Relative Ages of Galactic Globular Clusters: Clues to the Formation and Evolution of the Milky Way

The aim of this thesis was a study of the formation history of the Milky Way. With this objective, we have analyzed the oldest stellar systems, the Galactic globular clusters (GGC), and found as a main result that this formation was produced mainly in a rapid collapse of the halo.

This thesis is organized into three main sections: In the first one, a catalog of color‐magnitude diagrams (CMD), constituting the observational database, is presented. The second section presents the main result: the study of the Milky Way formation history on the basis of the relative ages of 34 GGCs from our catalog plus Palomar 12. In the third section, a detailed study of two especially young clusters (Palomar 1 and Palomar 12) is done.

The catalog (see A. Rosenberg, A. Aparicio, I. Saviane, & G. Piotto, 2000, A&AS, in press; A. Rosenberg, G. Piotto, I. Saviane, & A. Aparicio, 2000, A&AS, in press) represents the first large and homogeneous CCD CMD database of GGCs, comprising 52 nearby objects imaged in the V and I bands using only two telescopes (the ESO 91 cm Dutch telescope and the Isaac Newton Group 1 m Jacobus Kapteyn Telescope; one for each hemisphere). The observed clusters were NGC 104*, NGC 288*, NGC 362*, NGC 1261*, NGC 1851*, NGC 1904*, NGC 2298, NGC 2808*, E3, NGC 3201*, NGC 4372, NGC 4590*, NGC 4833, NGC 5053*, NGC 5139, NGC 5272*, NGC 5466*, NGC 5897*, NGC 5904*, NGC 5927, NGC 5986, NGC 6093*, NGC 6121*, NGC 6101, NGC 6171*, NGC 6205*, NGC 6218*, NGC 6254*, NGC 6266, NGC 6304, NGC 6341*, NGC 6352*, NGC 6362*, NGC 6366*, NGC 6397*, NGC 6496, NGC 6535*, NGC 6544, NGC 6541, NGC 6624, NGC 6626, NGC 6637, NGC 6638, NGC 6656*, NGC 6681*, NGC 6717, NGC 6723*, NGC 6752*, NGC 6779*, NGC 6809*, NGC 6838*, and NGC 7078*. This sample represents 75% of all known GGCs with (m− M)_V≤16.15 mag, covers most of the GGCs metallicity range (-2.2≤[Fe/H]≤ - 0.4), and spans Galactocentric distances from ∼1.2 to ∼18.5 kpc. For five clusters (NGC 4833, NGC 5986, NGC 6543, NGC 6638, and NGC 6779) we provide for the first time a CMD based on CCD data. The typical CMD spans from V = 22 to the tip of the red giant branch. Based on a large number of standard stars, the absolute photometric calibration is reliable to a 0.02 mag level in both filters. The homogeneity between the two telescopes has been tested by comparing the photometry of three clusters observed with both telescopes. Because of its homogeneity, this catalog is expected to represent a useful database for the measurement of the main absolute and relative parameters characterizing the CMD of GGCs (i.e., I. Saviane, A. Rosenberg, G. Piotto, & A. Aparicio, 2000, A&A, in press).

The main result of the thesis is the study of the Milky Way formation history based on the relative ages of 34 clusters from our catalog (marked with an asterisk in the previous paragraph) and Palomar 12. A set of distance‐ and reddening‐independent relative age indicators[δ(V - I)_@2.5 and ΔV^HB_TO] have been measured, and their relations with metallicity are compared to the relations predicted by two recent updated libraries of isochrones. Using these models and two independent methods, we have found that self‐consistent relative ages can be estimated for our GGC sample. In turn, this demonstrates that the models are internally self‐consistent.

From the relative age versus metallicity distribution (Fig. 1, upper panel), we conclude that (a) there is no evidence of an age spread for clusters with [Fe/H]< - 1.2, since all the clusters of our sample in this range are old and coeval; (b) for the intermediate‐metallicity group (-1.2≤[Fe/H]< - 0.9) there is a clear evidence of age dispersion, with clusters up to ∼25% younger than the older members; (c) the clusters within the metal‐rich group ([Fe/H]> - 0.9) seem to be coeval within the uncertainties (except Palomar 12), but younger (∼17%) than the bulk of the Galactic globulars. The latter result is however model dependent.

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From the Galactocentric distribution of the GGC ages (Fig. 1, lower panel), we can divide the GGCs in two groups, the old coeval clusters and the young clusters. The second group can be divided into two subgroups, the "very young clusters" and the "young, but model‐dependent clusters," which are within the intermediate‐ and high‐metallicity groups, respectively.

From this distribution, we present a possible scenario for the Milky Way formation: The GGC formation process started at the same zero age throughout the halo and originated in the current halo at least out to ∼20 kpc from the Galactic center. According to the present stellar evolution models, the metal‐rich globulars were formed at a later time (∼17% lower age). Additionally, significantly younger halo GGCs are found at any R_GC>8 kpc. For these, a possible scenario associated with mergers of dwarf galaxies with the Milky Way is suggested (A. Rosenberg, I. Saviane, G. Piotto, & A. Aparicio, 1999, AJ, 118, 2306).

Finally, a detailed study of two clusters (Palomar 1 and Palomar 12) is presented. For Palomar 1 we provide the first complete photometric (A. Rosenberg, I. Saviane, G. Piotto, A. Aparicio, & S. Zaggia, 1998, AJ, 115, 648) and spectroscopic (A. Rosenberg, G. Piotto, I. Saviane, A. Aparicio, & R. Gratton, 1998, AJ, 115, 658) investigation. The most relevant result is that Palomar 1 is the youngest Galactic globular cluster discovered so far. The second object, Palomar 12, was observed because it was known to be a young candidate, and we wanted to have a cross‐check of the age scale for the entire GGC sample. We found that Palomar 12 is 68% ± 10% younger than 47 Tucanae and M5 (A. Rosenberg, I. Saviane, G. Piotto, & E. V. Held, 1998, A&A, 339, 61).