2007PASP..119..939G

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 of the Lee diagram populated by dwarf spheroidal stars, favoring an extragalactic origin. Old globular clusters show the full range of eccentricities, while younger ones seem to have preferentially high eccentricities, again hinting at their extragalactic origin. However, the three outer halo second parameter clusters with well-determined orbits indicate they come from three independent systems. 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 α vs. [Fe/H] pattern of 12 of the 13 galaxies studied are similar to each other and very different from the Milky Way. Sgr appears to be the only possible exception. At the metal-poor end the extragalactic sample is only slightly deficient compared to the halo but begins to diverge by [Fe/H] ∼ -2 and the difference is particularly striking for stars with [Fe/H] ∼ -1. Only Sgr, the most massive dSph, has some stars similar in α-abundance to Galactic stars at intermediate metallicities, even the most extreme low-α subset most likely to have been accreted. It appears very unlikely that a significant fraction of the metal-rich halo could have come from disrupted dSphs of low mass. However, at least some of the metal-poor halo may have come from typical dSphs, and a portion of the intermediate metallicity and metal-rich halo may have come from very massive systems like Sgr. This argues against the standard hierarchical galaxy formation scenario and the Searle-Zinn paradigm for the formation of the Galactic halo via accretion of ``fragments'' composed of stars like those we see in typical present-day dSphs. 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. AGB stars are also more important in the chemical evolution of the dwarfs. 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. The intrinsic scatter in many elements, particularly the α-elements, indicates that the halo was also mixed on a surprisingly short timescale, a further problem for hierarchical formation theories.