Origin of the Galactic Halo: accretion vs. in situ formation

TL;DR

This study compares chemical abundance patterns of dwarf galaxy satellites with Galactic halo stars to investigate their origins, finding evidence that most halo stars formed in situ rather than from accreted satellites.

Contribution

It provides a detailed chemical evolution model showing how in situ formation dominates the Galactic halo, challenging the accretion hypothesis.

Findings

[O/Fe] ratios differ at higher metallicities, indicating different origins.

Discrepancies in neutron capture elements suggest in situ formation.

Chemical evolution depends on gas infall timescale and star formation thresholds.

Abstract

We test the hypothesis that the classical and ultra-faint dwarf spheroidal satellites of the our Galaxy have been the building blocks of the Galactic halo by comparing their [O/Fe] and [Ba/Fe] vs. [Fe/H] patterns with the ones observed in Galactic halo stars. The [O/Fe] ratio deviates substantially from the observed abundance ratios in the Galactic halo stars for [Fe/H] > -2 dex, while they overlap for lower metallicities. On the other hand, for the neutron capture elements, the discrepancy is extended at all the metallicities, suggesting that the majority of stars in the halo are likely to have been formed in situ. We present the results for a model considering the effects of an enriched gas stripped from dwarf satellites on the chemical evolution of the Galactic halo. We find that the resulting chemical abundances of the halo stars depend on the adopted infall time-scale, and the presence of a threshold in the gas for star formation.