSS photometric colours, Gaia DR2 parallaxes, MESA/MIST stellar isochrones, and the initial Pristine survey metallicities, following a Bayesian inference method. Chemical abundances are determined for 10 elements (Na, Mg, Ca, Sc, Ti, Cr, Fe, Ni, Y, Ba). Most stars show chemical abundance patterns that are similar to the normal metal-poor stars in the Galactic halo; however, we also report the discoveries of a new r-process rich star, a new CEMP-s candidate with [Y/Ba]>0, and a metal-poor star with very low [Mg/Fe]. The kinematics and orbits for all of the highly probable metal-poor candidates are determined by combining our precision radial velocities with Gaia DR2 proper motions. Some stars show unusual kinematics for their chemistries, including planar orbits, unbound orbits, and highly elliptical orbits that plunge deeply into the Galactic bulge (Rperi < 0.5 kpc); also, eight stars have orbital energies and actions consistent with the Gaia-Enceladus accretion event. This paper contributes to our understanding of the complex chemo-dynamics of the metal-poor Galaxy, and increases the number of known bright metal-poor stars available for detailed nucleosynthetic studies.

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arXiv2019

The Pristine Inner Galaxy Survey (PIGS) I: Tracing the kinematics of metal-poor stars in the Galactic bulge

Our Galaxy is known to contain a central boxy/peanut-shaped bulge, yet the importance of a classical, pressure-supported component within the central part of the Milky Way is still being debated. It should be most visible at low metallicity, a regime that has not yet been studied in detail. Using metallicity-sensitive narrow-band photometry, the Pristine Inner Galaxy Survey (PIGS) has collected a large sample of metal-poor ([Fe/H] < -1.0) stars in the inner Galaxy to address this open question. We use PIGS to trace the metal-poor inner Galaxy kinematics as function of metallicity for the first time. We find that the rotational signal decreases with decreasing [Fe/H], until it becomes negligible for the most metal-poor stars. Additionally, the velocity dispersion increases with decreasing metallicity for -3.0 < [Fe/H] < -0.5, with a gradient of -44 $\pm$ 4 km$\,$s$^{-1}\,$dex$^{-1}$. These observations may signal a transition between Galactic components of different metallicities and kinematics, a different mapping onto the boxy/peanut-shaped bulge for former disk stars of different metallicities and/or the secular dynamical and gravitational influence of the bar on the pressure-supported component. Our results provide strong constraints on models that attempt to explain the properties of the inner Galaxy.

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arXiv2019

The Pristine Survey -- VI. The first three years of medium-resolution follow-up spectroscopy of Pristine EMP star candidates

We present the results of a 3-year long, medium-resolution spectroscopic campaign aimed at identifying very metal-poor stars from candidates selected with the CaHK, metallicity-sensitive Pristine survey. The catalogue consists of a total of 1007 stars, and includes 146 rediscoveries of metal-poor stars already presented in previous surveys, 707 new very metal-poor stars with [Fe/H] < -2.0, and 95 new extremely metal-poor stars with [Fe/H] < -3.0. We provide a spectroscopic [Fe/H] for every star in the catalogue, and [C/Fe] measurements for a subset of the stars (10% with [Fe/H] < -3 and 24% with -3 < [Fe/H] < -2) for which a carbon determination is possible, contingent mainly on the carbon abundance, effective temperature and S/N of the stellar spectra. We find an average carbon enhancement fraction ([C/Fe] >= +0.7) of 41 +- 4% for stars with -3 < [Fe/H] < -2 and 58 +- 14% for stars with [Fe/H] < -3, and report updated success rates for the Pristine survey of 56 % and 23 % to recover stars with [Fe/H] < -2.5 and [Fe/H] < -3, respectively. Finally, we discuss the current status of the survey and its preparation for providing targets to upcoming multi-object spectroscopic surveys such as WEAVE.

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arXiv2019

The birth of the Milky Way as uncovered by accurate stellar ages with Gaia

Knowledge of ages for stars formed over a galaxy's lifetime is fundamental to understand its formation and evolution. However, stellar ages are difficult to obtain since they cannot be measured from observations, being comparison with stellar models (Soderblom 2010) required. Alternatively, age distributions can be derived applying the robust technique of colour-magnitude diagram fitting (Gallart et al. 2005), till now mainly employed to study nearby galaxies. The new distances to individual Milky Way stars from the Gaia mission (Brown et al. 2018) have allowed us to use this technique to derive ages from a thick disk colour-magnitude diagram, and from the enigmatic, two-sequenced colour-magnitude diagram of the kinematically hot local halo (Babusiaux et al. 2018), which blue-sequence has been linked to a major accretion event (Haywood et al. 2018, Helmi et al. 2018). Because accurate ages were lacking, the time of the merger and its role on our Galaxy's early evolution remained unclear. We show that the stars in both halo sequences share identical age distributions, and are older than the bulk of thick disc stars. The sharp halo age cut 10 Gyr ago can be identified with the accretion of Gaia-Enceladus. Along with state-of-the-art cosmological simulations of galaxy formation (Brook et al. 2012), these robust ages allow us to order the early sequence of events that shaped our Galaxy, identifying the red-sequence as the first stars formed within the Milky Way progenitor which, because of their kinematics, can be described as its long sought in-situ halo.

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