Alain Blanchard

The Universe on a Large Scale

To know the distribution of matter in the universe has been one of astronomers’ constant quests. It is pursued by studying more and more distant galaxies. The idea of a remote universe had been held since the middle of the eighteenth century. In the nineteenth century Herschel and Dreyer catalogued the many bright nebulae and noted how they were grouped. Recognition of the extragalactic realm of nature, mainly due to Shapley and Hubble in the years 1920–1930 (by the identification of Cepheids in Andromeda), like the discovery of the universal recession of galaxies by Hubble, opened the way to the development of extragalactic astronomy and to what would become observational cosmology. On extragalactic scales the galaxies, owing to the contrast in density that they represent (the mean density of our Galaxy within 10 kpc is approximately 2 × 10−24 g cm−3, while the mean density of the universe lies (probably) between 10−29 and 10−31g CM−3), are the most immediately discernible entities. Until recently they were thought to provide a reliable indicator of matter on a large scale, and their study led to the idea of a globally homogeneous and isotropic universe.

The Absorption-Line Systems of Quasars

Three years after the discovery of quasars (1963) the presence of narrow absorption lines in the spectrum of one of them, 3C 191 (with a redshift z e = 1.955) was observed for the first time. It is remarkable that 9 clearly significant lines associated with the same redshift z a = 1.9470 could be unambiguously identified. In Figure 10.1 the spectrum of another quasar, 1209+107, obtained with a photon-counting detector, is shown: in addition to the 3 emission lines (Lyα, N Vλ1240, and Si IVλ1393), 19 absorption lines were detected.

The Formation of Galaxies and Large Structures in the Universe

The formation of galaxies and large structures in the universe remains an unresolved problem. The starting point for current scenarios based on the idea of ‘gravitational instability’ is, however, simple. In a medium of uniform density a local density excess (overdensity) of matter will attract nearby matter by the effect of its own gravitation and this effect will accelerate. This type of model, which takes up the ideas of Jeans on the evolution of inhomogeneities of a static gravitational medium, predicts a very rapid (exponential) increase of such irregularities. In fact the application of it to a homogeneous, expanding universe of density ρ with some primordial irregularities ρ pett = ρ + δρ (where δρ/ρ« 1) shows that the increase is not as rapid. There is in effect competition between the growth of the perturbations, characterized by a gravitational collapse time (t eff ∝ (Gρpenrt)−1/2), and the expansion (t exp ∝ (Gα) −1/2), which tends to dilute all local overdensities. The result is that the growth of fluctuations in an expanding universe is slow. For baryons this increase only begins at recombination (z rec ≈ 1000) and in the linear phase the density contrast only grows by a factor (1 + z rec) ≈ 103. Now to obtain an excess δ = δρ/ρ = (ρ — ρ)/ρ 1 corresponding to existing objects (galaxies, clusters, and so on), an initial inhomogeneity δi of the order of 10−3 must be assumed. However, there is at least one observational constraint on the initial amplitude δi. It is given by observation of the 2.7 K cosmic background radiation. This radiation is homogeneous to better than 10−5 (δT/T ≤ 10−5), and as density fluctuations lead to temperature fluctuations (δT/T ≈ δρ/ρ), it follows that at the epoch of recombination the matter must have the same degree of inhomogeneity. This is the main difficulty that confronts models of galaxy formation. The appearance of grand unification theories (GUTs) and inflationary models (Chapter 13) partly resolves these problems by predicting in particular the existence of nonbaryonic particles (‘-inos’) of various masses. On the one hand there is the hidden contribution to the density parameter Ω 0 (predicted equal to 1 by inflationary theories), and on the other there is the source of fluctuations leading to large structures in the real universe. The growth of these fluctuations can begin before recombination; it is thus possible to postulate a δi compatible with observations of the cosmic microwave background. However, even in this setting, many difficulties still persist, but we shall nonetheless try to give a review of this rapidly evolving field.

The Classification and Morphology of Galaxies

The discovery of galaxies as such goes back to 1924. As a result of observations he made at the 2.5 m telescope at Mount Wilson, Edwin Hubble demonstrated definitively that certain nebulae do not form part of our Galaxy but are ‘island universes’, independent conglomerations of stars, gas, and dust. The historical confusion between the nebulae of ionized gas in the Milky Way and the external galaxies derives from the use of general catalogues, such as Messier’s (1794), which contains 39 galaxies out of 109 nebular objects or clusters of Galactic stars, and especially the New General Catalogue (NGC)(Dreyer 1890), which contains 7840 objects, of which 3200 are galaxies, and the Index Catalogue (IC)(1895–1910), which contains 5836 objects, of which 2400 are galaxies. The Harvard catalogue (Shapley and Ames 1932) contains only the brightest galaxies, with apparent magnitudes m < 13 (1249 objects in total). The apparent magnitude is equal to −2.5 log (luminosity). The eye can only perceive stars up to a magnitude m = 6. Up to an apparent magnitude m = 17.5 there are 500 000 galaxies, and up to m = 23, 109. The first step in getting to know the galaxies better is to describe the various types and classifications. We shall see throughout the book how important the classifications are, the different types of galaxy corresponding to different formation mechanisms and different environments.

Quasars and Other Active Nuclei

Radio astronomy was responsible for the discovery of the first quasars. As long ago as 1960 several radio sources in the 3C catalogue had been noticed on account of their remarkably small angular size and were therefore particularly suitable for a search for an associated optical object. For 3C 48, optical exposures of the corresponding field indicated an object with a stellar appearance; its spectrum showed very strong emission lines that at first could not be identified. In 1962 Hazard, Mackey, and Shimmins, using the telescope at Parkes, succeeded in locating the source 3C 273 with great precision (better than 1″), thanks to a lunar occultation. Analysis of the light-curve profile at the beginning and end of the occultation showed, moreover, the existence of two components, A and B, separated by 20″; the second, 3C 273B, coincides exactly with a stellarlike object (m V ≈ 13) whose spectrum also turned out to have very strong emission lines. Schmidt discovered that these were in fact hydrogen lines that have been redshifted by an amount z = Δλ/λ0 = 0.158. Hence it was realized that if 3C 273 obeys the Hubble law, it is at an extremely large distance and has an enormous intrinsic luminosity (of the order of 1047 ergs s−1).

The Kinematics and Masses of Galaxies

In the 1920s, several years before the spiral nebulae were even identified as entirely separate galaxies, it was discovered that they rotated. This discovery came about as a result of the inclination of absorption lines in the spectra of the central regions of, in particular, the galaxies M81 and M104. Up to the 1970s, rotation curves, or in other words the law describing how the rotation velocity V varies as a function of the distance to the galactic centre, were obtained solely at optical frequencies from the absorption lines of stars in the central regions and from the emission lines of H II regions in the outer regions. Subsequently interferometric radio observations at 21 cm (the HI line) with the Westerbork telescopes (in the 1970s) and the VLA (in the 1980s) enabled a large number of rotation curves to be quickly determined, at the same time with greater spectral resolution and an unequalled radial extension, the gaseous HI component generally exceeding the optical limit in the outer regions of galaxies. This confirmed in a striking way that the rotation curves remain flat at a large distance from the centre: does an enormous quantity of invisible mass then exist beyond the visible limits of a spiral galaxy?

Interactions between Galaxies

Gravitational interactions between galaxies are far more common than one might think, assuming that the space density of galaxies is uniform. The galaxies in effect group themselves in clusters, small groups, or pairs: far from being isolated systems they are formed and evolve in interaction with their environment and in particular with neighbouring galaxies.

The Galactic Interstellar Medium

Galaxies are formed mainly of stars, but these stars are immersed in a relatively diffuse and cold gaseous medium. Its density is on average of the order of 1 particle per cm3, 10 cm−3 in clouds of atomic hydrogen, and 1000 cm−3 in molecular clouds. Its temperature goes from 5 K in these latter regions up to 104 K in ionized regions heated by young stars. Hydrogen forms the bulk of the interstellar gas, helium around 25%. Other elements are present in trace amounts. The interstellar gas is enriched by heavy elements ejected by stars (in supernova explosions and stellar winds).

The Spiral Structure of Galaxies

About two-thirds of all galaxies are spiral galaxies, and a large number of them, more than two-thirds, have a regular spiral structure with two arms that can be followed continuously from the centre of the galaxy (the central bulge) to the extremities of the disc. This structure has for a long time posed a serious theoretical problem concerning its origin and persistence in galaxies. The density-wave theory and the amplification mechanism of these waves provide a beautiful solution to the problem in the majority of cases. Before going into the details of this theory (in Section 5.2), we must first tackle the problem of the gravitational stability of a galactic disc and define the main characteristics of the orbits of stars in a rotating disc (the theory of epicycles).