What is the age of the Galaxy's inner halo?

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Session 97 -- Milky Way
Oral presentation, Wednesday, 11, 1995, 2:00pm - 3:30pm

[97.04D] What is the age of the Galaxy's inner halo?

L.K.Fullton, B.W.Carney (University of North Carolina)

We are using globular clusters near the Galactic center to probe the formation history and chemical evolution of the Galaxy's central regions. Because star clusters are the systems for which the most reliable ages can be obtained, we have constructed color-magnitude diagrams of three globular clusters which lie within $\sim$4.5 kpc of the Galactic center. From these CMDs, we have derived accurate relative ages for the clusters with respect to previously studied clusters in the outer halo. We have found that the age-metallicity relationship of Carney, Storm \& Jones (1992, ApJ, 386, 663) continues into the inner halo, with NGC 5927, the most metal-rich cluster in our sample, being one of the youngest. We have derived a large age, 18 Gyr, for NGC 6723, a cluster 2.5 kpc from the Galactic center whose metallicity is similar to the mean metallicity of RR Lyraes in the bulge field called Baade's Window (BW). This age supports the claim by Lee (1992, AJ, 104, 1780) that the BW RR Lyraes are older than (or at least as old as) the oldest globulars. However, using the Holtzman, et al. (1993, AJ, 106, 1826) CMD of BW to derive the bulge age in a fashion consistent with that of the globulars, we find an age of $\sim$\nolinebreak[2] 10 Gyr for the bulk of the bulge population. If the BW RR Lyraes and NGC 6723 represent the first stages of bulge formation, then this result suggests that the bulge may have formed over a timescale as long as 8\nolinebreak[2] Gyr. We are currently analyzing high-resolution echelle spectra of giants in NGC 6723 and NGC 5927 to derive [Fe/H] and [CNO/Fe] ratios which are crucial for confirmation of our derived ages.

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