Chemical Abundances and Kinematics in Globular Clusters and Local Group Dwarf Galaxies and Their Implications for Formation Theories of the Galactic Halo

TL;DR

This review examines the chemical and kinematic properties of stars in globular clusters and dwarf galaxies to understand their roles in the formation of the Galactic halo, highlighting differences and possible origins.

Contribution

It provides a comprehensive comparison of chemical abundances and kinematics, proposing that the Galactic halo primarily formed from early accretion of massive satellites.

Findings

Outer halo clusters have high eccentricities, suggesting extragalactic origins.

Halo chemical abundances are distinct from those of dwarf galaxies studied.

Most metal-rich halo stars likely did not originate from low-mass dwarf galaxies.

Abstract

We review Galactic halo formation theories and supporting evidence, in particular kinematics and detailed chemical abundances of stars in some relevant globular clusters as well as Local Group dwarf galaxies. Outer halo red HB clusters tend to have large eccentricities and inhabit the area populated by dwarf spheroidal stars, favoring an extraGalactic origin. Old globulars show the full range of eccentricities, while younger ones seem to have preferentially high eccentricities, again hinting at their extraGalactic origin. We compare detailed abundances of a variety of elements between the halo and all dwarf galaxies studied to date, including both dwarf spheroidals and irregulars. The salient feature is that halo abundances are essentially unique. In particular, the general alpha vs. [Fe/H] pattern of 12 of the 13 galaxies studied are similar to each other and very different from the Milky Way. Sagittarius appears to be the only possible exception. It appears very unlikely that a significant fraction of the metal-rich halo could have come from disrupted dwarf galaxies of low mass. However, at least some of the metal-poor halo may have come from typical dwarfs, and a portion of the intermediate metallicity halo may have come from very massive systems. The chemical differences between the dwarfs and the halo are due to a combination of a low star formation efficiency and a high galactic wind efficiency in the former. The formation problem may be solved if the majority of halo stars formed within a few, very massive satellites accreted very early. However, any such satellites must either be accreted MUCH earlier than postulated, before the onset of SNe Ia , or star formation must be prevented to occur in them until only shortly before they are accreted.