The assembly history of the Galactic inner halo inferred from α-element patterns

Fernández-Alvar, Emma; Tissera, Patricia B.; Carigi, Leticia; Schuster, W. J.; Beers, Timothy C.; Belokurov, Vasily

doi:10.1093/mnras/stz443

Cited by 20 publications

(17 citation statements)

References 63 publications

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“…There is an apparent overlap in time and metallicity between the reported formation of the outer metal-poor low-[Mg/Fe] disc in our results and the signature of a major merger by a massive satellite found in the literature (Helmi et al 2018;Belokurov et al 2018;Myeong et al 2018;Di Matteo et al 2019;Gallart et al 2019;Fernández-Alvar et al 2019;Belokurov et al 2020;Feuillet et al 2020;Naidu et al 2020;Kordopatis et al 2020). Therefore, our results seem to be consistent with the arrival of a pristine gas-rich merger after the formation of the pre-existing old high-[Mg/Fe] population, which may have been diluted at different Galactic radii, leading to the formation of the second [Mg/Fe] sequence on longer timescales.…”

Section: Mass Accretionsupporting

confidence: 70%

The AMBRE Project: Solar neighbourhood chemodynamical constraints on Galactic disc evolution

Santos-Peral

Recio–Blanco

Kordopatis

et al. 2021

A&A

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Context. The abundance of α-elements relative to iron ([α/Fe]) is an important fossil signature in Galactic archaeology for tracing the chemical evolution of disc stellar populations. High-precision chemical abundances, together with accurate stellar ages, distances, and dynamical data, are crucial to infer the Milky Way formation history. Aims. The aim of this paper is to analyse the chemodynamical properties of the Galactic disc using precise magnesium abundance estimates for solar neighbourhood stars with accurate Gaia astrometric measurements. Methods. We estimated ages and dynamical properties for 366 main sequence turn-off stars from the AMBRE Project using PARSEC isochrones together with astrometric and photometric values from Gaia DR2. We use precise global metallicities [M/H] and [Mg/Fe] abundances from a previous study in order to estimate gradients and temporal chemodynamic relations for these stars. Results. We find a radial gradient of −0.099 ± 0.031 dex kpc−1 for [M/H] and +0.023 ± 0.009 dex kpc−1 for the [Mg/Fe] abundance. The steeper [Mg/Fe] gradient than that found in the literature is a result of the improvement of the AMBRE [Mg/Fe] estimates in the metal-rich regime. In addition, we find a significant spread of stellar age at any given [Mg/Fe] value, and observe a clear correlated dispersion of the [Mg/Fe] abundance with metallicity at a given age. While for [M/H] ≤ − 0.2, a clear age–[Mg/Fe] trend is observed, more metal-rich stars display ages from 3 up to 12 Gyr, describing an almost flat trend in the [Mg/Fe]–age relation. Moreover, we report the presence of radially migrated and/or churned stars for a wide range of stellar ages, although we note the large uncertainties of the amplitude of the inferred change in orbital guiding radii. Finally, we observe the appearance of a second chemical sequence in the outer disc, 10–12 Gyr ago, populating the metal-poor, low-[Mg/Fe] tail. These stars are more metal-poor than the coexisting stellar population in the inner parts of the disc, and show lower [Mg/Fe] abundances than prior disc stars of the same metallicity, leading to a chemical discontinuity. Our data favour the rapid formation of an early disc that settled in the inner regions, followed by the accretion of external metal-poor gas –probably related to a major accretion event such as the Gaia-Enceladus/Sausage one– that may have triggered the formation of the thin disc population and steepened the abundance gradient in the early disc.

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Section: Mass Accretionsupporting

confidence: 70%

The AMBRE Project: Solar neighbourhood chemodynamical constraints on Galactic disc evolution

Santos-Peral

Recio–Blanco

Kordopatis

et al. 2021

A&A

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“…One point that is worth mentioning here relates to recent results, for example, from Haywood et al (2018), Fernández-Alvar et al (2019), Myeong et al (2019), Di Matteo et al (2019), Gallart et al (2019), Belokurov et al (2020), who revived the idea that the low-angular momentum thick disc might be made up of stars from the proto-disc that are now part of the inner halo after having been heated by a major past accretion (see also, Gilmore et al 2002;Wyse et al 2006;Kordopatis et al 2013b). In particular, using the same input catalogue as we have in this work, Belokurov et al (2020) identified a small, yet non-negligible, prograde population of intermediate metallicity ([M/H] −0.7) with low angular momentum (v φ 100 km s −1 ), which they have dubbed "the Splash".…”

Section: Discussionmentioning

confidence: 98%

The chemodynamics of prograde and retrograde Milky Way stars

Kordopatis

Recio–Blanco

Schultheis

et al. 2020

A&A

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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, vϕ, 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 ∼ − 0.04 dex kpc−1 and −0.06 dex kpc−1, respectively, with little (if any) variation when different regions of the Galaxy are probed. The prograde stars show a vϕ − [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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“…All line-of-sight velocities used in this paper are from APOGEE. Following the study of Fernández-Alvar et al (2019a), we applied additional selection criteria only retaining stars with effective temperatures, T eff > 4000, and gravities, 1 < log(g) < 3.5. Finally, we also removed all APOGEE stars with ASCAPFLAG and STARFLAG warning of any problems with the determinations of the atmospheric parameters (specifically those with a warning about the reliability of the effective temperature, log(g), rotation, and having a very bright neighbour).…”

Section: Datamentioning

confidence: 99%

The Milky Way has no in-situ halo other than the heated thick disc

Matteo

Haywood

Lehnert

et al. 2019

A&A

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Previous studies based on the analysis of Gaia DR2 data have revealed that accreted stars, possibly originating from a single progenitor satellite, are a significant component of the halo of our Galaxy, potentially constituting most of the halo stars at [Fe/H] < −1 within a few kpc from the Sun and beyond. In this paper, we couple astrometric data from Gaia DR2 with elemental abundances from APOGEE DR14 to characterise the kinematics and chemistry of in-situ and accreted populations up to [Fe/H] ∼ −2. Accreted stars appear to significantly impact the galactic chemo–kinematic relations, not only at [Fe/H] < −1, but also at metallicities typical of the thick and metal-poor thin discs. They constitute about 60% of all stars at [Fe/H] < −1, the remaining 40% being made of (metal-weak) thick-disc stars. We find that the stellar kinematic fossil record shows the imprint left by this accretion event, which heated the old galactic disc. We are able to age-date this kinematic imprint, showing that the accretion occurred between nine and 11 Gyr ago, and that it led to the last significant heating of the galactic disc. An important fraction of stars with abundances typical of the (metal-rich) thick disc, and heated by this interaction, is now found in the galactic halo. Indeed, about half of the kinematically defined halo at few kpc from the Sun is composed of metal-rich thick-disc stars. Moreover, we suggest that this metal-rich thick-disc component dominates the stellar halo of the inner Galaxy. The new picture that emerges from this study is one where the standard, non-rotating in-situ halo population, the collapsed halo, seems to be more elusive than ever.

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The assembly history of the Galactic inner halo inferred from α-element patterns

Cited by 20 publications

References 63 publications

The AMBRE Project: Solar neighbourhood chemodynamical constraints on Galactic disc evolution

The AMBRE Project: Solar neighbourhood chemodynamical constraints on Galactic disc evolution

The chemodynamics of prograde and retrograde Milky Way stars

The Milky Way has no in-situ halo other than the heated thick disc

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