The New Boundaries of the Galaxy Concept

In Chapter 6 we outlined the research strategies implemented for the studies of high redshift galaxies, briefly addressed their structures, morphologies and star formation, discussed the ancestor problem and touched the theme of the origin of the first galaxies. With this Chapter we certainly enter in the modern view of galaxies, as members of a big society. Up to now we have essentially described and characterized the properties of nearby isolated galaxies, that are sufficiently relaxed to show their almost unperturbed properties. However the history of this society, not differently from the man society, is full of conflicts and battles for surviving. The present knowledge of the cosmic web, of the hierarchical nature of the structures in the Universe, and of the galaxy components have clearly demonstrated that gravitational interactions, gas accretion and outflows, as well as merging events and gravitational interactions are the true past history of galaxies. Galaxies change their structure and morphology across the Hubble time and many of their observed properties today depend on their past history, as well as in many cases on the environment in which they evolved. The title of this Chapter reflects our tentative effort of looking at galaxies along this viewpoint. What are the current boundaries of the galaxy concept? In other words, to what extent the structures we see today were present in the past and what are their limits in mass, luminosity, size, stellar populations and chemical enrichment across the Hubble time? What is the role of the dark matter? In which way the energetic phenomena observed in the galaxy nuclei and in SNe have changed the properties of galaxies? The suspect is that we have built the concept of galaxies looking only at their mature appearance and not at their whole life. Can we speak of man describing only its adult phase and not considering his interaction with the whole society? The following interviews will clarify several things connected to this new point of view. We start in Sec. 7.2 with a nice review of the effects of gravitational interactions and merging that will highlight the progresses done by simulations since the early Toomre’ pioneering work. We then address in Sec. 7.3 the problem of the disk assembling and the remarkable extended UV disks found in many galaxies. The current limits in the mass function of galaxies, from dwarfs to giants, are discussed in Sec. 7.4. The various effects of the presence of an AGN at the center of galaxies are analyzed in Sec. 7.5, while the activity of SNe is addressed in Sec. 7.6. Finally, Sec. 7.7 provide the different point of views on the role and nature of the dark matter.