My new textbook,
Astronomy and Cosmology,
has appeared in October 2006.
It is a translation of the German
in die extragalaktische Astronomie und Kosmologie
and is slightly expanded and updated.
Below please find an extended table of contents
Sample pages can be found at the Springer Website
The book has been reviewed
by Virginia Trimble.
A review of the German edition can be found here
||Additional online Material|
The Earth is orbiting around the Sun, which itself is orbiting around
the center of the Milky Way. Our Milky Way, the Galaxy, is the only
galaxy in which we are able to study astrophysical processes in detail.
Therefore, our journey through extragalactic astronomy will begin in
our home Galaxy, with which we first need to become familiar before
we are ready to take off into the depths of the Universe. Knowing the
properties of the Milky Way is indispensable for understanding other
The figure below shows the disk of the Milky Way in a large number of
wavebands, taken by a multitude of ground-based and satellite
The Milky Way is the only galaxy which we are able to examine in detail. We can resolve individual stars and analyze them spectroscopically. We can perform detailed studies of the interstellar medium (ISM), such as the properties of molecular clouds and star forming regions. We can quantitatively examine extinction and reddening by dust. Furthermore, we can observe the local dynamics of stars and gas clouds as well as the properties of satellite galaxies (such the Magellanic Clouds). Finally, the Galactic center at a distance of only 8 kpc gives us the unique opportunity to examine the central region of a galaxy at very high resolution. Only through a detailed understanding of our own Galaxy can we hope to understand the properties of other galaxies. Of course, we implicitly assume that the physical processes taking place in other galaxies obey the same laws of physics that apply to us. If this were not the case, we would barely have a chance to understand the physics of other objects in the Universe, let alone the Universe as a whole. We will return to this point shortly.
The Galactic disk rotates, with rotational velocity depending on the distance from the Galactic center. We can estimate the mass of the Galaxy from the distribution of the stellar light and the mean mass-to-light ratio of the stellar population, since gas and dust represent less than 10% of the mass of the stars. From this mass estimate we can predict the rotational velocity as a function of radius simply from Newtonian mechanics. However, the observed rotational velocity of the Sun around the Galactic center is significantly higher than would be expected from the observed mass distribution. From the visible matter in stars we would expect a rotational velocity of 160 km/s but we observe 220 km/s. This, and the shape of the rotation curve for larger distances from the Galactic center, indicates that our Galaxy contains significantly more mass than is visible in the form of stars. This additional mass is called dark matter. Its physical nature is still unknown. The main candidates are weakly interacting elementary particles like those postulated by some elementary particle theories, but they have yet not been detected in the laboratory. Macroscopic objects (i.e., celestial bodies) are also in principle possible candidates if they emit very little light. We will discuss experiments which allow us to identify such macroscopic objects and come to the conclusion that the solution of the dark matter problem probably can not be found in astronomy, but rather most likely in particle physics.
The insight that our Milky Way is just one of many galaxies in the Universe is less than 100 years old, despite the fact that many had already been known for a long time. The catalog by Charles Messier (1730-1817), for instance, lists 103 diffuse objects. Among them M31, the Andromeda galaxy, is listed as the 31st entry in the Messier catalog. Later, this catalogue was extended to 110 objects. John Dreyer (1852-1926) published the New General Catalog (NGC) that contains nearly 8000 objects, most of them galaxies. In 1912, Vesto Slipher found that the spiral nebulae are rotating, using spectroscopic analysis. But the nature of these extended sources, then called nebulae, was still unknown at that time; it was unclear whether they are part of our Milky Way or outside it.
The year 1920 saw a public debate (the Great Debate) between Harlow Shapley and Heber Curtis. Shapley believed that the nebulae are part of our Milky Way, whereas Curtis was convinced that the nebulae must be objects located outside the Galaxy. The arguments which the two opponents brought forward were partly based on assumptions which later turned out to be invalid, as well as on incorrect data. We will not go into the details of their arguments which were partially linked to the assumed size of the Milky Way since, only a few years later, the question of the nature of the nebulae was resolved.
In 1925, Edwin Hubble discovered Cepheids in Andromeda (M31). Using the period-luminosity relation for these pulsating starshe derived a distance of 285 kpc. This value is a factor of about 3 smaller than the distance of M31 known today, but it provided clear evidence that M31, and thus also other spiral nebulae, must be extragalactic. This then immediately implied that they consist of innumerable stars, like our Milky Way. Hubble's results were considered conclusive by his contemporaries and marked the beginning of extragalactic astronomy. It is not coincidental that at this time George Hale began to arrange the funding for an ambitious project. In 1928 he obtained six million dollars for the construction of the 5 meter telescope on Mt. Palomar which was completed in 1949.
This Chapter is about galaxies. We will confine the consideration here to `normal' galaxies in the local Universe; galaxies at large distances, some of which are in a very early evolutionary state, will be discussed in Chap.9, and active galaxies, like quasars for example, will be discussed later in Chap.5.
We will now begin to consider the Universe as a whole. Individual
objects such as galaxies and stars will no longer be the subject of
discussion, but instead we will turn our attention to the space and
time in which these objects are embedded. These considerations will
then lead to a world model, the model of our cosmos.
This chapter will deal with aspects of homogeneous cosmology. As we will see, the Universe can, to first approximation, be considered as being homogeneous. At first sight this fact obviously seems to contradict observations because the world around us is highly inhomogeneous and structured. Thus the assumption of homogeneity is certainly not valid on small scales. But observations are compatible with the assumption that the Universe is homogeneous when averaged over large spatial scales. Aspects of inhomogeneous cosmology, and thus the formation and evolution of structures in the Universe, will be considered later in Chap.7.
Cosmology is a very special science indeed. To be able to appreciate its peculiar role we should recall the typical way of establishing knowledge in natural sciences. It normally starts with the observation of some regular patterns which is then formulated as a physical `law'. Relations become physical laws if the predictions they make are confirmed again and again; the validity of such a law is considered more secure the more diverse the tests have been. However, relations that are found in our cosmos cannot be verified in other universes. Thus it is not possible to consider any property of our Universe as `typical' - we have no statistics on which we could base a statement like this. Despite this special situation, enormous progress has been made in understanding our Universe, as we will describe here and in subsequent chapters.
Cosmological observations are difficult in general, simply because the majority of the Universe (and with it most of the sources it contains) is very far away from us. Distant sources are very dim. This explains why our knowledge of the Universe runs in parallel with the development of large telescopes and sensitive detectors. Much of today's knowledge of the distant Universe became available only with the new generation of optical telescopes of the 8-meter class, as well as new and powerful telescopes in other wavelength regimes.
The most important aspect of cosmological observations is the finite speed of light. We observe a source at distance D in an evolutionary state at which it was a time D/c younger than today. Thus we can observe the current state of the Universe only very locally. Another consequence of this effect, however, is of even greater importance: due to the finite speed of light, it is possible to look back into the past. At a distance of 10 billion light years we observe galaxies in an evolutionary state when the Universe had only a third of its current age. Although we cannot observe the past of our own Milky Way, we can study that of other galaxies. If we are able to identify among them the ones that will form objects similar to our Galaxy in the course of cosmic evolution, we will be able to learn a great deal about the typical evolutionary history of such spirals.
The fact that our astronomical observations are restricted to sources which are located on our backward light cone implies that our possibilities to observe the Universe are fundamentally limited. If somewhere in spacetime there would be a highly unusual event, we will not be able to observe it unless it happens to lie on our backward light cone. Only if the Universe has an essentially `simple' structure we will be able to understand it, by combining astronomical observations with theoretical modeling. Luckily, our Universe seems to be basically simple in this sense.
The light of normal galaxies in the optical and near infrared part of the spectrum is dominated by stars, with small contributions by gas and dust. This is thermal radiation since the emitting plasma in stellar atmospheres is basically in thermodynamical equilibrium. To a first approximation, the spectral properties of a star can be described by a Planck spectrum whose temperature depends on the stellar mass and the evolutionary state of the star. As we have seen before, the spectrum of galaxies can be described quite well as a superposition of stellar spectra. The temperature of stars varies over a relatively narrow range. Only few stars are found with Temperatures larger than 40000 K, and those with temperatures below 3000 K hardly contribute to the spectrum of a galaxy, due to their low luminosity. Therefore, as a rough approximation, the light distribution of a galaxy can be described by a superposition of Planck spectra from a temperature range that covers about one decade. Since the Planck spectrum has a very narrow energy distribution around its maximum the spectrum of a galaxy is basically confined to a range between 4000 and 20000 Angstroms. If the galaxy is actively forming stars, young hot stars extend this frequency range to higher frequency, and the thermal radiation from dust, heated by these new-born stars, extends the emission to the far-infrared.
However, there are galaxies which have a much broader energy distribution. Some of these show significant emission in the full range from radio wavelengths to the X-ray and even Gamma range. This emission originates mainly from a very small central region of such an active galaxy which is called the active galactic nucleus (AGN). Active galaxies form a family of many different types of AGN which differ in their spectral properties, their luminosities and their ratio of nuclear luminosity to that of the stellar light.
Some classes of AGNs, in particular the quasars, belong to the most luminous sources in the Universe, and they have been observed out to the highest measured redshifts beyond z=6. The luminosity of quasars can exceed the luminosity of normal galaxies by a factor of a thousand. This luminosity originates from a very small region in space, well below 1 parsec. The optical/UV spectra of quasars are dominated by numerous strong and very broad emission lines, some of them emitted by highly ionized atoms. The processes in AGNs are among the most energetic in astrophysics. The enormous bandwidth of AGN spectra suggests that the radiation is nonthermal. As we will discuss, AGNs host processes which produce highly energetic particles and which are the origin of the nonthermal radiation.
After an introduction in which we will briefly present the history of the discovery of AGNs and their basic properties,we will describe the most important subgroups of the AGN family. Then we discuss several arguments which lead to the conclusion that the energy source of an AGN originates in accretion of matter onto a supermassive black hole (SMBH). In particular, we will learn about the phenomenon of superluminal motion, where apparent velocities of source components are larger than the speed of light. We will then consider the different components of an AGN where radiation in different wavelength regions is produced.
Of particular importance for understanding the phenomenon of active galaxies are the unified models of AGNs that will be discussed next. We will see that the seemingly quite different appearances of AGNs can all be explained by geometric or projection effects. Finally, we will consider AGNs as cosmological probes. Due to their enormous luminosity they are observable up to very high redshifts. These observations allow us to draw conclusions about the properties of the early Universe.
Galaxies are not uniformly distributed in space, but instead show a
tendency to gather together in galaxy groups and clusters of galaxies.
This effect can be clearly recognized in the projection of bright
galaxies on the sky. The Milky Way itself is a member of a group,
called the Local Group, which implies that we are living in a locally
overdense region of the Universe.
The transition between groups and clusters of galaxies is smooth. The distinction is made by the number of their member galaxies. Roughly speaking, an accumulation of galaxies is called a group if it consists of less than 50 bright members within a sphere of diameter 1.5 Mpc. Clusters have more than 50 members and diameters larger than about 2 Mpc.
Clusters of galaxies are the most massive gravitationally bound structures in the Universe. Typical values for the mass are 10^15 solar masses for massive clusters, whereas for groups, 3x10^13 solar masses is characteristic, with the total mass range of groups and clusters extending over a large range of values.
Originally, clusters of galaxies were characterized as such by the observed spatial concentration of galaxies. Today we know that, although the galaxies determine the optical appearance of a cluster, the mass contained in galaxies contributes only a small fraction to the total mass of a cluster. Through advances in X-ray astronomy, it was discovered that galaxy clusters are intense sources of X-ray radiation which is emitted by a hot gas located between the galaxies. This intergalactic gas (intracluster medium, ICM) contains more baryons than the stars seen in the member galaxies. From the dynamics of galaxies, from the properties of the X-ray emission of the clusters, and from the gravitational lens effect we deduce the existence of dark matter in galaxy clusters, dominating the cluster mass like it does for galaxies.
Clusters of galaxies play a very important role in observational cosmology. They are the most massive bound and relaxed (i.e., in a state of approximate dynamical equilibrium) structures in the Universe, as mentioned before, and therefore mark the most prominent density peaks of the large-scale structure in the Universe. Their cosmological evolution is therefore directly related to the growth of cosmic structures. Due to their high galaxy density, clusters and groups are also ideal laboratories for studying interactions between galaxies and their effect on the galaxy population. For instance, the fact that elliptical galaxies are preferentially found in clusters indicates the impact of the local galaxy density on the morphology and evolution of galaxies.
In Chap.4, we discussed homogeneous world models and
introduced the standard model of cosmology. It is based on the
cosmological principle, the assumption of a (spatially) homogeneous
and isotropic Universe. Of course, the assumption of homogeneity is
justified only on large scales because observations show us that our
Universe is inhomogeneous on small scales - otherwise no galaxies or
stars would exist.
The distribution of galaxies on the sky is not uniform or random, rather they form clusters and groups of galaxies. Also clusters of galaxies are not distributed uniformly, but their positions are correlated, grouped together in superclusters. The three-dimensional distribution of galaxies, obtained from redshift surveys, shows an interesting large-scale structure.
Even larger structures have been discovered. The Great Wall is a galaxy structure with an extent of 100 Mpc, which was found in a redshift survey of galaxies. Such surveys also led to the discovery of the so-called voids, nearly spherical regions which contain virtually no (bright) galaxies, and which have a diameter of typically 50 Mpc. The discovery of these large-scale inhomogeneities raises the question of whether even larger structures might exist in the Universe, or more precisely: does a scale exist, averaged over which the Universe appears homogeneous? The existence of such a scale is a requirement for the homogeneous world models to provide a realistic description of the mean behavior of the Universe.
To date, no evidence of structures with linear dimension larger than about 100 Mpc have been found. Hence, the Universe seems to be basically homogeneous if averaged over scales of about 200 Mpc. This `homogeneity scale' needs to be compared to the Hubble radius, which is about 3000 Mpc., so that after averaging, about 3000 independent volume elements exist per Hubble volume. This justifies the approximation of a homogeneous world model when considering the mean history of the Universe.
On small scales, the Universe is inhomogeneous. Evidence for this is the galaxy distribution projected on the sky, the three-dimensional galaxy distribution determined by redshift surveys, and the existence of clusters of galaxies, superclusters, `Great Walls', and voids. In addition, the anisotropy of the cosmic microwave background (CMB), with relative fluctuations of 1 part in 100000, indicates that the Universe already contained small inhomogeneities at redshift of about 1000. In this chapter, we will examine the evolution of such density inhomogeneities and their description.
We are now
ready to discuss the determination of the various cosmological
parameters. In the course of this discussion, we will describe a
number of methods, each of which is in itself useful for estimating
cosmological parameters, and we will present the corresponding results
from these methods. The most important aspect of this chapter is that
we now have more than one independent estimate for each cosmological
parameter, so that the determination of these parameters is highly
redundant. This very aspect is considerably more important than the
precise values of the parameters themselves, because it provides a
test for the consistency of the cosmological model.
We will give an example in order to make this point clear. We discussed how the cosmic baryon density can be determined from primordial nucleosynthesis and the observed ratio of deuterium to hydrogen in the Universe. Thus, this determination is based on the correctness of our picture of the thermal history of the early Universe, and on the validity of the laws of nuclear physics shortly after the Big Bang. As we will see later, the baryon density can also be derived from the angular fluctuations in the cosmic background radiation, for which the structure formation in a CDM model, discussed in the previous chapter, is needed as a foundation. If our standard model of cosmology was inconsistent, there would be no reason for these two values of the baryon density to agree - as they do in a remarkable way. Therefore, in addition to obtaining a more precise value of the baryon density from this comparison than from each of the individual methods alone, the agreement is also a strong indication of the validity of the standard model.
We will begin with the observation of the large-scale distribution of matter, the large-scale structure (LSS). It is impossible to observe the large-scale structure of the matter distribution itself; rather, only the spatial distribution of visible galaxies can be measured. Assuming that the galaxy distribution follows, at least approximately (which we will specify later), that of the dark matter, the power spectrum of the density fluctuations can be estimated from that of the galaxies. As we discussed in the previous chapter, the power spectrum in turn depends on the cosmological parameters. Then we will summarize some aspects of clusters of galaxies which are relevant for the determination of the cosmological parameters.
Type Ia supernovae will be used as cosmological tools, and we will discuss their Hubble diagram. Since SNIa are considered to be standard candles, their Hubble diagram provides information on the density parameters . These observations provided the first clear indication, around 1998, that the cosmological constant differs from zero. We will then analyze the lensing effect of the LSS , by means of which information about the statistical properties of the LSS of matter is obtained directly, without the necessity for any assumptions on the relation between matter and galaxies. As a matter of fact, this galaxy-mass relation can be directly inferred from the lens effect. We will then turn to the properties of the intergalactic medium and, in particular, we will introduce the Lyman-alpha forest in QSO spectra as a cosmological probe.
Finally, we will discuss the anisotropy of the cosmic microwave background. Through observations of the cosmic microwave background and their analysis, a vast amount of very accurate information about the cosmological parameters are obtained. In particular, we will report on the recent and exciting results concerning CMB anisotropies, and will combine these findings with the results obtained by other methods. This combination yields a set of parameters for the cosmological model which is able to describe nearly all observations of cosmological relevance in a self-consistent manner, and which today defines the standard model of cosmology.
The Hubble Deep Field (North) was at the time it was taken (end of 1995) the deepest optical image of the sky and triggered a enormous amount of work on the high-redshift Universe
several decades cosmologists have considered the determination of the
density parameter and the expansion rate of the Universe their prime
task, and now this goal has seemingly largely been achieved. However,
from this point on, the future evolution of the field of cosmology
will probably proceed in two directions. First, we will try to
uncover the nature of dark energy and to gain new insights into
fundamental physics along the way. Second, astrophysical cosmology is
much more than the mere determination of a few parameters. We want to
understand how the Universe evolved from a very primitive initial
state into what we are observing around us today - galaxies of
different morphologies, the large-scale structure of their
distribution, clusters of galaxies, and active galaxies. We seek to
study the formation of stars and of metals, and also the processes
that reionized the intergalactic medium.
The boundary conditions for studying these processes are now very well defined. A few years ago, the cosmological parameters in models of galaxy evolution, for instance, could vary freely because they had not been determined sufficiently well at that time. Today, a successful model needs to come up with predictions compatible with observations, but using the parameters of the standard model. There is little freedom left in designing such models. In other words, the stage on which the formation and evolution of objects and structure takes place is prepared, and now the cosmic play can begin.
Progress in recent years, with developments in instrumentation having played a vital role, has allowed us to examine the Universe at very high redshift. An obvious indication of this progress is the increasingly high maximum redshift of sources that can be observed. Besides larger telescopes, which enabled these deep images of the Universe, gaining access to new wavelength domains is of particular importance for our studies of the distant Universe. This can be seen, for example, from the fact that the optical radiation of a source at redshift 1 is shifted into the NIR. Because of this, near-infrared astronomy is about as important for galaxies at redshift 1 as optical astronomy is for the local Universe. Furthermore, the development of submillimeter astronomy has provided us with a view of sources that are nearly completely hidden to the optical eye because of strong dust absorption.
In this chapter, we will attempt to provide an impression of astronomy of the distant Universe, and shed light on some interesting aspects that are of particular importance for our understanding of the evolution of the Universe. This field of research is currently developing very rapidly, so we will simply address some of the main topics in this field today.