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 globulars 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 2nd 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 \alpha 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 ] \sim - 2 and the difference is particularly striking for stars with [ Fe/H ] \sim - 1 .
Only Sgr , the most massive dSph , has some stars similar in \alpha abundance to Galactic stars at intermediate metallicities , even the most extreme low \alpha 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 \alpha ’ s , indicates that the halo was also mixed on a surprisingly short timescale , a further problem for hierarchical formation theories .