COSMIC-LAB: Terzan 5 as a fossil remnant of the Galactic bulge formation
  epoch
  The formation and evolution of galaxy bulges is one of the most debated
topics in the modern astrophysics. One approach to address this issue is to
look at the Galactic bulge since it is the closest. According to some
theoretical models, our bulge may have built up from the merger of
substructures formed from the fragmentation of a gaseous disk in the early
phases of Galactic evolution. We may have discovered the remnant of one of
these substructures in the stellar system Terzan 5. In fact, Terzan 5 hosts two
stellar populations with quite different iron abundances, thus suggesting it
once was far more massive than today. Moreover, its peculiar chemistry
strikingly resembles that observed in the Galactic bulge. In this Thesis we
performed a detailed photometric and spectroscopic analysis of this cluster to
determine its formation and evolution. Form the photometric point of view we
built a high-resolution differential reddening map in the direction of the
system and we measured relative proper motions to separate its member
population from the contaminating bulge and disk stars. From the spectroscopic
point of view we measured abundances for more than 600 stars belonging to
Terzan 5 and its surroundings in order to build the largest
field-decontaminated metallicity distribution for this system. We find that the
metallicity distribution is extremely broad (>1 dex) and we discovered a third,
metal-poor and alpha-enhanced population with average [Fe/H]=-0.8 dex. The
striking similarity between Terzan 5 and the bulge in terms of their chemical
formation and evolution that is clearly revealed by this Thesis suggests that
Terzan 5 formed in situ with the Galactic bulge. In particular its metal-poor
populations trace the early stages of the bulge formation, while its most
metal-rich component may contain crucial information on the bulge more recent
evolution.
