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 similar to 1Gyr. 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 characterized 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 similar to 10(10)M(circle dot), distributing low [alpha/Fe] stars at intermediate radius.