The origin of galactic metal-rich stellar halo components with highly eccentric orbits

Fattahi, Azadeh; Belokurov, Vasily; Deason, Alis J.; Frenk, Carlos S.; Gómez, Facundo A.; Grand, Robert J. J.; Marinacci, Federico; Pakmor, Rüdiger; Springel, Volker

doi:10.1093/mnras/stz159

Cited by 131 publications

(159 citation statements)

References 108 publications

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“…Please note that in the discussion above, instead of over-simplified "the last major merger", a more appropriate nomenclature perhaps would be "the last major that reached the Solar neighborhood". Interestingly, the end phases of the mergers as measured here match well those reported in Fattahi et al (2019): the differences are typically within 0.3 Gyr.…”

Section: Auriga Simulationssupporting

confidence: 84%

“…Figure 10 shows the behaviour of the azimuthal velocity as a function of metallicity (first and second column) and age (third and fourth column) for stellar particles in Auriga 6, 23 and 27. Note that these galaxies are amongst the ones with a highly radial stellar halo component as shown in Fattahi et al (2019). Moreover, Auriga 23 is the clearest example of the [α/Fe] bimodality as described in Grand et al (2018a).…”

Section: Auriga Simulationsmentioning

confidence: 82%

“…The ICC Aurigaia datasets are produced with the method of Lowing et al (2015) to generate mock stars from simulation star particles. In our analysis, we include stars bound to the MW analogues (not to the satellites) and decrease the metallicity ([Fe/H]) of stars by a constant value of 0.5 dex in accordance with Fattahi et al (2019). Figure 10 shows the behaviour of the azimuthal velocity as a function of metallicity (first and second column) and age (third and fourth column) for stellar particles in Auriga 6, 23 and 27.…”

Section: Auriga Simulationsmentioning

confidence: 99%

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The biggest splash

Belokurov

Sanders

Fattahi

et al. 2020

Monthly Notices of the Royal Astronomical Society

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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.

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Section: Auriga Simulationssupporting

confidence: 84%

Section: Auriga Simulationsmentioning

confidence: 82%

Section: Auriga Simulationsmentioning

confidence: 99%

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The biggest splash

Belokurov

Sanders

Fattahi

et al. 2020

Monthly Notices of the Royal Astronomical Society

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247

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“…Efforts have been made to use hydrodynamical simulations to constrain the mass of the GES galaxy using metallicities and/or α-abundances to infer the mass of the accreted galaxies that contributed to the formation of the inner halo (Robertson et al 2005;Font et al 2006;Tissera et al 2013;Fattahi et al 2019;Mackereth et al 2018;Fernández-Alvar et al 2019;Vincenzo et al 2019). Our simulations support the notion that the chemical abundance patterns of the halo stars reflect the fact that a relatively massive accretion event occurred.…”

Section: Introductionsupporting

confidence: 60%

Explaining the chemical trajectories of accreted and in-situ halo stars of the Milky Way

Kawata

Gibson

Gallart

et al. 2020

Monthly Notices of the Royal Astronomical Society

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The Milky Way underwent its last significant merger ten billion years ago, when the Gaia-Enceladus-Sausage (GES) was accreted. Accreted GES stars and progenitor stars born prior to the merger make up the bulk of the inner halo. Even though these two main populations of halo stars have similar durations of star formation prior to their merger, they differ in [α/Fe]-[Fe/H] space, with the GES population bending to lower [α/Fe] at a relatively low value of [Fe/H]. We use cosmological simulations of a 'Milky Way' to argue that the different tracks of the halo stars through the [α/Fe]-[Fe/H] plane are due to a difference in their star formation history and efficiency, with the lower mass GES having its low and constant star formation regulated by feedback whilst the higher mass main progenitor has a higher star formation rate prior to the merger. The lower star formation efficiency of GES leads to lower gas pollution levels, pushing [α/Fe]-[Fe/H] tracks to the left. In addition, the increasing star formation rate maintains a higher relative contribution of Type II SNe to Type Ia SNe for the main progenitor population that formed during the same time period, thus maintaining a relatively high [α/Fe]. Thus the different positions of the downturns in the [α/Fe]-[Fe/H] plane for the GES stars are not reflective of different star formation durations, but instead reflect different star formation efficiencies. We argue that cosmological simulations match a wide range of independent observations, breaking degeneracies that exist in simpler models.

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“…This Plume is largely responsible for the marked change in radial anisotropy with increasing [Fe/H] in the RRL sample, which resembles the one Belokurov et al (2018) found in a sample of main-sequence stars. In all respects, the kinematics of the Plume RRL closely resemble as the 'Gaia-Sausage' or 'Gaia-Enceladus' , which appear to have their origin in a large dwarf galaxy that merged with the MW (see also Myeong et al 2018;Kruijssen et al 2019;Mackereth et al 2019a;Vincenzo et al 2019;Fattahi et al 2019). The MDF of the Plume RRL is skewed towards higher metallicity than the MDF of other high-energy RRL, many of which are members of either the prograde Helmi-Stream or retrograde groups that may be part of the 'Sequoia' galaxy (Myeong et al 2019).…”

Section: Discussionmentioning

confidence: 90%

Local RR Lyrae stars: native and alien

Zinn

Chen

Layden

et al. 2019

Monthly Notices of the Royal Astronomical Society

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Measurements of [Fe/H] and radial velocity are presented for 89 RR Lyrae (RRL) candidates within 6 kpc of the Sun. After the removal of two suspected non-RRL, these stars were added to an existing database, which yielded 464 RRL with [Fe/H] on a homogeneous scale. Using data from the Gaia satellite (Data Release 2), we calculated the positions and space velocities for this sample. These data confirm the existence of a thin-disc of RRL with [α/Fe]∼ solar. The majority of the halo RRL with large total energies have near zero angular momenta about the Z axis. Kinematically these stars closely resemble the Gaia-Sausage/Gaia-Enceladus stars that others have proposed are debris from the merger of a large galaxy with the Milky Way. The metallicity and period distributions of the RRL and their positions in the period-amplitude diagram suggest that this disrupted galaxy was as massive as the Large Magellanic Cloud and possibly greater.

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The origin of galactic metal-rich stellar halo components with highly eccentric orbits

Cited by 131 publications

References 108 publications

The biggest splash

The biggest splash

Explaining the chemical trajectories of accreted and in-situ halo stars of the Milky Way

Local RR Lyrae stars: native and alien

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