Abstract
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.
Highlights
The role that accretion events have played in the evolution of the Milky Way and the quest to find possible remnants that are at the origin of the old disc have been central topics in Galactic archaeology for over than five decades (e.g. Eggen et al 1962; Searle & Zinn 1978; Gilmore & Reid 1983; Chiba & Beers 2000; Gilmore et al 2002; Wyse et al 2006; Kordopatis et al 2011)
The advent of the Tycho-Gaia Astrometric Solution (TGAS) catalogue of the European space mission, Gaia (Gaia Collaboration 2016b,a), and, in particular, the second Gaia data release (GDR2, Gaia Collaboration 2018a), have enabled us to measure with much greater accuracy the positions and the 3D velocities of millions of stars in a volume several kiloparsecs wide
By comparing the two diagrams, especially at the turn-off and red giant branch (RGB) regions, we can already notice that the age range of the retrograde stars is smaller than that of the prograde stars, yet the large width of the turn-off as well as the width of the RGB, suggests that retrograde stars encompass a range of ages
Summary
The role that accretion events have played in the evolution of the Milky Way and the quest to find possible remnants that are at the origin of the old disc have been central topics in Galactic archaeology for over than five decades (e.g. Eggen et al 1962; Searle & Zinn 1978; Gilmore & Reid 1983; Chiba & Beers 2000; Gilmore et al 2002; Wyse et al 2006; Kordopatis et al 2011). Kordopatis et al 2017), a signature of inside-out formation and gas re-distribution in the primitive disc (Schönrich & McMillan 2017), or, as recently suggested by Minchev et al (2019), a correlation resulting from the superposition of mono[α/Fe] sub-populations with negative slopes (as in the thin disc). In the latter case, the measured positive correlation is due to the combination of several populations of different ages with different relative weights and proportions (the so-called Yule-Simpson paradox).
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