The assembly history of the Galactic inner halo inferred from alpha-patterns

TL;DR

This study investigates the origin of alpha-element abundance patterns in the Milky Way's inner halo, combining observational data from APOGEE with galaxy simulations to understand stellar formation history and accretion events.

Contribution

It provides new insights into the formation history of the Galactic inner halo by linking observed alpha-element gradients with simulated accretion and star formation histories.

Findings

Stars with [Fe/H] > -1.1 formed in two starbursts separated by ~1 Gyr.

The best-matching simulation involves accretion of low to intermediate-mass satellites.

Different simulated galaxies show contrasting alpha-pattern trends with galactocentric distance.

Abstract

We explore the origin of the observed decline in [O/Fe] (and [Mg/Fe]) with Galactocentric distance for high-metallicity stars ([Fe/H] > -1.1), based on a sample of halo stars selected within the Apache Point Observatory Galactic Evolution Experiment (APOGEE) fourteenth data release (DR14). We also analyse the characteristics of the [$\alpha$/Fe] distributions in the inner-halo regions inferred from two zoom-in Milky Way mass-sized galaxies that are taken as case studies. One of them qualitatively reproduces the observed trend to have higher fraction of $\alpha$-rich star for decreasing galactocentric distance; the other exhibits the opposite trend. We find that stars with [Fe/H] > -1.1 located in the range [15 - 30] kpc are consistent with formation in two starbursts, with maxima separated by about ~ 1 Gyr. We explore the contributions of stellar populations with different origin to the [$\alpha$/Fe] gradients detected in stars with [Fe/H] > -1.1. Our analysis reveals that the simulated halo that best matches the observed chemical trends is characterised by an accretion history involving low to intermediate-mass satellite galaxies with a short and intense burst of star formation, and contributions from a more massive satellite with dynamical masses about ~ 10$^{10}$M$_{\odot}$, distributing low-[$\alpha$/Fe] stars at intermediate radius.