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| Paper: |
Early and Late Evolution of the Milky Way |
| Volume: |
92, Formation of the Galactic Halo. . . . Inside and Out |
| Page: |
161 |
| Authors: |
Gilmore, Gerard |
| Abstract: |
A correlation between stellar orbital eccentricity and metallicity was interpreted by ELS as evidence for rapid collapse of the early halo into the disk. An important assumption of such models is that essentially all the gas which is today in the Milky Way's potential well has always been there. Since the later discovery of dark halos, and the development of dark matter--dominated models of galaxy formation, this assumption becomes unviable. Nonetheless, recent careful photometry by Twarog and Anthony-Twarog has proven that the correlation discussed by ELS is real, and requires an explanation. An important clue comes from determination of the rotation curve of the Galactic bulge, and hence calculation of the distributions of specific angular momentum in the major Galactic structural components. There are nearly identical distributions of specific angular momentum in both the thick disk and the old disk, and in both the metal-poor halo and the bulge, with the two distributions differing from each other substantially. Thus we may interpret the ELS/Twarog/Anthony-Twarog correlation as evidence for an early (monolithic) collapse of the halo, but into the bulge, not the disk. The halo/bulge and thick disk/thin disk parts of the Galaxy are essentially unrelated. The existence of the Sagittarius dSph well inside the Milky Way however raises the question of the importance of later mergers vis-a-vis the contents of the original halo. Perhaps the most remarkable property of the Sgr dSph is that it still exists as a discrete galaxy. Clearly, dSph are very robust against tidal disruption by the Galaxy. This is further evidence for their dominance by very cold dark matter. To test the universality of this conclusion, we have completed an extensive series of numerical calculations, showing, for the first time, that unrecognised binarism cannot explain the observed high velocity dispersion in the local dSph. The dSph really are extremely dark matter dominated, and extremely robust against destruction. This, together with direct evidence from studies of the age distribution of halo stars, supports the deduction that the present halo, and its close relation, the outer bulge, are dominated by the first stellar generations to form in the potential well which has since grown to today house the Milky Way. |
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