The biggest splash

Abstract

Using a large sample of bright nearby stars with accurate Gaia Data Release 2 astrometry and auxiliary spectroscopy we map out the properties of the principle Galactic components such as the "thin" and "thick" discs and the halo. We show that in the Solar neighborhood, there exists a large population of metal-rich ([Fe/H]> −0.7) stars on highly eccentric orbits. By studying the evolution of elemental abundances, kinematics and stellar ages in the plane of azimuthal velocity v φ and metallicity [Fe/H], we demonstrate that this metal-rich halo-like component, which we dub the Splash, is linked to the α-rich (or "thick") disc. Splash stars have little to no angular momentum and many are on retrograde orbits. They are predominantly old, but not as old as the stars deposited into the Milky Way in the last major merger. We argue, in agreement with several recent studies, that the Splash stars may have been born in the Milky Way's proto-disc prior to the massive ancient accretion event which drastically altered their orbits. We can not, however, rule out other (alternative) formation channels. Taking advantage of the causal connection between the merger and the Splash, we put constraints of the epoch of the last massive accretion event to have finished 9.5 Gyr ago. The link between the local metal-rich and metal-poor retrograde stars is confirmed using a large suite of cutting-edge numerical simulations of the Milky Way's formation.

The artemis simulations: stellar haloes of Milky Way-mass galaxies

“…3): on average, in the solar neighborhood, ≈ 70% of halo stars are predicted to have formed in situ, and the fractions remain high out to 30-40 kpc. This result is in good agreement with the Gaiaderived in situ halo fraction near the sun, which is estimated to be nearly 50% (Belokurov et al 2019).…”

“…Until recently, observational studies have targeted stars that were either metal-poor or kinematically distinct from the disc. With the recent confirmation of the importance of the in situ halo in the Milky Way (Belokurov et al 2019;Conroy et al 2019), and with strong evidence in favour of an important in situ component of the M31 halo (Dorman et al 2013), there is a need to provide accurate predictions for the nature of stellar haloes using cosmological simulations. Our study has provided detailed information about the structure of in situ and accreted components and has made a number of predictions which can be tested in the future.…”

“…Observations have found that the stellar halo of the Milky Way is well fitted by a broken power law, with an inner slope of ≈ −2.5, a steep outer slope of −4 to −5, and a break radius of ≈ 25-30 kpc (Watkins et al 2009;Deason et al 2011;Sesar et al 2011;Xue et al 2015). However, as a word of caution, the reported inner slope for the Milky Way was derived from samples that may have missed a significant fraction of the in situ component, which has only been uncovered recently using Gaia data 7 (Belokurov et al 2019). In terms of the outer slope, some studies also find a steeper decline in the density profile at large radii, with a slope of ≈ −6 outside ≈ 50 kpc (Ivezić 2000;Deason et al 2014).…”

“…Observational results indicate that the stellar halo of the Milky Way has a 'dual nature', with a more metal-rich, flattened and slightly rotating inner component and a more metal-poor, rounder and non-rotating outer one (Carollo et al 2007). A more recent analysis of the stellar halo using Gaia DR2 data has revealed that approximately half of the total halo population in the Solar neighborhood is likely to have been born in situ (Belokurov et al 2019). A significant population of in situ stars in the inner halo has also been revealed by the H3 survey (Conroy et al 2019).…”

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

“…This scenario explains the higher metallicity of the red sequence compared to the blue sequence: chemical evolution progresses more rapidly in larger galaxies. Belokurov et al (2020) named these 'kicked out' halo stars the 'Splash', and analysed the extensive data set that combined Gaia DR2 with data from several different spectroscopic surveys, including APOGEE DR14, LAMOST DR2, RAVE DR5, Gaia-ESO DR3, GALAH DR2, and SEGUE, which is compiled and value added in Sanders & Das (2018). They thoroughly analysed the chemical abundances, kinematics, and ages of the stars in the halo and thick and thin discs.…”