Evidence for an old galactic bulge from RR lyrae stars in Baade's window: Implications for the formation of the galaxy and the age of the universe

Recent observations suggest that the peak of the metallicity distribution for RR Lyrae variables in the Galactic nuclear bulge is more metal-rich than that found in the halo of the Galaxy. It is shown that this is what one would expect if the radial variation in horizontal-branch (HB) morphology observed in the halo continues to the very center of the Galaxy. Interpreted as an age effect, as supported by recent work, this provides evidence, for the first time, that the oldest stellar population (i.e., RR Lyraes) in the Galactic nuclear bulge is older than that in the halo by 1.3±0.3 Gyr. Other possibilities can be ruled out from the analyses of the periods of RR Lyrae variables and/or other observations. The inferred radial gradient in age implies that the bulge was the first part of our Galaxy to form, and then served as a nucleus around which the rest of the Galaxy was built up from the inside out. This information, together with the radial variation in age dispersion among the halo population, suggests that the protogalactic halo fragments hypothesized by Searle & Zinn [ApJ, 225, 357 (1978)] may actually be a part of the protogalactic subsystems that have been the building blocks not just of the halo, but of the entire Galaxy. This is consistent with the idea that galaxies formed as clusters and mergers of many gas-rich subsystems, rather than by the rapid collapse of single large protogalactic clouds. The similarity between elliptical galaxies and the bulges of spiral galaxies suggests then that the origin of the Hubble sequence is due, at least in part, to the differences in the overall frequency of collisions and mergers among the subsystems in different environments. A further implication of the present result is that the globular clusters found in the halo of the Galaxy are not the oldest objects in the Galaxy. The lower limit on the age of the Universe is at least ∼1 Gyr older than the age of the oldest halo population.