Introduction

Introduction

Not long after Edwin Hubble established that galaxies are ‘island universes’ similar to our home galaxy, the Milky Way, he realized that a few of these external galaxies are considerably closer to us than any others. In 1936 he first coined the term ‘Local Group’ in his famous book The Realm of the Nebulae to identify our nearest galactic neighbors. More than 60 years later, the galaxies of the Local Group remain particularly important to astronomers because their proximity allows us to obtain our most detailed views of the properties of normal galaxies beyond our own. These nearby systems also provide our clearest views of how galaxies interact with one another in the relatively small volume of space of the Local Group.

The brightest members of the Local Group are so close to us that on a clear, dark night away from city lights it is possible to see them with the unaided eye: in the southern hemisphere the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) shine brightly, while in the north the Andromeda and Triangulum Galaxy can be seen as faint smudges of light in the sky. These two galaxies are, in fact, the most distant objects visible with the naked eye. From both hemispheres, the gossamer glow of the Milky Way reveals the presence of billions of stars spread throughout the thin disk of our home galaxy. These five galaxies constitute the most luminous and massive members of the Local Group.

Although the existence, if not the true nature, of the five naked-eye Local Group galaxies and the Milky Way has been known to humans since antiquity, the first member identified telescopically was Messier 32 (more typically referred to as M32) by G-J Le Gentil in 1749. Since then, astronomers have steadily identified additional members of the Local Group. By the time Hubble first introduced the concept of the Local Group in 1936, he was able to list 11 galaxies that he considered to be members of the group. At present (2015), 47 galaxies can be cataloged as probable members of the Local Group; these systems are listed on the last tab of this section along with the dates of discovery for each. Remarkably, more Local Group members have been found in the past 40 years than in all previous human history. Also, the era of discovery is almost certainly not over as future surveys uncover more members or as new nearby galaxies are found serendipitously.

Why are the galaxies of the Local Group so difficult to identify? The principal reason is that, apart from our Milky Way Galaxy and the large Andromeda and Triangulum galaxies (known also as M31 and M33, respectively), the known members of the Local Group are Dwarf Galaxies. By definition, these systems have low intrinsic luminosities. They usually also exhibit very low surface brightness (see Low Surface Brightness Galaxies). This property is a measure of how spread out the galaxy’s light is on the sky. In the case of nearly every dwarf galaxy member of the Local Group, the surface brightnesses are lower than that of the night sky, lending them a ghostly appearance and making them very difficult to detect, even at close range (Unlike the apparent brightness of a galaxy, the surface brightness of an extended object does not change as a function of distance—at least for distances up to a few hundred million light-years. This makes low-surface-brightness galaxies difficult to detect anywhere. Consequently, a very large number of lowsurface-brightness galaxies may still remain hidden throughout the universe, enough possibly to fundamentally change our views of the distribution and numbers of galaxies in the universe.)

Much of the recent success in finding new Local Group members is due to the availability of the many large-scale photographic surveys of the sky carried out since the seminal Palomar Sky Survey of the 1950s. Soon after these surveys were begun, visual searches of the photographic plates identified new nearby galaxies. Starting in the 1970s, automated measurements and analyses of the plates from these surveys helped uncover nearly all of the most recently discovered Local Group members. However, even the most complete optical survey cannot find all of the galaxies in the sky. For example, searches for galaxies near the bright band of the Milky Way itself are severely hindered by the high stellar density in this part of the sky and by the clouds of gas and dust within the plane of our Galaxy. This Interstellar Matter effectively blocks all optical light from distant objects, making it impossible to find galaxies lurking in the background. Ongoing and planned surveys in the infrared and radio wavelengths can penetrate the haze of the Milky Way by detecting radiation that is unaffected by dust obscuration. These searches are almost certain to reveal several new Local Group members in coming years.

Another complication in producing a complete census of the Local Group is the uncertainty involved with defining the group’s boundary. The best way to establish this is to determine which local galaxies are gravitationally bound to one another. Since M31 and the Milky Way dominate the mass of all probable Local Group members, this process requires a good estimate of the masses of these two giant galaxies. In addition, we need accurate information on the distances and motions of individual candidate Local Group galaxies to determine whether they are physically bound to the M31–Milky Way system. Table 1 lists the 47 galaxies that appear to be likely members of the Local Group based on this approach.

Although well defined, this method of identifying and counting members of the Local Group is highly uncertain. For example, apart from a few of the nearest galaxies, we cannot measure the Proper Motion—the angular movement across the sky—of external galaxies. A galaxy that may be moving towards or away from us at a moderate speed may be moving very rapidly across our line of sight. Thus, some of the galaxies in the table that we believe are bound to the Local Group may actually only be ‘passing through the neighborhood’. A second problem is that distances to local galaxies are notoriously difficult to determine reliably. Methods that work for the Magellanic Clouds may not be applicable to other nearby galaxies, and vice versa. It is quite common that galaxies once believed to be Local Group members are later removed from the list of members as we refine our estimates of their distances and motions. Indeed, two of the galaxies Hubble first proposed to be Local Group members are now known to be located much further away. Thus we can not only expect additions to the table of Local Group members as new galaxies are discovered but also subtractions from the list as we learn that some of the galaxies are not members after all.

Local Group Global Properties

Global Properties of Local Group Galaxies

The individual galaxies of the Local Group span a large range of basic properties. The luminosities of Local Group galaxies range from a minimum of about 250,000 times the luminosity of the SUN, to a maximum of more than 20 billion times the luminosity of the Sun, a range of a factor of 75,000. It is also clear that the galaxies of the Local Group span a very large range in size even though their extreme dimensions are impossible to determine precisely. The smallest systems in the Local Group are approximately 1000 ly in diameter, while the luminous parts of the giant galaxies M31 and the Milky Way span over 100,000 ly from end to end. This range is comparable with the size range exhibited by mammals, from the tiny bumblebee bat to the blue whale. Figure 1 illustrates the relative sizes of the visible portions of most of the Local Group galaxies.

Figure 1

Virtually every major galaxy type is represented in the Local Group. M31, the Milky Way and M33 are all examples of Spiral Galaxies, but each represents a slightly different subclass of this family of galaxies. M31, for example, exhibits a prominent central bulge and well-defined spiral arms throughout the thin disk of the galaxy; it is classified as an Sb galaxy. M33 has a very weak, possibly non-existent central bulge and very poorly defined spiral arms; it is an Sc galaxy.

Although it is clear that the Milky Way is a spiral galaxy— the thin structure of the Milky Way and the existence of a concentration of stars and Globular Clusters towards the constellation Sagittarius all confirm this classification—the specific spiral subtype is extremely difficult to determine reliably from our unfavorable vantage point inside the Galaxy. Various indirect indicators suggest that our Galaxy can be classified as an Sbc galaxy, intermediate between the properties described for M31 and M33. Some recent studies at radio, infrared and optical wavelengths also suggest that our Galaxy contains an elongated central bar composed of old, metal-rich stars. If true, then the Milky Way is an example of a Barred Spiral Galaxy, and its specific subtype is SBbc, where the upper-case ‘B’ denotes a barred system. All three galaxies are typically considered ‘giant’ spirals, despite the fact that M33 is only about 10% as luminous or massive as M31. By comparison, our Galaxy is about half as luminous and massive as M31.

All the remaining galaxies of the Local Group are dwarf systems of various types. The galaxies that exhibit the most variation in appearance and in their global properties are the DWARF IRREGULAR GALAXIES. The most massive example of this type of galaxy in the Local Group is the LMC, one of the satellites of the Milky Way and the second closest external galaxy to the Sun. The LMC is a massive dwarf, and its global properties place it near the ill-defined boundary separating dwarf irregular galaxies from small spirals. Careful studies reveal features in the LMC normally found in spiral galaxies, such as a central bar-like concentration of older stars, and some evidence of indistinct spiral arms. The LMC’s companion, the SMC, is a true irregular with little sign of large-scale structure. The remaining irregular galaxies of the Local Group are identified in the table. All of these systems are substantially smaller and less luminous than the LMC.

The principal reason that dIrr galaxies appear to have such chaotic structures is that they typically form stars in small clumps called associations embedded within the galaxies. Because young stars are typically very luminous, they are often the most prominent stars seen in optical images of these types of dwarf galaxies. If the star-forming regions are irregularly distributed—and they usually are—they give a strong impression that the entire galaxy has a highly distorted, chaotic overall structure.

This apparent lack of large-scale organization in dIrr galaxies is somewhat of an illusion produced by the luminous young stars that generally make up a minority of the total stellar population of such galaxies. Observations in the red and infrared are most sensitive to the light output of the dominant population of old and middle-aged stars in these galaxies. When astronomers try to determine the distribution of these oldest stars, even the most irregular dwarfs exhibit a much smoother, more symmetric appearance. An analogy is the surface of a pot of slowly boiling soup: although bubbles erupt at different locations at different times, the overall distribution of the soup is relatively uniform. The active star-forming regions of irregulars correspond to the bubbles: very prominent when active, but short lived and all interspersed in a more uniform medium.

With the striking exception of the Magellanic Clouds, the dIrr galaxies of the Local Group tend to be found far from the two large galaxies (M31 and the Milky Way) of the Local Group. A few of the least luminous dwarf irregulars are also among the most metal-poor galaxies known. Because stars produce heavy elements which they then eject back into the interstellar medium at the ends of their lives, the low abundances of such elements in the smaller dwarfs suggest that these galaxies are relatively pristine, unevolved systems. These galaxies may be forming stars in large quantities for the first time in their lives. As such, these galaxies are invaluable ‘living fossils’ that can tell us of the properties of gas and stars in the early universe.


Image of IC 10

One particularly interesting Local Group irregular galaxy is IC 10. This galaxy is forming stars at an unsustainably rapid rate. If it were to continue it would soon exhaust its raw materials for making stars (gas and dust) in only a few million years. The implication is that unless it started out with an astoundingly large reservoir of gas from which to form stars, IC 10 has probably been caught during a particularly active—but short-lived— phase in its star-formation history. Such galaxies that appear to form stars at unsustainably high rates are known as Starburst Galaxies. IC 10 is probably the closest example of a true starburst galaxy, although one active region of star formation in the LMC—the 30 Doradus Region—exhibits similar characteristics on a subgalactic scale.

The remaining dwarfs of the Local Group are ellipsoidal systems. These galaxies are characterized by a roughly circular or elliptical outline on the sky, and by a smooth, centrally concentrated distribution of light. One of these, M32, is considered to be an example of a true Dwarf Elliptical Galaxy. As such, it represents the low-luminosity end of the very large family of elliptical galaxies which includes some of the largest, most luminous and most massive individual galaxies known. M32 is also noteworthy because it appears to harbor a massive Black Hole in its extremely bright nucleus, it may be a unique local example of a galaxy with no ancient stars and, as a companion of M31, it shows distortions that indicate a strong gravitational interaction with its massive parent.

The remaining ellipsoidal Local Group galaxies are known as Dwarf Spheroidal Galaxies (or dSph galaxies). About half of all galaxies in the Local Group are of this type and they are apparently similarly common in other groups and clusters. Consequently, these dim, unassuming galaxies probably represent the most common type of galaxy in the entire universe. Within the Local Group, dSph galaxies are typically found in the company of a larger parent galaxy. For example, of the 13 close companions of the Milky Way, nine are dSph satellites. Virtually all the remaining dSph galaxies of the Local Group are found near M31. The most luminous dSph galaxy, NGC205,


is a highly distorted companion of M31. The two lowest-luminosity galaxies known are dSph companions of the Milky Way: the Draco and Ursa Minor systems. These galaxies emit less light than some individual globular clusters— massive, compact star clusters typically found within extended halos surrounding elliptical, spiral and larger dwarf galaxies. Nevertheless, the large dimensions and large masses (most in the form of matter that we cannot see directly) of galaxies such as Draco and Ursa Minor distinguish them from the more compact clusters.

Five of the dwarf galaxies of the Local Group are difficult to classify as ellipsoidal or irregular systems because they exhibit some of the properties of both. These ‘transition’ galaxies may represent the late stages of star-formation episodes in dwarf irregulars fortuitously caught during the period when the youngest stars begin to fade from prominence. This picture is consistent with the star-formation histories we measure for transition systems and with the growing evidence that dSph and dIrr galaxies have undergone complicated star-formation histories over the entire lifetime of the universe. A few hundred million years after periods of active star formation, a dSph galaxy that may initially have looked like a dIrr system could become a transition galaxy. This intimate relation between dIrr and dSph galaxies in which ‘transition’ systems act as a ‘missing’ evolutionary link remains controversial. Nonetheless, there is little doubt that some dSph galaxies looked a lot like low-luminosity dIrr systems at some point(s) in the past.

Galaxies are not the only inhabitants of intergalactic space within the Local Group. There is growing evidence of the existence of isolated clouds of gas, usually in the form of neutral hydrogen, distributed throughout the group. This material may represent gas expelled from other galaxies or may correspond to primordial matter that has yet to collapse into small stellar systems such as globular clusters or dwarf galaxies. Some remote globular star clusters could plausibly be ‘free-floating’ members of the Local Group that were ejected from their parent galaxies during past interactions of individual galaxies within the group.

One prominent class of normal galaxy is not found within the Local Group: giant elliptical galaxies. This is not entirely surprising; since large ellipticals are relatively rare in the local universe, we would have been ‘lucky’ to find one closeby. Moreover, giant ellipticals tend to be found in regions with a high density of galaxies, a manifestation of the so-called Galaxy Morphology–Density Relation. Because the Local Group is a loose, low-density collection of galaxies, it would have been unusual— although not impossible—for it to contain one or more big elliptical galaxies. The nearest giant elliptical galaxy to us is probably MAFFEI 1. The slightly more distant Centaurus A (NGC 5128) is somewhat easier to study because it is not located so close to the Milky Way in the sky and suffers far less obscuration by interstellar dust in the plane of our Galaxy.

Local Group Motions

The Motions of Local Group Galaxies

The motions of galaxies in the Local Group appear to violate Hubble’s Law that the universe is uniformly expanding. The reason for this has to do with the definition of the Local Group as the collection of nearby galaxies that are gravitationally bound to one another. Just as the planets of the solar system do not appear to be expanding away from us because they are bound to the Sun, the mutual attraction of galaxies within the Local Group has overcome the universal expansion in our immediate neighborhood. The most striking example of this is M31, which is currently approaching our galaxy at nearly 50 km s−1: if on a true collision course, the two galaxies will meet in about 8–10 billion years. More likely, M31 and the Milky Way make up a binary system in which the two galaxies orbit their common center of gravity. Inevitably, tidal effects will cause the two galaxies to merge into one giant system at some time in the distant future. Many of the other galaxies of the Local Group also exhibit motions towards us, signifying that they too are either in orbit about the Milky Way or currently moving along orbits about M31 or other Local Group galaxies that cause them to move towards us at the present time. In all of these cases, the orbital motions are larger than the universal expansion velocities we would expect to measure for such nearby galaxies based on Hubble’s law.

The mutual attraction of M31 and the Milky Way can be used to estimate the mass of the Local Group. Much as a ball thrown in the air first rises, stops and then falls, M31 and the Milky Way are now falling towards each other after their initial movement apart after the Big Bang. If one measures the relative velocities and locations of the two galaxies and one estimates how long it has been since they were together—essentially the age of the universe estimated from the Hubble constant or from the ages of the oldest stars—it is possible to estimate the combined mass of M31 and the Milky Way. This sort of analysis was first carried out by FD Kahn and L Woltjer in 1959. Modern applications of this technique reveal that the combined mass of M31 and the Milky Way is in the range between 500 billion and 2 trillion times the mass of the Sun. Because virtually all the matter of the Local Group is located in these two giant spiral galaxies, this is also our best estimate of the total mass of the group.

As massive as the Local Group is, the total light output of all of the galaxies in the group M31 and the Milky Way is equivalent to ‘only’ 30 billion times the luminosity of the Sun. This is unusual because for normal stars the ratio of their mass (in units of the mass of Sun, or 2 × 1030 kg) and their luminosity (also in units of the luminosity of the Sun, or 4 × 1026 W) is around 1. For the Local Group the ratio is much higher, approximately 50. This suggests that the group is dominated by ‘Dark Matter’ which contributes to the local gravitational field but remains invisible at any wavelength of Electromagnetic Radiation. The large massto-light ratio of the Local Group implies that only 2–5% of the total matter in the group is visible to us through emitted or reflected radiation.

We can also measure the masses of Local Group galaxies individually by determining the range of velocities of individual stars and gas clouds that orbit within individual systems. So long as Newton’s law of gravity remains valid over the physical scales of the galaxies, astronomers find that Local Group galaxies typically exhibit mass-to-light ratios in the range 3–100. Thus, the dark matter within the Local Group is not spread out uniformly throughout the group, but is instead concentrated about the individual galaxies we detect optically. This suggests that possibility that the Local Group may contain some nearby examples of ‘dark’ galaxies consisting of only dark matter with no luminous material. No such systems have been detected yet. The dwarf galaxies of the Local Group do offer one important clue about the nature of dark matter. Certain types of dark matter that have been postulated in the past—in particular Neutrinos cannot account for the surprisingly high mass densities required to account for the total masses of these small galaxies.

Local Group Galaxy Distribution

The Distribution of Local Group Galaxies

In general, each subgroup corresponds to a set of gravitationally bound galaxies. The 13 satellites of the Milky Way, for example, are all likely to be in orbit about our Galaxy while M31 maintains a similar stable of small companion galaxies. This situation is analogous to the moons that orbit individual planets of the Solar System: although Jupiter and Saturn orbit the Sun, both planets possess their own large families of bound satellites. In the case of the Local Group, the subgroups are weakly bound together, and there is evidence that some satellites may be occasionally ‘swapped’ from one subgroup to another. For example, some models of the dynamical evolution of the Local Group suggest that the Magellanic Clouds and perhaps the Leo I dSph galaxy may have first formed near M31, but are now satellites of our Galaxy.

Some Local Group galaxies show unmistakable signs of strong mutual gravitational interactions. As noted above, the Milky Way and M31 have overcome the initial expansion and are now falling in towards one another. The close companions of M31 — NGC 205 and M32 — both show global distortions that may be due to strong tidal effects induced by their close passage to their parent galaxy (figure 3 shows the distortions induced in NGC 205 by its close passage by M31). The Magellanic Clouds reveal considerable evidence that they too have interacted in the past. For example, the Magellanic Stream is a long arc of neutral gas that was probably ejected from one or both of the Clouds as they passed close to one another and passed the Milky Way. Because the SMC is the least massive component of the interacting pair, it has suffered the most. One clear sign of this is the fact that the SMC is elongated significantly along our line of sight, resembling a cigar whose long axis is roughly pointed towards us. This elongation reflects the stretching of the galaxy as it is literally pulled apart by its abusive neighbor. Detailed models of the dynamical evolution of the Magellanic Clouds indicate that they will both fall into the Milky Way in the next few billion years. Although the Clouds will be completely disrupted in the process, even the much-larger Milky Way will be affected as its disk is puffed up by the energy injected by the infalling dwarfs.

Perhaps the most spectacular example of an interaction within the Local Group involves the Sagittarius dSph galaxy. Discovered only in 1994, the Sagittarius Dwarf Galaxy is now known to be the closest galaxy to the Milky Way. Sadly for Sagittarius, this is too close, and the dwarf is being severely torn apart by the gravitational tidal forces exerted on it by our much more massive Galaxy. A clear indication that this process has already begun is the fact that Sagittarius has been stretched along a long arc that currently is known to extend nearly all of the way around the sky. Like the Magellanic Clouds, Sagittarius does not have long to live as a separate galaxy before it too is completely disrupted and merges into the main body of the Milky Way.

Local Group Cosmological Implications

Local Group Cosmological Implications

Although most studies of the origin of the universe focus on radiation coming from extremely distant galaxies, the Local Group also plays a critical role in cosmological studies. For example, with the advent of large ground-based and space-based telescopes and the use of high-sensitivity electronic detectors, it has become possible to measure the complete fossil record of star formation in many of the galaxies of the Local Group. These investigations provide a critical point of comparison with studies aimed at understanding how galaxies formed in the first place by studying objects detectable at the edge of the visible universe. So far, observations of Local Group systems reveal that normal galaxies have formed stars in unexpectedly complex and varied ways. Some nearby dwarf spheroidal galaxies are composed almost entirely of ancient stars formed soon after the big bang, but most of these galaxies formed large numbers of stars over long periods of time extending as recently as the past few hundred million years. The dwarf irregulars continue to form stars today, in some cases perhaps for the first time. Astronomers are just now beginning to try to reconcile these detailed observations of Local Group galaxies with studies of larger numbers of extremely remote galaxies.

The Local Group also offers insights into the question of whether small galaxies have merged over time to form larger systems such as M31, the Milky Way or the Magellanic Clouds. Some of the mergers can be studied today such as the disruption of the Sagittarius dwarf and the interactions of the Magellanic Clouds and our Galaxy described above. However, most mergers that led to the formation of larger galaxies must have occurred long ago when the universe was still quite young; we are only now beginning to understand how to disentangle evidence of these past encounters within our own Galaxy. The merger histories of Local Group galaxies will eventually shed new light on the conditions of the early universe and on the nature of the dark matter that helped drive the formation of galaxies in the first place.

On an even larger scale, the Local Group itself seems to interact significantly with other nearby groups of galaxies. One of these, the Sculptor Group, appears to be strongly elongated along a line projecting back to the Local Group. Because our group is considerably more massive, it is probable that the gravitational force of the Local Group has significantly distorted the Sculptor Group. One result of this interaction is the lack of a clear boundary between the Sculptor Group and our own. Some galaxies traditionally associated with Sculptor are occasionally catalogued with the Local Group and vice versa. Unlike the Local Group, Sculptor lacks any giant galaxies, but does contain a number of lower-luminosity spirals and many dwarf galaxies. On roughly the opposite side of the sky the M81–Maffei Group represents another nearby concentration of galaxies that may have a significant effect on the evolution and internal motions of the Local Group. Unlike the Sculptor Group, the M81–Maffei Group contains some giant galaxies, including the large spiral M81 and the elliptical galaxy Maffei 1. As with the Sculptor Group, some of the galaxies assigned to the M81– Maffei Group have been considered at times to be Local Group members and vice versa. On top of the interactions with neighboring groups, the Local Group is also ‘falling’ into the nearby Virgo Cluster of galaxies. The attempt to understand the details and implications of these local interactions and large scale motions remains a very active field of modern research.