DOUBLE and MULTIPLE GALAXIES

Adapted from The Astronomy and Astophysics Encyclopedia and P. Hodge and J.A. Graham

1.GALAXIES, LOCAL GROUP

Our Milky Way galaxy is a member of a small group of galaxies that forms a modest density enhancement in the universe of galaxies. This group, which includes about 25 galaxies, is called the Local Group and is similar to many other loose clusters of galaxies in nearby extragalactic space. Its importance comes from the fact that all of its members are near enough to us to resolve well into their individual stellar and interstellar components, and thus we can study them in great detail. This fact allows the Local Group to be the testing ground for many of our ideas, for example, about the distance scale, stellar populations, and galaxy evolution.

• PROPERTIES OF THE GROUP

Number of Members

If the Local Group were to be observed from a distant galaxy, it would seem to include only seven or so members, because there are only about that many that would be conspicuous from such a vantage point. However, there are many small, faint members, and a total census would have to include at least 25 galaxies. Table 1 lists them and gives certain of their vital statistics.

The true number of members remains unknown, and there are three reasons for this. First, there are parts of the sky, especially those areas obscured by the Milky Way dust, that have not yet been searched for members. We know that no large spiral galaxy member lies hidden, because we could detect such a galaxy by its radio emission (especially its neutral hydrogen emission), even if its optical image were completely absorbed by Milky Way dust. However, an elliptical galaxy, perhaps one of low luminosity like the Sculptor dwarf, would not be easy to find because it would emit virtually no radio radiation. Considering the size of the area obscured by the Milky Way, we would expect no more than one or two hidden elliptical galaxies that might have been missed by searches thus far.

The second reason that the list may be incomplete is that there might be objects, like the extremely inconspicuous Ursa Minor dwarf, that are simply too faint and sparse to have been found. The most distant dwarf elliptical galaxies are And I, II, and III, which might not have been found easily if their discoverer, Sidney van den Bergh, had not been specifically searching the Andromeda area for them. Other such galaxies, in other parts of the sky and perhaps even a little farther away, might still await discovery. If these types of galaxies are approximately uniformly spaced in the Local Group, there could be as many as 50-100 of these objects within its boundaries. However, it is believed that there is a higher than average density of them near our galaxy, because of its large mass, and that there may be only a few undiscovered examples in the more distant parts of the group.

The third reason that the number is uncertain is the fact that the "boundaries" of the group are not clearly defined. There are several small galaxies, mostly irregular galaxies, that lie at distances of about 1 Mpc (3 million light years), and it is not always clear whether they are members of the group or merely field galaxies.

Types

Within the Local Group there are examples of all three main types of galaxies: spirals, ellipticals, and irregulars. The three spirals (the Milky Way, the Andromeda galaxy, and M33) are the most luminous galaxies of the group. The Magellanic Clouds and other irregular galaxies are also fairly bright. There are no giant, luminous elliptical galaxies, however, even though such galaxies are often conspicuous members of more populous clusters of galaxies. The elliptical members include some of intermediate brightness, like M32, and eight extremely faint dwarf ellipticals.

Size

The Local Group is very small, when compared to the famous galaxy clusters like those in Virgo and Coma, which span hundreds of millions of light years. From our perspective, the group has a diameter of approximately 3 million light years; that is, the most distant certain members are about that distance from the Milky Way galaxy. We are not yet sure, however, just where to draw the boundaries of the group, especially because the distances to some of the more distant dwarf irregular galaxies are not yet reliably known.

Dynamics

The best way to decide on membership in the Local Group has been to measure the velocities of all nearby galaxies and then to see which seem to be moving together in space. If a certain galaxy has a velocity that is very different from the rest, then it is probably an interloper. Our fellow members of the group do not seem to be participating in the Hubble flow (the general expansion of the universe), because they are all gravitationally held together, at least loosely, in the group. Various tests of the stability of the group have been made over the years, with somewhat uncertain results. It appears likely from these studies that the group is a fairly stable dynamical entity, but that it is held together principally by its interstellar dark matter. Most of the visible mass of the group is contained in just two members: the Milky Way and the Andromeda galaxy, and these are falling towards each other at a velocity of 300 km s^-1. The group is probably not collapsing, but rather these two galaxies are probably in highly elongated orbits around the group center of mass, which is somewhere between them. The velocities of all members are probably balanced by the distribution of dark matter in the group, about which we know very little.

Member of the Virgo Cloud

The Local Group is not a simple, isolated clump of galaxies, but is one of a large number of groups that belong at least peripherally, to a giant complex of galaxies called the Virgo cloud. The center of the Virgo cloud is the Virgo cluster, a large, massive, irregular cluster of many hundreds of galaxies. Studies of the velocities of expansion of the universe in our neighborhood show that the Local Group is falling toward the Virgo cloud with a velocity of a few hundred kilometers per second, which is probably caused by the large gravitational field of the Virgo galaxies. We are thus gravitationally attached to Virgo as one of its outlying members.

• DESCRIPTION OF THE MEMBERS

Spiral Galaxies

The Milky Way galaxy and the Andromeda galaxy (frequently referred to as M31) are the most luminous and massive members of the group. M31 is a Hubble Sb type, with a luminous, large bulge of older stars, surrounded by a less-luminous disk of gas, dust, and younger stars, arranged in spiral arms. Its diameter is about 200,000 ly and its mass is approximately 700 billion times that of the sun. Among the many objects that have been studied within it are about 300 globular clusters, 400 open clusters, numerous dust clouds, gas clouds, stellar associations, supernovae remnants, and other components.

The Milky Way galaxy is of Hubble type Sb/Sc, with a somewhat less-conspicuous central bulge and a brighter disk and looser arms than M31. We do not know its total luminosity, because we cannot see it from a distant perspective and because so much of its visible light is obscured by the dust in the disk in which we are enveloped, but indirect evidence suggests that it may be roughly twice as bright as M31. The total mass is approximately 500 billion suns, though this is uncertain because of the unknown distribution of dark matter in our galaxy.

M33 is a Hubble type Sc galaxy; it is smaller and fainter than the other two spirals, being only about as bright visually as a few billion suns. It contains many blue luminous stars in its complex, thick spiral arms, and has several spectacular glowing gas clouds (giant HII regions).

Elliptical Galaxies

There are only four moderately bright elliptical galaxies in the Local Group and all four are companions to the Andromeda galaxy. Two very close companions are M32 and NGC 205, both of which are seen super- imposed on the outer parts of M31. M32 is a nearly circular galaxy with a population of exclusively very old stars, whereas NGC 205 is more elongated in shape and contains a small but remarkable population of young stars, with accompanying dust and gas. The other two companions, NGC 147 and NGC 185, are somewhat fainter and more distant from M31.

All other known elliptical members of the Local Group are classed as dwarf ellipticals; they are very low in density, faint in luminosity, and small in size, with typical brightnesses being about a million suns and typical diameters being only about 10 thousand light years. These objects contain primarily old stars, though some show evidence of star formation that took place not too long ago (only 7 or 8 billion years ago, compared to the oldest stars, with ages of 15 billion years).

Irregular Galaxies

The two best studied irregular galaxies in the Local Group are the two Magellanic Clouds, galaxies that are so bright that they were well known to the early explorers and were named after the great navigator Ferdinand Magellan. They are in the southern skies and can only be observed well from below the equator. The biggest of the two, the Large Magellanic Cloud, is only about 150 thousand light years away (some astronomers cite a distance of 160,000 or even 170,000 ly) and the Small Magellanic Cloud is not much farther. They can be studied in great detail because of this proximity. We have information about nearly every kind of star and interstellar object in these two galaxies and they have played an important role in helping our understanding of such fundamental issues as stellar evolution and the extragalactic distance scale. The Large Cloud has a truly giant HII region as one of its most conspicuous features, a huge complex of massive stars, stars being formed, gas, and dust, called 30 Doradus, popularly known as the Tarantula nebula. In 1987 a bright supernova explored near it, causing a sensation among astronomers by being the first nearby supernova in over 300 years. This star, Supernova 1987a, was observed in optical and radio wavelengths, as well as in x-rays, and even its neutrinos were detected, making it a veritable bonanza for our understanding of the supernova phenomenon.

2. MAGELLANIC CLOUDS

The Magellanic Clouds are the nearest of the external galaxies. Each is an independently evolving star system, actively forming stars at the present time but also containing some which are as old, about 15 billion years, as any that we know. Their importance is many-fold but two aspects especially stand out. First, they act as a mirror to our own Milky Way galaxy and provide a guide as to how it would appear if we could view it from a vantage point high above its dusty disk. Second, we can make use of them to tell us about other galaxies far too remote for any sort of detailed study. The Magellanic Clouds are fundamentally important for the calibration of extragalactic distance indicators. They represent one of the few opportunities we have to intercompare rare objects like the most luminous blue supergiant stars, variable stars, star clusters, and HII regions directly with common stars similar to the Sun, all at the same distance and all comparatively unobscured by interstellar dust. With firm calibrations in hand, we can then confidently proceed to more distant systems where only the very brightest objects may be identifiable.

• POPULATIONS

Even with a small telescope trained on one or other of the two Magellanic Clouds, there is an immediate sense of looking into the heart of a galaxy. It is the young population which is immediately the most striking. These are the massive stars which expend energy so profusely that their nuclear fuel is used up after only a few million years. They tend to clump in close groups of associations and often illuminate the surrounding gas to form bright HII regions. These are the classic markers of Population I as defined by Walter Baade in the middle of this century. The older Population II is much less conspicuous, contributing a faint substratum of stars which have long ago arranged themselves in extended, rather uniform distributions.

Population I and Population II are very much extreme categories and we have learned, after Baade's fundamental work, that there is a continuous transition between them. Stars and clusters of all ages covering a wide range of chemical composition are found in both the Magellanic Clouds and the Galaxy. As well as discussing the two extreme groups, it is appropriate to give special mention to this intermediate population as it is comparatively so prominent in the Magellanic Clouds.

Population I

Among the representatives of Population I, the brightest stars provide a unique opportunity to study the evolution of massive stars and the upper limit to the mass that a star can have and remain stable. Stars like this are sufficiently rare and widespread that this is an impossible job to do within the Milky Way. For most of this century, it was thought that a mass of 70-80 times that of the Sun was the maximum that a star could have and remain vibrationally stable. In the last decade, largely through Magellanic Cloud research, it has been shown that stellar winds dampen incipient instabilities very effectively and that stellar masses 100-200 times that of the Sun are not only possible but probable.

All massive stars end as supernovae and we were incredibly privileged in 1987 to witness in the Large Magellanic Cloud the brightest supernova since the invention of the telescope nearly 400 years ago. Among many other things, this event founded a whole new science of extrasolar neutrino astronomy and provided indisputable observational evidence that nucleosynthesis actually occurs inside stars. SN 1987A was formerly a normal, undistinguished blue supergiant star in one of the rich Population I regions of the Large Magellanic Cloud.

Also included among Population I are the Cepheid variable stars. Cepheids have become one of the standard distance indicators for galactic and extragalactic research through their period-luminosity relation and its validity from galaxy to galaxy. Fortunately, encouraging progress is being made in removing this uncertainty by observing many of the Cepheids in the Magellanic Clouds. With new instrumentation, the accuracy of the brightness and color measurements is being refined and observations in the infrared are proving especially useful.

Neutral and molecular hydrogen gas has a close association with Population I. 21-cm radio surveys have shown that each Magellanic Cloud has an abundant supply remaining for future generations of stars. Molecular hydrogen is harder to detect as we rely mainly on measurements at mm wavelengths of carbon monoxide, a tracer molecule. Both are present in regions where luminous stars are now forming in the Magellanic Clouds.

• POPULATION II

When we come to study the older populations of the Magellanic Clouds, we look past the brilliant associations with their blue supergiants and HII regions, past the Cepheid variables, and the numerous open star clusters until we see in each Cloud only the faint amorphous background which is made up of stars and planetary nebulae a billion years or more old. Relieving the general uniformity, old globular star clusters similar in appearance to those of our own galaxy are seen but, except for the occasional nova, every star in this old population is faint. Yet it is this component which forms the structural backbone of each Cloud, accounts for most of the mass and thereby determines the internal dynamics. Of the oldest objects, those which tell us that the Magellanic Clouds have existed for as long as our galaxy, the short period RR Lyrae stars are perhaps the easiest Population II objects to discover. Most information about the chemical composition of Population II comes from observing red giants in the oldest globular clusters. As in our galaxy, there is a good correlation between age and heavy element abundance although, in neither Cloud does the heavy element enrichment reach the level that is found in the youngest galactic stars.

Study of the old populations is important for investigating the origin of the Magellanic Clouds and the differences between them and the Galaxy at the earliest epochs. The distribution of faint red stars on photographs taken with wide field Schmidt telescopes is a guide to the mass distribution within each body. Magellanic Cloud novae occur two or three times a year and, as the list lengthens, these too are giving us a better idea about where most of the matter in each Cloud is located.

• INTERMEDIATE POPULATIONS

Both Magellanic Clouds have been found to have a major component of intermediate age which spans the two extremes of Population I and II. It is much more developed than the analogous age group in the Milky Way. One example is the large number of rich populous star clusters. In our own galaxy, such clusters are invariably old with ages in excess of 10 billion years. In the Magellanic Clouds, similarly aged clusters exist but they are outnumbered by a strong representation of populous clusters in the 1-10 billion year age range. Partly through their dynamical history, but more directly through their star forming history, the Magellanic Clouds have been able to create and to maintain massive clusters like this at all times. In the Milky Way, populous, globular clusters were only made at the earliest epoch. Similar intermediate age clusters either never formed or have long since been destroyed.

The distribution of the more numerous open clusters that appear at all times in all three systems gives us some hints. Recent work comparing age distributions of complete samples show that the Cloud clusters can survive to much longer lifetimes as they are evidently not subject to disrupting forces as strong as those that exist in the Galaxy.

Independent of such effects, we find that even in the general field, the representation of intermediate age stars is proportionally much larger than in the Milky Way. Apparently bursts of star formation have been occurring throughout the lifetime of each Magellanic Cloud which are much greater than anything we find in the Milky Way. It is from the study of intermediate populations more than from any other that we can observe the long-term effects of differing star-forming histories and apply the knowledge gained to more distant stellar systems which cannot be resolved into individual stars.

• DYNAMICS

A fundamental difference between the Magellanic Clouds and our own galaxy is that neither Cloud has a strong central concentration of stars which can maintain dynamical order in the rest of the system. Both Magellanic Clouds are very vulnerable, not only to gravitational interaction with the Galaxy but also to gravitational interaction with each other.

The Large Magellanic Cloud rotates in a fairly regular manner. The best measurements come from young supergiants and from planetary nebulae as well as from neutral hydrogen. To reconcile the observed rotation (line of nodes) with the distribution (major axis orientation) of the old populations, a transverse motion of the Large Cloud amounting to about 300 km s^-1 is required. This is sufficiently large to be directly measurable now from the proper motions of Cloud stars with reference to background galaxies. However, detailed interpretation of the rotation curve is hampered by the mutual gravitational interactions mentioned above.

Observation constrains the two clouds to form a bound system which is currently orbiting our galaxy. At their last approach , approximately 200 million years ago, substantial damage was done to the Small Magellanic Cloud which is still apparent in its very disordered structure today. A long stream of gaseous material was drawn out of the Small Cloud by the interaction which is observed in neutral hydrogen radiation and has been called the Magellanic Stream. It has no associated stellar component. This close approach and others which have taken place at earlier epochs may have given rise to the bursts of star formation in both clouds which we now observe as the intermediate age population. The calculation for this intercloud orbit is remarkably explicit and it tells us that there have been several such close encounters in the last 10 billion years. However, there is still some doubt as to whether the orbit of the Small Cloud around the Galaxy is bound or unbound. The fact that the Magellanic Clouds have survived for the last 10 billion years indicates that there can have been no approaches much closer than the one we are experiencing now.

• SOLUTION

As independently evolving galaxies, the Large and the Small Magellanic Clouds are being gradually enriched with heavy elements. A finite step in this direction is taking place before our eyes as we observe the remnant of the 1987 supernova dispersing into interstellar space. Dynamically, the Small Cloud may be breaking up as a result of its last encounter with the Large Magellanic Cloud. Never held together very tightly, it is now strung out mostly along the line of sight over a distance of about 20 kpc. This is of the same order as the current distance between it and the Large Cloud. The Large Magellanic Cloud remains stable as shown by its well-defined rotation curve. Up until the present, the evolution of both Clouds has differed greatly from that of our galaxy. As low-mass galaxies from the beginning, neither underwent a major rapid collapse with a concurrent burst of star formation. This is shown by the lack of a strong central concentration of RR Lyrae stars, novae, and planetary nebulae to the degree that we see in the Galaxy. Instead star formation, and consequently heavy element enrichment, has proceeded at a much more gradual pace, with star bursts irregularly occurring every 10 million years or so wherever and whenever there is enough raw material. These are punctuated by system-wide star forming events whenever the two Clouds approach closely to each other. Through the work done over the past decade, the history of the Magellanic Clouds has become a lot clearer and we have been able to see how they relate to more massive star systems like our own galaxy.

 Compiled by G.T.Petrov, 2004