Before the Bar: The Hidden Bulge of the Milky Way

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The aim of this proposal is to use old, metal-poor stars to trace a possible classical bulge in the Milky Way. We believe to have finally uncovered the long sought pressure-supported bulge component by probing the oldest luminous bulge stars, the RR Lyrae stars (RRLs), and we propose to use spectroscopy to understand its origin, via dynamics, stellar chemical compositions and by performing dynamical simulations. Our pilot survey has revealed that the bulge RRLs are the first bulge population known to exhibit hot kinematics and to have null or negligible rotation, as is expected for a classical bulge. They are therefore completely distinct from the prominent rotating bar/bulge population. Their abundance distribution is more metal rich than the stellar halo, yet nearly 1 dex more metal poor than the bulge/bar population. Their stellar pulsation properties argue powerfully for their being distinct to any Milky Way component and perhaps related to M31. The difference of the kinematics of the bulge RRLs from the bulge giants, which dominate the stellar mass in the Galactic bulge, demonstrates that the Milky Way has a composite bulge. Therefore, there were two formation epochs for the Galactic bulge: an early epoch that is preserved in the kinematics of the ancient RRLs, and a later epoch in which the massive disk, which has evolved into the bar, experienced a burst of star formation. These bulge RRLs may be the oldest stars in the Galaxy, serving as a nucleus around which the rest of the Galaxy was built up from the inside out. The case is now strong to undertake more investigation of this new stellar population. A detailed chemo-dynamic analysis will allow an understanding of this ancient stellar population in the Galactic bulge, which is a population that pre-dated the formation of the ``bar" that dominates the central mass in the present day.