Galaxies. Группа авторов

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Galaxies - Группа авторов


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galaxies existed. Hubble recognized such galaxies as elliptical galaxies, where the luminosity distribution is defined by a regular decline from a bright center to the faint outer regions. Ellipticals were thought to be characterized by no other features but their isophotal shapes, which ranged from round (E0) to a flattening approaching that of disk-shaped galaxies (E7). In fact, Hubble believed the sequence of E galaxy shapes blended smoothly into the domain of disk-shaped galaxies, which he split into two parallel sequences of normal and barred spirals characterized by the degree of central concentration, the degree of resolution of the spiral arms into what were likely to be star-forming regions and the degree of openness of the spiral arms. Hubble (1936) illustrated these features in his famous “tuning fork” of galaxy morphologies (Figure 1.1). Although this view is now obsolete, the tuning fork is still an effective way of binning galaxies into astrophysically meaningful classes.

      In stellar astronomy, temporal terms are often used to describe certain kinds of stars. Stars of spectral classes O, B and A tend to be young and are known as “early-type” stars, while those of spectral classes K and M tend to be older and are called “late-type” stars. Those of spectral classes F and G are known as “intermediate-type” stars. Hubble found it convenient to use similar terms for galaxies, calling galaxies on the left part of the tuning fork early-type galaxies and those on the right part late-type galaxies. In general, E and S0 galaxies are said to be early-type galaxies, S0/a and Sa galaxies are called early-type spirals, Sc-Sm galaxies are called late-type spirals and Sab-Sbc galaxies are called intermediate-type spirals. Unlike for stars, no real temporal meaning was to be implied by these terms.

      The value of the Hubble classification system, as well as its limitations, can only be appreciated by examining large numbers of galaxies and attempting to classify them within that system. The small number of astronomers who actually did this, such as Sandage, de Vaucouleurs, van den Bergh and Morgan, all saw the need for modifications or even alternative views, which led to major revisions of the system. This article uses images from the EFIGI project (Baillard et al. 2011; de Lapparent et al. 2011) to illustrate galaxies of different types within the framework of the CVRHS classification system (Buta et al. 2015). The classifications are from Buta (2019).

      de Vaucouleurs (1963) also introduced the underline notation for stages, as in SA(s)ab, which means “closer to Sb than to Sa”. Although this notation can be applied directly as for families and inner varieties, it has appeared mainly in classifications based on averages of multiphase (or repeat) efforts designed to check for consistency (e.g. Buta et al. 2015). This usage of underlines for stages, families and varieties considerably increases the number of “cells” in the CVRHS as compared to the VRHS, and one may well question whether some of these cells might not have any entrants. Buta (2019) showed that for the EFIGI sample, the cells of the CVRHS system are roughly evenly occupied among late S0s and early-type spirals, but not among later-type spirals. This means the classification volume lacks the symmetry depicted in Figure 1.3.

      In the following sections, the different types of galaxies are described in more detail.

      For more than two decades, elliptical galaxies were the subject of the most intense extragalactic research, spawning many meetings and research papers that underscored how deceptive the simple appearance of these galaxies is. When studied in detail both photometrically and spectroscopically, it becomes clear that the history of ellipticals is likely very complex. The main issues examined in detail are as follows: isophote shapes and orientations; characteristics of luminosity profiles; the presence of dust, rings, and peculiar structures; how low-luminosity ellipticals connect to high luminosity ellipticals; and the characteristic scaling relations between measured quantities such as physical size, surface brightness and central velocity dispersion.


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