The Anatomy of Galaxies

Just after the WWII Astronomy started to live its “Golden Age”, not differently to many other sciences and human activities, especially in the west side countries. The improved resolution of telescopes and the appearance of new efficient light detectors (e.g. CCDs in the middle eighty) greatly impacted the extragalactic researches. The first morphological analysis of galaxies were rapidly substituted by “anatomic” studies of their structural components, star and gas content, and in general by detailed investigations of their properties. As for the human anatomy, where the final goal was that of understanding the functionality of the organs that are essential for the life of the body, galaxies were dissected to discover their basic structural components and ultimately the mystery of their existence. The first morphological studies and photometric analyses already identified many galaxy structures, that only later on stellar and gas kinematics were able to identify as separate and independent galaxy components. Despite these efforts many questions are still open today. What define a galaxy component? Does exist a structural component common to all galaxies? How much galaxy components are the product of nature and what can be attributed to nurture? What in synthesis define a galaxy system? And why some galaxies obey scaling relations and others do not? In this chapter we interview several outstanding extragalactic astronomers to find an answer to the above questions. At the end we will have a much clear view of what astronomers actually mean when they speak of galaxy components. Section 4.3 deals with one of the most debated issues about galaxy structure: the bulge component. What define it? how and when it was formed? to what extent bulges are naturally produced during the collapse of the protogalaxy and what can be attributed to secular evolution and merging events? In the same section we highlight the main properties of galaxy disks, briefly reviewing their structures and kinematics. Again, some underlying questions remain to be answered: how disks evolve with time? why galaxies disk exhibit different substructures (bars, lenses, rings, spiral arms) and different light profiles? in which way they chemically evolve? Why spiral arms are not equal in all galaxies? Section 4.4 and 4.5 have been explicitly dedicated to the typical disk substructures. Although not present in all disk-like objects, these structures have an important role in galaxy evolution. Are they transient phenomena occurring on the disk components? How much they contribute to the general evolution of the galaxies and in which way? We will see what the current models are able to predict up to now. Originally Sect. 4.6 had to address the properties of the less known galaxy component: the halo. Its structure and stellar/non-stellar content is indeed poorly known still today. The halo is the more difficult component to define and study, and would merit by itself an entire book. Actually a detailed analysis of this component is possible only in the MW and its neighborhoods. We therefore decided to focus our interviews on one important physical effect connected with halos: the gravitational lensing phenomenon. Why it is so important and what can we deduce about halos from this analysis? In the same section we also ask a question on the most typical stellar aggregate present in galaxy halos: the Globular Clusters. They are the oldest stellar systems and could therefore testify about the first epoch of galaxy formation. In Sect. 4.7 we remind that galaxies often contain a non negligible amount of gas and dust. However in the whole Universe the percentage of cold gas represents a very small fraction of the global energy density. So, why should we care of it? Interestingly we will discover that this material can teach us a lot of things about galaxy evolution and the origin of the first galaxies. Following such discussion, Section 4.8, will introduce the theme of the hot baryon component of galaxies, mainly visible in the X-ray domain. What can we learn about galaxies from these studies? Section 4.9 ends our discussion of the main galaxy components addressing the most neglected one: the magnetic field. This is not a classical component made of stars and gas and normally we cannot see it without measuring the polarization of light coming from galaxies; this has been always a very difficult job. However, today we are gaining more and more insights of the importance of magnetic fields in galaxies, in particular for their connection with the star formation process. With Sect. 4.10 we review again the concept of stellar population and its connection with the structural components of galaxies. As it emerges from several parts of this book the various generations of stars do not reside everywhere in the galaxy body and can also migrate across it. Why then stars seems to know very well where they should reside? How the different populations of stars were chemically enriched? Why the Mass-to-Light ratio is so entangled with the galaxy Mass? We finally discuss the main scaling relations that galaxies have shown to obey in Sect. 4.11. The physical origin of many of such relations is still not completely understood. We will address in particular the relations known as Fundamental Plane and Tully-Fisher. These relations tell us that galaxy form and evolve following well defined rules, but we miss some fundamental physics behind them. Their existence seems to indicate that a strong fine-tuning exists between the star formation history and the structures which host the stellar populations. We will also discuss the linearity of these scaling relations and their consequences for understanding the processes which gave origin to the present morphological classes of galaxies