Читать книгу Galaxies - Группа авторов - Страница 12

Galaxies are complex gravitational systems whose structure has been influenced not only by how they formed but also by the environment into which they were born. A century ago, getting a classifiable image of a single galaxy was a major effort involving long exposures of photograph plates. Today, there are classifiable images of literally millions of galaxies available through the Internet. Although it is not obvious how any galaxy arrived at its current morphological state, examination of the details of large numbers of galaxies have led to important physical insights into the roles played by both internal and external processes. It is for this reason that classical galaxy morphology and classification have survived into the modern era.

Galaxy morphology and classification began with simple descriptive terms for angular size, brightness and central concentration based on visual observations (e.g. Dreyer 1888). Although large 19th-Century reflectors did reveal through visual observation some genuine aspects of galaxy morphology, it was photography that led to the classification systems of the early-to-mid 20th Century, including those of Wolf (1908), Reynolds (1920), Hubble (1926, 1936), Lundmark (1926) and Morgan (1958). On the plates of the time, which were relatively more sensitive to blue light than to red light, details of spiral arms, disks and bulges could be used as classification criteria. In spite of great observational and theoretical progress in extragalactic studies during the past century, modern galaxy classification is still basically tied to the system proposed by Hubble (1936; Figure 1.1), only now the system is applied using digital images rather than photographic plates. The main reason the Hubble system has survived for nearly a century is that the aspects Hubble focused on (degree of central concentration, pitch angle and resolution of spiral arms, amount of distinct structure) correlated with measured properties of galaxies, such as luminosities, colors, stellar populations, HI content and global star formation history. This gave his view an astrophysically relevant edge that has driven much of extragalactic research since his time.

Figure 1.1. The Hubble (1936) “tuning fork” representation of galaxy morphology

This chapter describes the different classes of galaxies within the framework of the “Comprehensive de Vaucouleurs revised Hubble–Sandage” (CVRHS) classification system, a visual system that follows the precepts of de Vaucouleurs (1959), who proposed a personal revision of the Hubble–Sandage (HS) classification (Sandage 1961; Figure 1.2) that he believed provided a better description of galaxies without being too unwieldy. For many galaxies, the CVRHS classification is no more complicated than a de Vaucouleurs (1959) VRHS classification. However, the system has been designed to take into account more details that are of astrophysical interest today and which have become more noticeable and relevant in the era of digital astronomical imaging. These details include lenses, nuclear rings and bars, ansae bars, boxy/peanut bulges, boxy and disky elliptical galaxies, special outer rings and pseudorings, dust lanes and galactic disk warps.

Figure 1.2. The Hubble–Sandage (Sandage 1961) revised “tuning fork” representation of galaxy morphology, including stages of S0 galaxies