›I NAT- Lectures on Astrophysics. Collapsing or Colliding Systems: Solving the Galactic Puzzle
One of the most relevant issues in astrophysics is the study of how galaxies form and evolve. These systems are believed to be the building blocks of the universe and the complete understanding of their nature requires efforts in different fields in astronomy and physics. The study of galaxies is not important only for its own since it involves the knowledge of processes acting on the smallest scales of elementary particles as well as on the largest structures of the universe. All these subjects should be viewed from different perspectives and analyzed with tools from different branches in astrophysics. By combining the results from different studies, a coherent picture is emerging when it comes to the properties and nature of galaxies of different morphologies. Stellar populations, chemical enrichment, gas flows, dynamics, star formation, and many other processes and properties have been put together providing explanations and insights into the differences and similarities among galaxies, but their formation and evolution are still matter of debate. The two most popular scenarios proposed to the formation and evolution of galaxies apparently contradict each other: the monolithic scenario proposes that large galaxies formed through the central collapse of a gas cloud at high redshifts, whereas the hierarchical one states that large galaxies formed via the merger of minor blocks in more recent epochs. Both scenarios have advantages and drawbacks, depending on the observational constraint to which the theory is compared. The results of cosmological models reproduce very well the observed properties of the universe at large scales, but when individual galaxies are taken into account, models adopting the monolithic formation provide better results. In order to obtain a complete picture of this field, one should put together the possible largest amount of data and results of all kinds of simulations and theories, examining and comparing all the information obtained.