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

Abstract

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 [$\alpha$/Fe]-[Fe/H] space, with the GES population bending to lower [$\alpha$/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 [$\alpha$/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 [$\alpha$/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 [$\alpha$/Fe]. Thus the different positions of the downturns in the [$\alpha$/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.

Accepted version MNRAS

References in corpus (9)

• Star Formation and Feedback in Smoothed Particle Hydrodynamic Simulations--I. Isolated Galaxies
• Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo
• The Dual Origin of Stellar Halos
• A Superbubble Feedback Model for Galaxy Simulations
• The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage
• Age dating of an early Milky Way merger via asteroseismology of the naked-eye star $\nu$ Indi
• The tale of the tail -- disentangling the high transverse velocity stars in Gaia DR2
• The origin of low [alpha/Fe] ratios in extremely metal-poor stars
• The Role of Feedback in Shaping the Structure of the Interstellar Medium

Cited by in corpus (11)

• Stellar migrations and metal flows -- Chemical evolution of the thin disc of a simulated Milky Way analogous galaxy
• Bimodality of [α/Fe]-[Fe/H] distributions is a natural outcome of dissipative collapse and disc growth in Milky Way-type galaxies
• The challenge of simultaneously matching the observed diversity of chemical abundance patterns in cosmological hydrodynamical simulations
• High-precision chemical abundances of Galactic building blocks. The distinct chemical abundance sequence of Sequoia
• The stellar halo in Local Group Hestia simulations III. Chemical abundance relations for accreted and in-situ stars
• The nature of the Milky Way's stellar halo revealed by the three integrals of motion
• A high fidelity Milky Way simulation with Kraken, Gaia-Enceladus, and Sequoia analogues: clues to their accretion histories
• Applying machine learning to Galactic Archaeology: how well can we recover the origin of stars in Milky Way-like galaxies?
• Stellar substructures in the Galactic disc and halo: Properties, origins, and evolution
• Machine Learning for Galactic Archaeology: A chemistry-based neural network method for identification of accreted disc stars
• Rapid early gas accretion for the inner Galactic disc