In the Dark

If you were wondering where I got yesterday’s piece from, the answer is that I fired up my old laptop and found it among a lot of old papers there. And by “old laptop”, I mean really old laptop: I bought it in 1995! Anyway, since I haven’t got time to write anything today here is another piece I wrote a long time ago but have only recently unearthed. This one is about Galaxies. It’s a lot longer than yesterday’s effort, but like that one I can’t remember what it was for. Still, some of you might find it interesting. The piece ends with a reference to galaxies observed as they were in the distant past, rather like the article itself!

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A galaxy is a collection of stars, held together by their mutual gravitational attraction and orbiting around their common centre.  Galaxies range in size from dwarf systems of perhaps a few million stars, to giants containing up to a thousand billion. The Sun and all the stars visible in the night sky to the naked eye belong to one such galaxy, our own Galaxy the Milky Way. Although principally recognized through the light given off by their component stars, galaxies also contain other material such as clouds of gas and dust, and significant quantities of dark matter whose nature is not yet understood.

Only stars inside our Galaxy can be resolved with the naked eye; these stars have been studied and catalogued since antiquity. Ancient astronomers  also knew of the existence of a diffuse band of light crossing the sky they could not resolve into individual stars; we now call this the Milky Way. The word galaxy is derived from the Greek galaktos, meaning “milk”. The existence of galaxies other than our own is a much more recent discovery. While even relatively nearby stars appear as point sources of light, the light from other galaxies appears as cloudy and diffuse much like small fragments of the Milky Way. The generic term for a such sources is nebula, the latin word for “mist”.

A Persian astronomer, al-Sufi, in the 10^th century AD described such a faint patch of light in the constellation Andromeda which is now known to be another galaxy, but it was only in the 18^th Century that a systematic catalogue of  nebulae was compiled, by the French astronomer Charles Messier. Not all the objects he found were other galaxies – some were clouds of dust and gas inside our own – but the Messier catalogue contained 32 objects that we now know to be galaxies, including al-Sufi’s object, which was number 31 in his list. The Andromeda nebula is known to this day as M31. With the increasing power of astronomical telescopes, the list of known nebulae grew to thousands even before the use of astronomical photography became widespread. William and Caroline Herschel and, later, their son John played a leading role in identifying and cataloguing such objects in the early 19^th century.

While the existence of large numbers of these nebulae was well established by the start of the 20^th Century, their nature remained controversial. Since their distances could not be directly measured, it was possible that they could be inside our own galaxy. Many astronomers believed that the spiral structure seen in some of them, for example M31, suggested that they represented the formative stages of planetary systems like our own Solar System inside the Milky Way. Others argued that the nebulae were very much more distant than that, and were “island universes” on a much larger scale. This debate was only resolved in the 1920s, when Edwin Hubble was able to measure the distances to some nebulae using variable stars called Cepheids. He found them to be far too distant to be inside the Milky Way. This discovery established galaxies as the basic building-blocks of the Universe and gave rise to the field of extragalactic astronomy. Astronomers now estimate that there are as many galaxies in our observable Universe as there are individual stars in our own galaxy, i.e. around a hundred billion.

Galaxies come in a rich variety of shapes and sizes, but there are three basic types: Galaxies come in three basic types: spiral (or disk), elliptical and irregular. Hubble proposed a morphological classification, or taxonomy, for galaxies in which he envisaged the three basic types (spiral, elliptical and irregular) as forming a sequence which in the past was often assumed to represent various evolutionary stages of a galaxy . Although it is now not thought the interpretation as an evolutionary sequence is correct, Hubble’s nomenclature is still commonly used.

Spiral galaxies account for more than half the galaxies observed  in our neighbourhood.  These contain a bright central nucleus surrounded by a flattened disk that sometimes contains beautiful spiral arms. Hubble divided these galaxies into classes labelled as normal (S) or barred (SB) depending on whether the prominent spiral arms emerge directly the nucleus, or originate at the ends of a luminous bar projecting symmetrically through it . Spirals often contain copious amounts of dust, and the spiral arms containing many young stars givin them a noticeably blue colour.  The normal and barred spirals S and SB are further subdivided into a, b or c depending on how tightly the spiral arms are wound up.

The elliptical galaxies (E), which account for only around 10% of observed bright galaxies, are elliptical in shape and have no discernible spiral structure. They are usually red in colour, have very little dust and show no signs of active star formation. The further classification of elliptical galaxies into En depends on the degree of elongation of the galaxy: E0 is nearly spherical; E7 is cigar-shaped. Ellipticals tend to occur in regions of space where there are many other galaxies, giving rise to the idea that they might originally have been spiral galaxies but have lost their spiral structure through mergers or interactions with other galaxies.

The shapes and colours of elliptical galaxies resemble the corresponding properties of spiral nuclei. Elliptical galaxies cover a broad range in mass, from a few hundred thousand to a thousand billion times the mass of the Sun. Spiral galaxies seem to have a smaller spread in mass, typically weighing in at about a hundred billion times the mass of the Sun.

Lenticular, or S0 galaxies, were added later by Hubble to bridge the gap between normal spirals and ellipticals. Around 20% of galaxies we see have this morphology. They are more elongated than elliptical galaxies but have neither bars nor spiral structure.

Irregular galaxies have no apparent structure. They are relatively rare, and are often faint and small so are consequently very hard to see. Their irregularity may stem from the fact that they are have such small masses that the material within them is relatively loosely bound and may have been disturbed by the environment in which they sit.

The classification of galaxies proposed by Hubble applies to “normal” galaxies whose light output is dominated by radiation their constituent population of stars. Stars predominantly emit visible light, which occupies a relatively narrow part of the spectrum of electromagnetic radiation. Spiral galaxies also contain dust which is heated by starlight and radiates in the infra-red. Active galaxies are characterized by the prodigious amounts of energy they emit in regions of the spectrum normal galaxies cannot reach, particularly in radio and X-rays. Much of the energy broadcast by active galaxies is associated with the relatively small nucleus of the galaxy, so the term Active Galactic Nuclei (AGN) is often used to describe these regions. Sometimes the central nucleus is accompanied by a jet of material being ejected at high velocity into the surrounding intergalactic medium. The different types of active galaxy include Seyfert galaxies, radio galaxies, BL Lac objects, and quasars.

Seyfert galaxies are usually spiral galaxies with no radio emission and no evidence of jets. They do, however, emit radiation over a continuous range of frequencies from infra-red to X-rays. Splitting their optical light up into its characteristic spectrum reveals the presence of strong and variable emission lines.  One can see such lines in ordinary stellar spectra and consequently in the spectra of normal galaxies, but they are much more prominent in active galaxies. Radio galaxies, on the other hand, are more commonly elliptical galaxies. These objects are extremely dramatic in their appearance, frequently having two lobes of  radio-emitting material extending far away from the central compact nucleus. There is also sometimes the appearance of a jet of material, extending from the core into the radio lobes. It appears that material is ejected from the nucleus along the jet, eventually being slowed  down by its interaction with the intergalactic medium and forming the radio lobes. The central parts of radio galaxies seem to have properties similar to those of Seyfert galaxies.

BL Lac objects have spectra with no emission lines, but they emit strongly in all wavebands from radio to X-ray frequencies. Their main characteristic, however, is their extremely strong and rapid variability. It is thought that a possible explanation for these objects is that the observer is seeing a jet of material travelling head-on at close to the velocity of light.

The first quasars to be found were detected by their strong radio emission, but they were found to be so small that, like stars but unlike other galaxies, they could not be resolved with optical telescopes. For this reason they became known as quasi-stellar radio sources, or quasars for short. Later on, other such objects were found which did not emit radio waves at all, so the name was changed to quasi-stellar object or QSO, but the name quasar has in any case stuck. It seems that only one in about two hundred quasars is actually radio-loud, but the quasars are still the most powerful of all the active galaxy types.

These different kinds of objects were discovered at different times by different people and were originally thought to be entirely different phenomena. Now, however, there is a unified model in which these structures are all interpreted as having basically similar structure but a different orientation to the observer’s line-of-sight. The engine which powers the activity is thought to be a supermassive black hole, with a mass up to about 100 million solar masses. This seems very large, but is actually just a small fraction of the mass of the host galaxy, which may be a thousand times larger. Material surrounding the black hole is attracted towards it and undergoes a process of accretion, gradually spiralling in and being swallowed. As it spirals in, it forms an accretion disk around the black hole. This disk can be very hot, producing the X-ray radiation frequently seen in AGN, but its presence prevents radiation being transmitted through it. Radiation tends therefore to be beamed out of the poles of the nucleus and does not appear from the equatorial regions which are obscured by the disk. When the beamed radiation interacts with material inside the host galaxy or in the surrounding medium, it forms jets or radio lobes. Depending on the thickness of the disk, the size of the `host’ galaxy ,the amount of gas and dust surrounding the nucleus and the orientation at which the whole system is viewed one can, at least qualitatively, account for the variety of properties listed above.

It is not known what fraction of normal galaxies undergoes activity at some stage in their careers. Although active galaxies are relatively uncommon in our neighbourhood, this may simply be because the active phase lasts for a very short time compared to the total life of a galaxy. For example, if activity only lasts only one-thousandth of the total lifetime, we would expect only to see one in a thousand galaxies at any one time displaying the symptoms. It is perfectly possible, therefore, that the kind of extreme activity displayed by these galaxies is merely a phase through which all galaxies pass. If so, this would suggest that all galaxies should possess a massive black hole at their centre, which is no longer powering an accretion disk because there is insufficient gas left in the surrounding regions. Recent studies using the ultra-high resolution available on the Hubble Space Telescope suggest that most normal galaxies may indeed have black holes in their centres.

A somewhat milder form of activity is displayed by starburst galaxies which, as their name suggests are galaxies undergoing a vigorous period of star formation. Such activity is not thought to involve an active galactic nucleus, but is probably triggered by a tidal interaction between two galaxies moving closely past each other.

The stars in a galaxy exert gravitational forces on each other. This not only holds the galaxy together, it also causes the stars to move. The internal dynamical properties of galaxies are extremely important because they allow astronomers to work out how much matter is there.

In spiral galaxies, the component stars orbit roughly in a plane about the central nucleus. It is this bulk rotation that is responsible for the flattened shape of these systems. Much the same state of affairs applies in the Solar System, with all the planets moving in roughly circular orbits about the Sun. In the case of a disk galaxy that lies edge-on to the observer, stars on one side will be approaching while those on the other will be receding. These motions cause a Doppler shift in the light from different parts of the disk: one side will have a spectrum that is shifted towards blue colours, while the other side will be shifted to the red. One can therefore use spectroscopic methods to plot a graph showing how the rotation speed of material  varies with distance from the centre of rotation. Such a curve is called a rotation curve.  These curves show that the matter in spiral galaxies has a roughly constant velocity out to tens of thousands of light years from the centre. This is surprising because the planets of the Solar System have orbital speeds that fall off quite rapidly with distance from the Sun. Most of the mass of the Solar System lies in the Sun, which is near the centre of motion. Most of the light produced in a galaxy is likewise produced in the central regions. If all the mass in a galaxy were where the stars are, i.e. in the middle, the rotation speed should fall off the further out from the centre one looked. The simplest interpretation of this strange behaviour is that galaxies contain a large amount of material that does not produce starlight and which is not as concentrated in the centre of the galaxy as the stars. To make this work requires galaxies to be embedded in a diffuse halo of dark matter that is about ten times as large as the luminous part of the disk and containing perhaps ten times as much matter.

Dynamical studies of elliptical galaxies are more complicated because the stellar motions within them are not those of simple rotation. Nevertheless, these objects too reveal evidence for dark matter in similar quantity to that in spiral galaxies.

It is thought that less than 10 per cent of the total mass of a galaxy is in visible stars, but the form of the mysterious dark matter is not at all understood. The best candidate at the moment is some form of exotic particle left over from the Big Bang, usually called a WIMP (Weakly Interacting Massive Particle), although no such particle has yet been directly detected.

Galaxies are the basic building blocks of the Universe. They are not, however, the largest structures one can see. They tend not to be isolated, but cluster together. The distribution of nebulae on the sky was thought to be non-uniform even in the days of the Herschels, but it is only in the 20^th century that it has become possible to map their three-dimensional positions in a systematic fashion.

The technique used to explore the large-scale distribution of galaxies is based on the discovery of the expanding universe usually attributed to Edwin Hubble, who built  on earlier work by Vesto Slipher. Slipher had discovered that lines in the optical spectra of galaxies were systematically shifted towards the longer wavelength, red end of the electromagnetic spectrum. Hubble extended this study by looking at these redshifts in tandem with the distances he had estimated for the galaxies. He found, to his surprise, that the redshift of a galaxy came out to be proportional to its distance. Contrary to popular belief, Hubble never really interpreted this himself as the result of cosmic expansion but the empirical correlation between redshift and distance now known as Hubble’s Law is the cornerstone of the big-bang cosmology. It is now accepted that the redshift of the galaxies arises from their motion away from the observer, similar to the Doppler shift that causes a change of pitch in a receding police siren. While the accurate determination of extragalactic distances remains difficult, measuring redshifts is rather straightforward. Hubble’s law has been used to chart the pattern traced out by millions of individual galaxies from their spectral shifts.

The general term used to describe a physical  aggregation of many galaxies is a cluster of galaxies, or galaxy cluster. Clusters can be systems of greatly varying size and richness. Our galaxy, the Milky Way,  is a member of the Local Group of galaxies which is a rather small cluster of galaxies of which the only other large member is the Andromeda galaxy (M31). On the other extreme, there are the so-called rich clusters of galaxies, also known as Abell clusters, which contain many hundreds or even thousands of galaxies in a region just few million light years across: prominent nearby examples of such entities are the Virgo and Coma clusters. In between these two extremes, galaxies appear to be distributed in systems of varying density.

Individual galaxy clusters are not the largest structures in the Universe. The distribution of galaxies on scales larger than around 30 million light years also reveals a wealth of complexity. Galaxies are not simply distributed in blobs, like the Abell clusters, but often lie in extended linear structures called filaments, such as the Perseus-Pisces chain, or flattened sheet-like structures like the Great Wall. The latter object is roughly two-dimensional concentration of galaxies, discovered in 1988 by astronomers from the Harvard-Smithsonian Center for Astrophysics. This structure is at least 200 million light years by 600 million light years in size, but is less than 20 million light years thick. It contains many thousands of galaxies and has a mass of at least 10¹⁶ solar masses.  The interconnecting network of filaments and sheets is aptly called the “cosmic web”, with rich clusters appearing where the parts of the web join together.

Rich clusters are clustered into enormous loosely-bound agglomerations called superclusters, containing anything from around ten rich clusters to more than 50. The most prominent known supercluster is called the Shapley concentration, while the most nearby is the Local Supercluster, a flattened structure in the plane of which the Local Group is moving. Superclustering is known to exist on scales up to 300 million light years, and superclusters may contain as much as 10¹⁷ solar masses of material or more.

These overdense structures are complemented by vast underdense regions known as voids, many of which appear to be roughly spherical.  These regions containing very many fewer galaxies than average, or even no galaxies at all. Voids with density less than 10% of the average density on scales of up to 200 million light years have been found in large-scale redshift surveys.

The existence of galaxies, clusters of galaxies and the overall complexity of large-scale structure in the Universe around us must be contrasted with the extreme simplicity of the very early Universe. Observations of the cosmic microwave background, relic radiation left over from the early stages of the Big Bang, suggest that the initial state of the Universe was almost featureless, with variations in density from place to place of less than one part in a hundred thousand.

The process that is thought to have transformed these smooth beginnings into the clumpiness we see today is called gravitational instability. If the universe were initially exactly smooth, it would have remained so as it expanded and cooled. But if there were small initial variations in density, these would become amplified. A small patch of the Universe that was more dense than average would exert a slightly greater gravitational pull on its surroundings than an average patch. This would cause material to flow in, making it even denser. This, in turn, would make it pull even more than average. This starts a runaway process by which small initial ripples can turn into dense clumps.

This basic idea has been around since it was first suggested by Sir James Jeans more than a hundred years ago, but it is only in the last ten years or so that a convincing picture has been put together explaining how it works in the expanding Universe. According the modern theories, most of the matter in the Universe is in the form of exotic particles left over from the primordial fireball phase that was the Big Bang. These particles are thought to be very slow-moving and are consequently called Cold Dark Matter (CDM). These particles cluster together via the process of gravitational instability, first forming small objects with the mass of a very small dwarf galaxy (around one hundred thousand solar masses). These small seed objects then progressively merge into larger objects in a hierarchical fashion, eventually forming galaxy-sized and cluster-sized dark matter clumps. These form gravitational wells into which gaseous matter falls and becomes trapped. Stars  form as gas clouds cool and fragment in the dark matter clumps. All this happens within a continuous sequence of interaction, disruption and merging. The whole process is extremely complicated, but extensive computer simulations show that the structure produced is very similar to the cosmic web revealed by observations, at least in the essential details.

Further support for these theoretical ideas is provided by observations of galaxies so distant that it has taken their light a large fraction of the age of the Universe to reach us. Looking at such objects allows astronomers to see galaxies in the process of formation.