Formation of normal galaxies: the Milky Way halo

1. The relative ages of inner-halo globular clusters
   1. The largest homogeneous photometric database of Galactic globular clusters has been assembled (34 objects), based on observational runs carried out along two years, both with Southern and Northern telescopes.
   2. This database (+ Pal 12) has been used to measure a set of distance and reddening independent relative age indicators, which have been compared to the relations predicted by two recent updated libraries of isochrones.
   3. Using these models and two independent methods, we have found that self-consistent relative ages can be estimated for the GGC sample. In turn, this demonstrates that the models are internally self-consistent. Based on the relative age vs. metallicity distribution, the following conclusions have been reached.
   4. There is no evidence of an age spread for clusters with [Fe/H], all the clusters of our sample in this range being old and coeval.
   5. For the intermediate metallicity group ( there is a clear evidence of age dispersion, with clusters up to younger than the older members.
   6. The clusters within the metal rich group ([Fe/H]) seem to be coeval within the uncertainties (except Pal 12), but younger () than the bulk of the Galactic globulars. The latter result is totally model dependent.
   7. From the Galactocentric distribution of the GGC ages, GGCs can be divided in two groups: The old coeval clusters, and the young clusters.
   8. From this distribution, a possible scenario for the Milky Way formation has been presented: The GC formation process started at the same zero age throughout the halo, at least out to  kpc from the Galactic center. According to the present stellar evolution models, the metal-rich globulars are formed at a later time ( lower age). And finally, significantly younger halo GGCs are found at any  kpc. For these, a possible scenario associated with mergers of dwarf galaxies to the Milky Way is suggested.
2. Young clusters in the Galactic halo
   1. Palomar 1
      1. A color magnitude diagram down to , and a luminosity function complete down to have been constructed.
      2. A distance modulus has been obtained, indicating that Pal 1 is located  kpc above the Galactic disk and  kpc from the Galactic center.
      3. The metallicity has been determined using the equivalent widths of the CaII triplet on medium resolution spectra. From the comparison of the luminosity corrected equivalent widths in four stars of Palomar 1 with those of a sample of stars in each of three calibration clusters (M2, M15, and M71), [Fe/H]= on the Zinn & West (1984) scale has been found, or [Fe/H]= on the Carretta & Gratton (1997) scale. The radial velocity has been also obtained, Km/s .
      4. A comparison with 47 Tuc and M71 has shown that Pal 1 must be significantly younger. The best fitting Bertelli et al. (1994) isochrones have given an age between 6.3 and 8 Gyrs, which would make Pal 1 the youngest Galactic globular cluster so far identified. The luminosity function shows some evidence for mass segregation, consistent with the very short relaxation time. The global mass function can be fitted with a power law of slope . This mass function is anomalously flat, suggesting either a strong dynamical evolution or an initial mass function significantly different from most of the other halo globular clusters.
   2. Palomar 12
      1. Broadband , CCD photometry for stars, covering a field of arcmin, has been obtained. From these data, a color-magnitude diagram from the red giant branch tip to mag below the cluster's turn-off has been constructed.
      2. From a comparison with the color magnitude diagrams of 47 Tuc and M5, and using different theoretical models, it has been confirmed that Pal 12 is younger, finding an age that of both template clusters.
      3. Revised structural parameters have also been obtained: and .

Los Angeles, October 20, 2000