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

Understand

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.