87 3. The World of Galaxies The insight that our Milky Way is just one of many gal- which the two opponents brought forward were partly axies in the Universe is less than 100 years old, despite based on assumptions which later turned out to be in- the fact that many had already been known for a long valid, as well as on incorrect data. We will not go into the time. The catalog by Charles Messier (1730–1817), for details of their arguments which were partially linked instance, lists 103 diffuse objects. Among them M31, to the assumed size of the Milky Way since, only a few the Andromeda galaxy, is listed as the 31st entry in years later, the question of the nature of the nebulae was the Messier catalog. Later, this catalog was extended resolved. to 110 objects. John Dreyer (1852–1926) published In 1925, Edwin Hubble discovered Cepheids in An- the New General Catalog (NGC) that contains nearly dromeda (M31). Using the period-luminosity relation 8000 objects, most of them galaxies. In 1912, Vesto for these pulsating stars (see Sect. 2.2.7) he derived Slipher found that the spiral nebulae are rotating, using a distance of 285 kpc. This value is a factor of ∼ 3 spectroscopic analysis. But the nature of these extended smaller than the distance of M31 known today, but it sources, then called nebulae, was still unknown at that provided clear evidence that M31, and thus also other time; it was unclear whether they are part of our Milky spiral nebulae, must be extragalactic. This then imme- Way or outside it. diately implied that they consist of innumerable stars, The year 1920 saw a public debate (the Great Debate) like our Milky Way. Hubble’s results were considered between Harlow Shapley and Heber Curtis. Shapley conclusive by his contemporaries and marked the begin- believed that the nebulae are part of our Milky Way, ning of extragalactic astronomy. It is not coincidental whereas Curtis was convinced that the nebulae must that at this time George Hale began to arrange the fund- be objects located outside the Galaxy. The arguments ing for an ambitious project. In 1928 he obtained six Fig. 3.1. Galaxies occur in differ- ent shapes and sizes, and often they are grouped together in groups or clusters. This cluster, ACO 3341, at aredshiftofz = 0.037, contains nu- merous galaxies of different types and luminosities Peter Schneider, The World of Galaxies. In: Peter Schneider, Extragalactic Astronomy and Cosmology. pp. 87–140 (2006) DOI: 10.1007/11614371_3 © Springer-Verlag Berlin Heidelberg 2006 3. The World of Galaxies 88 million dollars for the construction of the 5-m telescope structure. They are subdivided according to their on Mt. Palomar which was completed in 1949. ellipticity ≡ 1 − b/a, where a and b denote the This chapter is about galaxies. We will confine the semimajor and the semiminor axes, respectively. El- consideration here to “normal” galaxies in the local lipticals are found over a relatively broad range in Universe; galaxies at large distances, some of which are ellipticity, 0 ≤ 0.7. The notation En is commonly in a very early evolutionary state, will be discussed in used to classify the ellipticals with respect to , Chap. 9, and active galaxies, like quasars for example, with n = 10; i.e., an E4 galaxy has an axis ratio will be discussed later in Chap. 5. of b/a = 0.6, and E0’s have circular isophotes. • Spiral galaxies consist of a disk with spiral arm struc- ture and a central bulge. They are divided into two 3.1 Classification subclasses: normal spirals (S’s) and barred spirals (SB’s). In each of these subclasses, a sequence is de- The classification of objects depends on the type of ob- fined that is ordered according to the brightness ratio servation according to which this classification is made. of bulge and disk, and that is denoted by a, ab, b, This is also the case for galaxies. Historically, optical bc, c, cd, d. Objects along this sequence are often re- photometry was the method used to observe galaxies. ferred to as being either an early-type or a late-type; Thus, the morphological classification defined by Hub- hence, an Sa galaxy is an early-type spiral, and an ble is still the best-known today. Besides morphological SBc galaxy is a late-type barred spiral. We stress ex- criteria, color indices, spectroscopic parameters (based plicitly that this nomenclature is not a statement of on emission or absorption lines), the broad-band spec- the evolutionary stage of the objects but is merely tral distribution (galaxies with/without radio- and/or a nomenclature of purely historical origin. X-ray emission), as well as other features may also be • Irregular galaxies (Irr’s) are galaxies with only weak used. (Irr I) or no (Irr II) regular structure. The classi- fication of Irr’s is often refined. In particular, the 3.1.1 Morphological Classification: sequence of spirals is extended to the classes Sdm, The Hubble Sequence Sm, Im, and Ir (m stands for Magellanic; the Large Magellanic Cloud is of type SBm). • S0 galaxies are a transition between ellipticals and Figure 3.2 shows the classification scheme defined by spirals. They are also called lenticulars as they are Hubble. According to this, three main types of galaxies lentil-shaped galaxies which are likewise subdivided exist: into S0 and SB0, depending on whether or not they • Elliptical galaxies (E’s) are galaxies that have nearly show a bar. They contain a bulge and a large en- elliptical isophotes1 without any clearly defined veloping region of relatively unstructured brightness which often appears like a disk without spiral arms. 1Isophotes are contours along which the surface brightness of a sources is constant. If the light profile of a galaxy is elliptical, Ellipticals and S0 galaxies are referred to as early- then its isophotes are ellipses. type galaxies, spirals as late-type galaxies. As before, Fig. 3.2. Hubble’s “tuning fork” for galaxy classification 3.1 Classification 89 these names are only historical and are not meant to frequency interval that is located in the optical and NIR describe an evolutionary track! sections of the spectrum. In addition to these, other galaxies exist whose spec- Obviously, the morphological classification is at least tral distribution cannot be described by a superposition partially affected by projection effects. If, for instance, of stellar spectra. One example is the class of active the spatial shape of an elliptical galaxy is a triaxial galaxies which generate a significant fraction of their ellipsoid, then the observed ellipticity will depend on luminosity from gravitational energy that is released in its orientation with respect to the line-of-sight. Also, the infall of matter onto a supermassive black hole, as it will be difficult to identify a bar in a spiral that is was mentioned in Sect. 1.2.4. The activity of such ob- observed from its side (“edge-on”). jects can be recognized in various ways. For example, Besides the aforementioned main types of galaxy some of them are very luminous in the radio and/or morphologies, others exist which do not fit into the in the X-ray portion of the spectrum (see Fig. 3.3), or Hubble scheme. Many of these are presumably caused they show strong emission lines with a width of several by interaction between galaxies (see below). Further- thousand km/s if the line width is interpreted as due more, we observe galaxies with radiation characteristics to Doppler broadening, i.e., Δλ/λ = Δv/c.Inmany that differ significantly from the spectral behavior of cases, by far the largest fraction of luminosity is pro- “normal” galaxies. These galaxies will be discussed duced in a very small central region: the active galactic next. nucleus (AGN) that gave this class of galaxies its name. In quasars, the central luminosity can be of the order of 13 ∼ 10 L, about a thousand times as luminous as the to- 3.1.2 Other Types of Galaxies tal luminosity of our Milky Way. We will discuss active The light from “normal” galaxies is emitted mainly by galaxies, their phenomena, and their physical properties stars. Therefore, the spectral distribution of the radiation in detail in Chap. 5. from such galaxies is in principle a superposition of Another type of galaxy also has spectral properties the spectra of their stellar population. The spectrum of that differ significantly from those of “normal” galaxies, stars is, to a first approximation, described by a Planck namely the starburst galaxies. Normal spiral galaxies function (see Appendix A) that depends only on the like our Milky Way form new stars at a star-formation star’s surface temperature. A typical stellar population rate of ∼ 3M/yr which can be derived, for instance, covers a temperature range from a few thousand Kelvin from the Balmer lines of hydrogen generated in the up to a few tens of thousand Kelvin. Since the Planck HII regions around young, hot stars. By contrast, el- function has a well-localized maximum and from there liptical galaxies show only marginal star formation or steeply declines to both sides, most of the energy of none at all. However, there are galaxies which have such “normal” galaxies is emitted in a relatively narrow a much higher star-formation rate, reaching values of Fig. 3.3. The spectrum of a quasar (3C273) in comparison to that of an elliptical gal- axy. While the radiation from the elliptical is concentrated in a narrow range span- ning less than two decades in frequency, the emission from the quasar is observed over the full range of the electromagnetic spectrum, and the energy per logarithmic frequency interval is roughly constant.