The dual origin of the Galactic thick disc and halo from the gas-rich Gaia–Enceladus Sausage merger

Grand, Robert J. J.; Kawata, Daisuke; Belokurov, Vasily; Deason, Alis J.; Fattahi, Azadeh; Fragkoudi, Francesca; Gómez, Facundo A.; Marinacci, Federico; Pakmor, Rüdiger

doi:10.1093/mnras/staa2057

Cited by 104 publications

(126 citation statements)

References 94 publications

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“…These stars, if proven to really exist, could have been formed from local gas at the moment of the accretion with the Gaia-Enceladus-Sausage progenitor (see Maiolino et al 2017;Gallagher et al 2019, for star formation activity within galactic outflows). This population would therefore offer us an undeniable sample of locally born retrograde stars in order to precisely date and weigh this merger (see, Grand et al 2020). The Gaia data release 3, which will become public at the end of 2020, will first confirm whether these peculiar stars are indeed counter-rotating.…”

Section: Discussionmentioning

confidence: 99%

“…Recently, Fragkoudi et al (2020), Belokurov et al (2020) and Grand et al (2020) revived the idea that the thick disc may have formed after a violent gas-rich merger (e.g. Brook et al 2004).…”

Section: Retrograde Starsmentioning

confidence: 99%

“…In this context, because of the compressible nature of the gas, some of the newborn stars could have been formed on counter-rotating or radial orbits. The time and mass of this gas-rich merger, according to Grand et al (2020), could be then inferred from the properties and the fraction of the locally born retrograde stars. Figure 11 shows the magnesium abundance as a function of metallicity for all of the retrograde stars in the APOGEE DR16 sample.…”

Section: Retrograde Starsmentioning

confidence: 99%

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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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Section: Discussionmentioning

confidence: 99%

Section: Retrograde Starsmentioning

confidence: 99%

Section: Retrograde Starsmentioning

confidence: 99%

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The chemodynamics of prograde and retrograde Milky Way stars

Kordopatis

Recio–Blanco

Schultheis

et al. 2020

A&A

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“…High-resolution numerical simulations also suggested several thick and thin disc formation scenarios, including violent gas-rich mergers at high-redshift (e.g., Brook et al 2004;Grand et al 2018Grand et al , 2020, accretion of high-[ /Fe] stars (Abadi et al 2003;Kobayashi & Nakasato 2011;Tissera et al 2012), vertical heating from satellite merging events (e.g., Quinn et al 1993;Villalobos & Helmi 2008) and turbulence in clumpy high-redshift gas-rich disc (Noguchi 1998;Bournaud et al 2009; Beraldo e Silva et al 2020). The recent popular view is that the thick disc formation precedes the thin disc formation and the earlier disc was smaller and thicker, i.e.…”

Section: Introductionmentioning

confidence: 98%

Unveiling the distinct formation pathways of the inner and outer discs of the Milky Way with Bayesian Machine Learning

Ciucă

Kawata

Miglio

et al. 2021

Monthly Notices of the Royal Astronomical Society

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We develop a Bayesian Machine Learning framework called BINGO (Bayesian INference for Galactic archaeOlogy) centred around a Bayesian neural network. After being trained on the APOGEE and Kepler asteroseismic age data, BINGO is used to obtain precise relative stellar age estimates with uncertainties for the APOGEE stars. We carefully construct a training set to minimise bias and apply BINGO to a stellar population that is similar to our training set. We then select the 17,305 stars with ages from BINGO and reliable kinematic properties obtained from Gaia DR2. By combining the age and chemo-kinematical information, we dissect the Galactic disc stars into three components, namely, the thick disc (old, high-[α/Fe], [α/Fe] ≳ 0.12), the thin disc (young, low-[α/Fe]) and the Bridge, which is a region between the thick and thin discs. Our results indicate that the thick disc formed at an early epoch only in the inner region, and the inner disc smoothly transforms to the thin disc. We found that the outer disc follows a different chemical evolution pathway from the inner disc. The outer metal-poor stars only start forming after the compact thick disc phase has completed and the star-forming gas disc extended outwardly with metal-poor gas accretion. We found that in the Bridge region the range of [Fe/H] becomes wider with decreasing age, which suggests that the Bridge region corresponds to the transition phase from the smaller chemically well-mixed thick to a larger thin disc with a metallicity gradient.

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“…Our understanding of the formation and dynamical evolution of dense stellar systems, such as globular clusters (hereafter GCs) and nuclear star clusters (hereafter NSCs), has a crucial impact on galactic archaeology (see e.g. Belokurov et al 2018;Myeong et al 2018Myeong et al , 2019Massari et al 2019;Ibata et al 2019;Di Matteo et al 2019;Chung et al 2019;Grand et al 2020), multi-messenger astronomy (where it allows us to better constrain compact object mergers; see e.g. Belczynski et al 2002;Banerjee et al 2010;Bae et al 2014;Ziosi et al 2014;Breivik et al 2016;Rodriguez et al 2016;Hurley et al 2016;Askar et al 2017;Chatterjee et al 2017;Arca Sedda et al 2018;Kremer et al 2018;Di Carlo et al 2019;Bouffanais et al 2019;Rastello et al 2019;Antonini & Gieles 2020), cosmology and supermassive black hole science (with star clusters acting both as nurseries of intermediate-mass black hole seeds and a delivery mechanism to the galactic centres; see e.g.…”

Section: Introductionmentioning

confidence: 99%

Introducing a new multi-particle collision method for the evolution of dense stellar systems

Cintio

Pasquato

Kim

et al. 2021

A&A

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Context. Stellar systems are broadly divided into collisional and non-collisional categories. While the latter are large-N systems with long relaxation timescales and can be simulated disregarding two-body interactions, either computationally expensive direct N-body simulations or approximate schemes are required to properly model the former. Large globular clusters and nuclear star clusters, with relaxation timescales of the order of a Hubble time, are small enough to display some collisional behaviour and big enough to be impossible to simulate with direct N-body codes and current hardware. Aims. We aim to introduce a new method to simulate collisional stellar systems and validate it by comparison with direct N-body codes on small-N simulations. Methods. The Multi-Particle Collision for Dense Stellar Systems (mpcdss) code is a new code for evolving stellar systems with the multi-particle collision method. Such a method amounts to a stochastic collision rule that makes it possible to conserve the exact energy and momentum over a cluster of particles experiencing the collision. The code complexity scales with N log N in the number of particles. Unlike Monte Carlo codes, mpcdss can easily model asymmetric, non-homogeneous, unrelaxed, and rotating systems, while allowing us to follow the orbits of individual stars. Results. We evolved small (N = 3.2 × 10 4) star clusters with mpcdss and with the direct-summation code nbody6, finding a similar evolution of key indicators. We then simulated different initial conditions in the 10 4 − 10 6 star range. Conclusions. mpcdss bridges the gap between small collisional systems that can be simulated with direct N-body codes and large non-collisional systems. In principle, mpcdss allows us to simulate globular clusters such as Omega Centauri and M54, and even nuclear star clusters, which is beyond the limits of current direct N-body codes in terms of the number of particles.

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The dual origin of the Galactic thick disc and halo from the gas-rich Gaia–Enceladus Sausage merger

Cited by 104 publications

References 94 publications

The chemodynamics of prograde and retrograde Milky Way stars

The chemodynamics of prograde and retrograde Milky Way stars

Unveiling the distinct formation pathways of the inner and outer discs of the Milky Way with Bayesian Machine Learning

Introducing a new multi-particle collision method for the evolution of dense stellar systems

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