The Milky Way and the Local Group

Abstract

The beauty and the charm of the Milky May (MW) have been celebrated by countless poets and writers of many Countries along the centuries (see e.g. the beautiful anthology of Piero Boitani [46]). The stellar nature of the MW was firstly observed by Galileo. In 1610 in the Sidereus Nuncius [126] Galileo wrote that the MW is “nient’altro che una congerie di innumerevoli Stelle, disseminate a mucchi; ch`e in qualunque regione di essa si diriga il cannocchiale, subito una ingente folla di Stelle si presenta alla vista, delle quali parecchi si vedono abbastanza grandi e molto distinte; ma la moltitudine delle piccole `e del tutto inesplorabile”. In the same paragraph, Galileo remarked that observations with his telescope, for the first time, wipe out centuries of philosophical discussions about the nature of the MW. Three more centuries have been necessary to complete a second radical Copernican Revolution that displaces the solar system from being roughly at the center of the MW and project this latter in the vast Universe populated by billions of similar spiral galaxies (see Chapter1). The first part of this Chapter is dedicated to the crescendo of studies that have progressively contributed to our understanding of the MW. Section 2.2, by James Lattis, reviews the fundamental step in recognizing the Galaxy as a “normal” spiral, before the WWII. Piet van der Kruit in Section 2.3, goes into the evolution of our knowledge of the MW structure and of kinematics. This Section, starting from the achievements of Jaan Ort in the 1920s about the Galaxy differential rotation, brings us to the modern view of theMWand how this reverberates in general on the understanding of spiral galaxies. The Galaxy is made by stars of different families. The fundamental concepts of stellar populations is introduced in Section 2.4 by Antonella Vallenari with an historical evolution up to the use of stars in the definition of MW structure and the clock they provide to understand the MW evolutionary path. The dust properties in the MW are discussed in Section 2.5 by Daniela Calzetti. The second part of the Chapter is dedicated to the problem of the extragalactic distance scale. As mentioned in Chapter 1 there was a long term debate on this issue that has seen two different schools of thought on opposite positions. Barry Madore introduces the debate about the determination of the distance scale and the use of adequate primary and secondary indicators for the purpose. The third part of the Chapter is dedicated to the nearby galaxies of theMWand to the exploration from inside of a gravitationally bound group of galaxies in an evolving environment. It is worth to mention that the MW and the companions in the socalled Local Group (LG) are basically the only galaxies we can study resolving their stellar components. In this sense, the MW and the LG galaxies are a sort of Rosetta stone for understanding extragalactic objects. They are of overwhelming importance even for understanding distant galaxies since among the recently discovered ultra faint dwarfs (UFD) might lurk the debris of first galaxies (see the contribution of Volker Bromm in Chapter 6. In Section 2.7 Valentina Karachentseva describes the population of galaxies members of the LG, their discovery along this century of research and their properties. A zoom on the Andromeda galaxy M31 is provided by Rodrigo Ibata in Section 2.7.1, while the dwarf galaxy populations, with the widest number of members in the LG is presented by Carme Gallart in Section 2.7.2. An overall view of the co-evolution of the galaxies members of the LG is presented in Section 2.8 by Rodrigo Ibata and Goerge Lake. In Section 2.9 Brent Tully brings us just outside the LG discussing the present view of the structure and motions in the nearby Universe. The Chapter closes with what we call the “lessons for the future” in Section 2.10. The road toward a modern vision of The Galaxy and a precise calibration of the galaxy distances was and remains hard.