Local Group Encyclopedia of Astronomy & Astrophysics

Home , Dwarf elliptical galaxy, Local Group, Maffei 1, Messier 32, Sculptor Group, Triangulum Galaxy

eaa.iop.org DOI: 10.1888/0333750888/1667

Local Group Mario L Mateo

From Encyclopedia of Astronomy & Astrophysics P. Murdin

ISBN: 0333750888

Institute of Physics Publishing Bristol and Philadelphia

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Terms and Conditions Local Group E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS

Local Group of the night sky, lending them a ghostly appearance and making them very difficult to detect, even at close range1. Not long after EDWIN HUBBLE established that galaxies are Much of the recent success in finding new Local ‘island universes’ similar to our home galaxy, the MILKY Group members is due to the availability of the many large- WAY, he realized that a few of these external galaxies are scale photographic surveys of the sky carried out since the considerably closer to us than any others. In 1936 he seminal Palomar Sky Survey of the 1950s. Soon after these first coined the term ‘Local Group’ in his famous book surveys were begun, visual searches of the photographic The Realm of the Nebulae to identify our nearest galactic plates identified new nearby galaxies. Starting in the neighbors. More than 60 yr later, the galaxies of the 1970s, automated measurements and analyses of the plates Local Group remain particularly important to astronomers from these surveys helped uncover nearly all of the most because their proximity allows us to obtain our most recently discovered Local Group members. However, detailed views of the properties of normal galaxies beyond even the most complete optical survey cannot find all of our own. These nearby systems also provide our clearest the galaxies in the sky. For example, searches for galaxies near the bright band of the Milky Way itself are severely views of how galaxies interact with one another in the hindered by the high stellar density in this part of the relatively small volume of space of the Local Group. sky and by the clouds of gas and dust within the plane The brightest members of the Local Group are so close of our Galaxy. This INTERSTELLAR MATTER effectively blocks to us that on a clear, dark night away from city lights it is all optical light from distant objects, making it impossible possible to see them with the unaided eye: in the southern to find galaxies lurking in the background. Ongoing and hemisphere the LARGE MAGELLANIC CLOUD (LMC) and SMALL planned surveys in the infrared and radio wavelengths can MAGELLANIC CLOUD (SMC) shine brightly, while in the north penetrate the haze of the Milky Way by detecting radiation the ANDROMEDA and TRIANGULUM GALAXY can be seen as faint that is unaffected by dust obscuration. These searches are smudges of light in the sky. These two galaxies are, in fact, almost certain to reveal several new Local Group members the most distant objects visible with the naked eye. From in coming years. both hemispheres, the gossamer glow of the Milky Way Another complication in producing a complete census reveals the presence of billions of stars spread throughout of the Local Group is the uncertainty involved with the thin disk of our home galaxy. These five galaxies defining the group’s boundary. The best way to establish constitute the most luminous and massive members of the this is to determine which local galaxies are gravitationally Local Group. bound to one another. Since M31 and the Milky Way Although the existence, if not the true nature, of dominate the mass of all probable Local Group members, the five naked-eye Local Group galaxies and the Milky this process requires a good estimate of the masses of Way has been known to humans since antiquity, the first these two giant galaxies (see below). In addition, we member identified telescopically was Messier 32 (more need accurate information on the distances and motions typically referred to as M32) by G-J Le Gentil in 1749. of individual candidate Local Group galaxies to determine Since then, astronomers have steadily identified additional whether they are physically bound to the M31–Milky Way members of the Local Group. By the time Hubble first system. Table 1 lists the 43 galaxies that appear to be likely introduced the concept of the Local Group in 1936, he was members of the Local Group based on this approach. able to list 11 galaxies that he considered to be members of Although well defined, this method of identifying and the group. At present (1999), 43 galaxies can be catalogued counting members of the Local Group is highly uncertain. as probable members of the Local Group; these systems are For example, apart from a few of the nearest galaxies, we listed in table 1 along with the dates of discovery for each. cannot measure the PROPER MOTION—the angular movement Remarkably, more Local Group members have been found across the sky—of external galaxies. A galaxy that may in the past 30 yr than in all previous human history. Also, be moving towards or away from us at a moderate speed the era of discovery is almost certainly not over as future may be moving very rapidly across our line of sight. Thus, surveys uncover more members or as new nearby galaxies some of the galaxies in the table that we believe are bound are found serendipitously. to the Local Group may actually only be ‘passing through the neighborhood’. A second problem is that distances Why are the galaxies of the Local Group so difficult to to local galaxies are notoriously difficult to determine identify? The principal reason is that, apart from our Milky reliably. Methods that work for the Magellanic Clouds Way Galaxy and the large Andromeda and Triangulum may not be applicable to other nearby galaxies, and vice galaxies (known also as M31 and M33, respectively), the known members of the Local Group are DWARF GALAXIES.By 1 Unlike the apparent brightness of a galaxy, the surface definition, these systems have low intrinsic luminosities. brightness of an extended object does not change as a function They usually also exhibit very low surface brightness of distance—at least for distances up to a few hundred million light-years. This makes low-surface-brightness galaxies difficult (see LOW SURFACE BRIGHTNESSES GALAXIES). This property is a to detect anywhere. Consequently, a very large number of low- measure of how spread out the galaxy’s light is on the sky. surface-brightness galaxies may still remain hidden throughout In the case of nearly every dwarf galaxy member of the the universe, enough possibly to fundamentally change our views Local Group, the surface brightnesses are lower than that of the distribution and numbers of galaxies in the universe.

Table 1. Galaxies of the Local Group.

Year of RA Declination Distance Luminosity Mass Galaxy Other name discovery (2000) (2000) Type Subgroup (Mly) VT (106L) (106M)

◦ M31 NGC 224 – 00h42.7m +41 16 SbI–II M31 2.5 3.4 25 000 700 000 ◦ Milky Way – 17h45.7m −29 01 Sbc MW 0.03 – 8 300 350 000 ◦ M33 NGC 598 – 01h33.9m +30 40 ScII–III M31 2.7 5.9 3 000 30 000 ◦ LMC – 05h23.6m −69 45 IrrIII–IV MW 0.16 0.4 2 100 20 000 ◦ SMC NGC 292 – 00h52.7m −72 50 IrrIV–V MW 0.19 2.0 580 1 000 ◦ WLM DDO 221 1923 00h02.0m −15 28 IrrIV–V LGC 3.0 10.4 500 150 ◦ M32 NGC 221 1749 00h42.7m +40 52 E2 M31 2.6 8.1 380 2 120 ◦ NGC 205 M110 1864 00h40.4m +41 41 E5p/dSph–N M31 2.6 8.1 370 740 ◦ NGC 3109 DDO 236 1864 10h03.1m −26 10 IrrIV–V N3109 4.1 9.9 160 6 550 ◦ IC 10 UGC 192 1895 00h20.4m +59 18 dIrr M31 2.7 11.6 160 1 580 ◦ NGC 185 UGC 396 1864 00h39.0m +48 20 dSph/dE3p M31 2.0 9.1 130 130 ◦ NGC 147 DDO 3 1864 00h33.2m +48 31 dSph/dE5 M31 2.3 9.4 130 110 ◦ NGC 6822 DDO 209 1864 19h44.9m −14 48 IrrIV–V LGC 1.6 9.1 94 1 640 ◦ IC 5152 1895 22h02.7m −51 18 dIrr LGC 5.2 11.2 70 400 ◦ IC 1613 DDO 8 1906 01h04.9m +02 08 IrrV M31 2.3 9.6 64 795 ◦ Sextans A DDO 75 1942 10h11.1m −04 43 dIrr N3109 4.7 11.3 56 395 ◦ Sextans B DDO 70 1955 10h00.0m +05 20 dIrr N3109 4.4 11.4 41 885 ◦ Sagittarius 1994 18h55.1m −30 29 dSph-N MW 0.08 4.0 18 – ◦ Fornax 1938 02h40.0m −34 27 dSph MW 0.45 7.6 16 68 ◦ Pegasus DDO 216 1958 23h28.6m +14 45 dIrr/dSph LGC 3.1 12.0 12 58 ◦ EGB 0427+63 UGCA 92 1984 04h32.0m +63 36 dIrr M31 4.2 13.9 9.1 – ◦ SagDIG UKS1927-177 1977 19h30.0m −17 41 dIrr LGC 3.4 13.5 6.9 9.6 ◦ And VII Cassiopeia 1998 23h26.5m +50 42 dSph M31 2.5 15.2 5.7 – ◦ UKS2323-326 UGCA 438 1978 23h26.5m −32 23 dIrr LGC 4.3 13.8 5.3 – ◦ Leo I DDO 74 1955 10h08.5m +12 19 dSph MW 0.81 10.1 4.8 22 ◦ And I 1972 00h45.7m +38 00 dSph M31 2.6 12.8 4.7 – ◦ GR 8 DDO 155 1956 12h58.7m +14 13 dIrr GR8 4.9 14.4 3.4 7.6 ◦ Leo A DDO 69 1942 09h59.4m +30 45 dIrr MW 2.2 12.8 3.0 11 ◦ And II 1972 01h16.5m +33 26 dSph M31 1.7 12.7 2.4 – ◦ Sculptor 1938 01h00.2m −33 43 dSph MW 0.26 8.5 2.2 6.4 ◦ Antlia 1985 10h04.1m −27 20 dIrr/dSph N3109 4.1 14.8 1.7 12 ◦ And VI Peg dSph 1998 23h51.7m +24 36 dSph M31 2.7 14.1 1.4 – ◦ LGS 3 Pisces 1978 01h03.9m +21 53 dIrr/dSph M31 2.6 14.3 1.3 13 ◦ And III 1972 00h35.3m +36 31 dSph M31 2.5 14.2 1.1 – ◦ And V 1998 01h10.3m +47 38 dSph M31 2.6 15.0 1.0 – ◦ Phoenix 1976 01h51.1m −44 27 dIrr/dSph MW 1.4 13.2 0.9 33 ◦ DDO 210 Aquarius 1959 20h46.8m −12 51 dIrr/dSph LGC 2.6 14.7 0.8 5.4 ◦ Tucana 1985 22h41.8m −64 25 dSph LGC 2.9 15.2 0.6 – ◦ Leo II DDO 93 1950 11h13.5m +22 09 dSph MW 0.66 12.0 0.6 9.7 ◦ Sextans 1990 10h13.1m −01 37 dSph MW 0.28 10.3 0.5 19 ◦ Carina 1977 06h41.6m −50 58 dSph MW 0.33 10.9 0.4 13 ◦ Ursa Minor DDO 199 1955 15h09.2m +67 13 dSph MW 0.21 10.3 0.3 23 ◦ Draco DDO 208 1955 17h20.3m +57 55 dSph MW 0.27 10.9 0.3 22

versa. It is quite common that galaxies once believed to be Group span a very large range in size even though their Local Group members are later removed from the list of extreme dimensions are impossible to determine precisely. members as we refine our estimates of their distances and The smallest systems in the Local Group are approximately motions. Indeed, two of the galaxies Hubble first proposed 1000 ly in diameter, while the luminous parts of the giant to be Local Group members are now known to be located galaxies M31 and the Milky Way span over 100 000 ly from much further away. Thus we can not only expect additions end to end. This range is comparable with the size range to the table of Local Group members as new galaxies are exhibited by mammals, from the tiny bumblebee bat to discovered but also subtractions from the list as we learn the blue whale. Figure 1 illustrates the relative sizes of the that some of the galaxies are not members after all. visible portions of most of the Local Group galaxies. Virtually every major galaxy type is represented in Global properties of the galaxies of the Local the Local Group. M31, the Milky Way and M33 are all Group examples of SPIRAL GALAXIES, but each represents a slightly The individual galaxies of the Local Group span a large different subclass of this family of galaxies. M31, for range of basic properties. The luminosities of Local Group example, exhibits a prominent central bulge and well- galaxies range from a minimum of about 250 000 times defined spiral arms throughout the thin disk of the galaxy; the luminosity of the SUN, to a maximum of more than it is classified as an Sb galaxy (see GALAXIES: CLASSIFICATION). 20 billion times the luminosity of the Sun, a range of a M33 has a very weak, possibly non-existent central bulge factor of 75 000. It is also clear that the galaxies of the Local and very poorly defined spiral arms; it is an Sc galaxy.

Figure 1. A family portrait of many of the members of the Local Group. The galaxies are shown to scale and to roughly the correct range of relative brightness. The Milky Way is shown as it may appear to a viewer located outside the galaxy. This montage was produced by Bruno Binggeli of the University of Basel.

Although it is clear that the Milky Way is a spiral galaxy— a central bar-like concentration of older stars, and some the thin structure of the Milky Way and the existence of a evidence of indistinct spiral arms. The LMC’s companion, concentration of stars and GLOBULAR CLUSTERS towards the the SMC, is a true irregular with little sign of large-scale constellation Sagittarius all confirm this classification—the structure. The remaining irregular galaxies of the Local specific spiral subtype is extremely difficult to determine Group are identified in the table. All of these systems are reliably from our unfavorable vantage point inside the substantially smaller and less luminous than the LMC. Galaxy. Various indirect indicators suggest that our The principal reason that dIrr galaxies appear to Galaxy can be classified as an Sbc galaxy, intermediate have such chaotic structures is that they typically form between the properties described for M31 and M33. Some stars in small clumps called associations embedded within recent studies at radio, infrared and optical wavelengths the galaxies. Because young stars are typically very also suggest that our Galaxy contains an elongated central luminous, they are often the most prominent stars seen bar composed of old, metal-rich stars. If true, then the in optical images of these types of dwarf galaxies. If the Milky Way is an example of a BARRED SPIRAL GALAXY, and its star-forming regions are irregularly distributed—and they specific subtype is SBbc, where the upper-case ‘B’ denotes usually are—they give a strong impression that the entire a barred system. All three galaxies are typically considered galaxy has a highly distorted, chaotic overall structure. ‘giant’ spirals, despite the fact that M33 is only about Figure 2 illustrates the appearance of a dwarf irregular 10% as luminous or massive as M31. By comparison, our galaxy Sextans A, both from a ground-based image and Galaxy is about half as luminous and massive as M31. from an image taken with the HUBBLE SPACE TELESCOPE. The All the remaining galaxies of the Local Group are luminous young blue and red stars are distributed non- dwarf systems of various types. The galaxies that exhibit uniformly, lending this galaxy its highly disorganized the most variation in appearance and in their global appearance. properties are the DWARF IRREGULAR GALAXIES. The most This apparent lack of large-scale organization in dIrr massive example of this type of galaxy in the Local Group galaxies is somewhat of an illusion produced by the is the LMC, one of the satellites of the Milky Way and luminous young stars that generally make up a minority of the second closest external galaxy to the Sun. The LMC the total stellar population of such galaxies. Observations is a massive dwarf, and its global properties place it in the red and infrared are most sensitive to the light near the ill-defined boundary separating dwarf irregular output of the dominant population of old and middle- galaxies from small spirals. Careful studies reveal features aged stars in these galaxies. When astronomers try to in the LMC normally found in spiral galaxies, such as determine the distribution of these oldest stars, even the

Figure 2. (Left panel) A ground-based color image of the Local Group irregular galaxy Sextans A. The entire central region of the galaxy is included. Notice how the bright clumps in the body of the galaxy are dominated by blue–white stars. Such stars are very hot and young objects indicating that these clumps are regions of relatively recent star formation in the galaxy. This image was taken by Diedre Hunter of Lowell Observatory. This figure is reproduced as Color Plate 9. (Right panel) An image of the central region of Sextans A obtained with the Hubble Space Telescope. In this image the individual stars in the clump located to the left of the galaxy’s center in the left-hand panel are now easily apparent. Note too how red stars are more common away away from the bright clump of blue stars; these redder stars trace star formation in the galaxy long ago and show clearly that star formation has gone on over an extended period in Sextans A. This complex spatial and temporal star formation history is common for most of the dwarfs of the Local Group, especially the irregular galaxies. The HST photograph was provided by Robbie Dohm-Palmer of the University of Minnesota and the University of Michigan. This figure is reproduced as Color Plate 10. most irregular dwarfs exhibit a much smoother, more dust) in only a few million years. The implication is that symmetric appearance. An analogy is the surface of a pot unless it started out with an astoundingly large reservoir of slowly boiling soup: although bubbles erupt at different of gas from which to form stars, IC 10 has probably been locations at different times, the overall distribution of the caught during a particularly active—but short-lived— soup is relatively uniform. The active star-forming regions phase in its star-formation history. Such galaxies that of irregulars correspond to the bubbles: very prominent appear to form stars at unsustainably high rates are known when active, but short lived and all interspersed in a more as STARBURST GALAXIES. IC 10 is probably the closest example uniform medium. of a true starburst galaxy, although one active region of star With the striking exception of the Magellanic Clouds, formation in the LMC—the 30 Doradus Region—exhibits the dIrr galaxies of the Local Group tend to be found far similar characteristics on a subgalactic scale. from the two large galaxies (M31 and the Milky Way) of the The remaining dwarfs of the Local Group are Local Group. A few of the least luminous dwarf irregulars ellipsoidal systems. These galaxies are characterized by are also among the most metal-poor galaxies known. a roughly circular or elliptical outline on the sky, and by a Because stars produce heavy elements which they then smooth, centrally concentrated distribution of light. One eject back into the interstellar medium at the ends of their of these, M32, is considered to be an example of a true lives, the low abundances of such elements in the smaller DWARF ELLIPTICAL GALAXY. As such, it represents the low- dwarfs suggest that these galaxies are relatively pristine, luminosity end of the very large family of elliptical galaxies unevolved systems. These galaxies may be forming stars which includes some of the largest, most luminous and in large quantities for the first time in their lives. As such, most massive individual galaxies known. M32 is also these galaxies are invaluable ‘living fossils’ that can tell us noteworthy because it appears to harbor a massive BLACK of the properties of gas and stars in the early universe. HOLE in its extremely bright nucleus, it may be a unique One particularly interesting Local Group irregular local example of a galaxy with no ancient stars and, as galaxy is IC 10. This galaxy is forming stars at an a companion of M31, it shows distortions that indicate a unsustainably rapid rate. If it were to continue it would strong gravitational interaction with its massive parent. soon exhaust its raw materials for making stars (gas and The remaining ellipsoidal Local Group galaxies are

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Local Group E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS 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 (figure 3). 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 (see HALO, GALACTIC). 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 Figure 3. An image of NGC 205, a dwarf spheroidal companion the entire lifetime of the universe. A few hundred million of M31. The bright galaxy to the lower right of the figure is M31 years after periods of active star formation, a dSph galaxy itself. Note how close to the larger galaxy NGC 205 appears to that may initially have looked like a dIrr system could be and how the outer extent of NGC 205 appears distorted from become a transition galaxy. This intimate relation between the smooth elliptical shape apparent near the galaxy core. This dIrr and dSph galaxies in which ‘transition’ systems act distortion is due to the strong tides that are induced in NGC 205 as a ‘missing’ evolutionary link remains controversial. during its close passage to its much larger parent, M31. M32 and the Sagittarius dwarf also show evidence of tidal disruption by Nonetheless, there is little doubt that some dSph galaxies M31 and the Milky Way, respectively. The Magellanic Clouds looked a lot like low-luminosity dIrr systems at some show clear evidence of tidal distortion due to mutual point(s) in the past. interactions over the past few billion years. This image was Galaxies are not the only inhabitants of intergalactic obtained by Paul Harding, Heather Morrison and Anne Fry of space within the Local Group. There is growing evidence Case Western Reserve University. 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 to find one closeby. Moreover, giant ellipticals tend to other galaxies or may correspond to primordial matter be found in regions with a high density of galaxies, a that has yet to collapse into small stellar systems such as manifestation of the so-called GALAXY MORPHOLOGY–DENSITY globular clusters or dwarf galaxies. Some remote globular RELATION. Because the Local Group is a loose, low-density star clusters could plausibly be ‘free-floating’ members collection of galaxies, it would have been unusual— of the Local Group that were ejected from their parent although not impossible—for it to contain one or more big galaxies during past interactions of individual galaxies elliptical galaxies. The nearest giant elliptical galaxy to us within the group. is probably MAFFEI 1. The slightly more distant Centaurus A One prominent class of normal galaxy is not found (NGC 5128) is somewhat easier to study because it is not within the Local Group: giant elliptical galaxies. This is located so close to the Milky Way in the sky and suffers not entirely surprising; since large ellipticals are relatively far less obscuration by interstellar dust in the plane of our rare in the local universe, we would have been ‘lucky’ Galaxy.

The motions of galaxies within the Local Group We can also measure the masses of Local Group The motions of galaxies in the Local Group appear galaxies individually by determining the range of to violate HUBBLE’S LAW that the universe is uniformly velocities of individual stars and gas clouds that orbit expanding. The reason for this has to do with the within individual systems. So long as Newton’s law definition of the Local Group as the collection of nearby of gravity remains valid over the physical scales of the galaxies that are gravitationally bound to one another. galaxies, astronomers find that Local Group galaxies Just as the planets of the solar system do not appear typically exhibit mass-to-light ratios in the range 3–100 to be expanding away from us because they are bound (see table 1). Thus, the dark matter within the Local to the Sun, the mutual attraction of galaxies within the Group is not spread out uniformly throughout the group, Local Group has overcome the universal expansion in our but is instead concentrated about the individual galaxies immediate neighborhood. The most striking example of we detect optically. This suggests that possibility that the this is M31, which is currently approaching our galaxy at Local Group may contain some nearby examples of ‘dark’ − nearly 50 km s 1: if on a true collision course, the two galaxies consisting of only dark matter with no luminous galaxies will meet in about 8–10 billion years. More likely, material. No such systems have been detected yet. The M31 and the Milky Way make up a binary system in which dwarf galaxies of the Local Group do offer one important the two galaxies orbit their common center of gravity. clue about the nature of dark matter. Certain types of dark Inevitably, tidal effects will cause the two galaxies to merge matter that have been postulated in the past—in particular into one giant system at some time in the distant future. NEUTRINOS—cannot account for the surprisingly high mass Many of the other galaxies of the Local Group also exhibit densities required to account for the total masses of these motions towards us, signifying that they too are either in small galaxies. orbit about the Milky Way or currently moving along orbits about M31 or other Local Group galaxies that cause them The distribution of galaxies within the Local to move towards us at the present time. In all of these cases, Group the orbital motions are larger than the universal expansion Figure 4 shows a stereoscopic picture of the Local Group. velocities we would expect to measure for such nearby It is quite clear from this three-dimensional image of the galaxies based on Hubble’s law. Local Group that the group’s volume is not uniformly The mutual attraction of M31 and the Milky Way can filled with galaxies. Instead, most of the galaxies appear to be used to estimate the mass of the Local Group. Much congregate into three clumps; the remaining galaxies are as a ball thrown in the air first rises, stops and then falls, spread out into a larger ‘cloud’ of galaxies that occupies a M31 and the Milky Way are now falling towards each other large fraction of the total volume of the Local Group. The after their initial movement apart after the BIG BANG.Ifone smaller ‘subgroups’ correspond to M31 and its satellites, measures the relative velocities and locations of the two the Milky Way and its companions, and the ‘NGC3109 galaxies and one estimates how long it has been since subgroup’ comprising five small galaxies that are well they were together—essentially the age of the universe separated from M31 and the Milky Way. The galaxies estimated from the Hubble constant or from the ages of of the more extended cloud—the so-called ‘Local Group the oldest stars—it is possible to estimate the combined Cloud’—are not obviously associated with any single mass of M31 and the Milky Way. This sort of analysis was larger galaxy. Table 1 lists the subgroup with which each first carried out by F D Kahn and LWoltjerin 1959. Modern Local Group galaxy is most likely associated. applications of this technique reveal that the combined In general, each subgroup corresponds to a set of mass of M31 and the Milky Way is in the range between 500 gravitationally bound galaxies. The 13 satellites of the billion and 2 trillion times the mass of the Sun. Because Milky Way, for example, are all likely to be in orbit about virtually all the matter of the Local Group is located in our Galaxy while M31 maintains a similar stable of small these two giant spiral galaxies, this is also our best estimate companion galaxies. This situation is analogous to the of the total mass of the group. moons that orbit individual planets of the Solar System: As massive as the Local Group is, the total light output although Jupiter and Saturn orbit the Sun, both planets of all of the galaxies in the group M31 and the Milky Way possess their own large families of bound satellites. In the is equivalent to ‘only’ 30 billion times the luminosity of case of the Local Group, the subgroups are weakly bound the Sun. This is unusual because for normal stars the ratio together, and there is evidence that some satellites may of their mass (in units of the mass of Sun, or 2 × 1030 kg) be occasionally ‘swapped’ from one subgroup to another. and their luminosity (also in units of the luminosity of the For example, some models of the dynamical evolution of Sun, or 4 × 1026 W) is around 1. For the Local Group the the Local Group suggest that the Magellanic Clouds and ratio is much higher, approximately 50. This suggests that perhaps the Leo I dSph galaxy may have first formed near the group is dominated by ‘DARK MATTER’ which contributes M31, but are now satellites of our Galaxy. to the local gravitational field but remains invisible at any Some Local Group galaxies show unmistakable signs wavelength of ELECTROMAGNETIC RADIATION. The large mass- of strong mutual gravitational interactions. As noted to-light ratio of the Local Group implies that only 2–5% above, the Milky Way and M31 have overcome the initial of the total matter in the group is visible to us through expansion and are now falling in towards one another. emitted or reflected radiation. The close companions of M31—NGC 205 and M32—both

Figure 4. (Top panel) A stereo map of the Local Group showing its approximate three-dimensional structure. The view is from a point located about 7 million ly directly in front of the Sun’s current motion about the center of the Milky Way. The large cross near the center of each panel denotes the position of the Milky Way, while the large off-center cross is M31. The smaller cross represents M33. The open squares show the locations of the dwarf ellipticals and dwarf spheroidal galaxies; the solid squares denote the dwarf irregulars; and the small ×s are the transition galaxies. The MW, M31 and NGC 3109 groups are easily visible; the latter is on the ‘far’ side of the figure. The large diffuse ‘bubble’ of galaxies extending to the lower right and towards the viewer is the Local Group Cloud. (Lower panel) A guide to the identities of individual galaxies in the stereo image. The systems are denoted with abbreviations that should—in conjunction with the names listed in table 1—uniquely define each galaxy. The galaxies closest to M31 and the Milky Way are not identified in order to minimize confusion in those crowded regions of the figure. show global distortions that may be due to strong tidal massive component of the interacting pair, it has suffered effects induced by their close passage to their parent galaxy the most. One clear sign of this is the fact that the SMC is (figure 3 shows the distortions induced in NGC 205 by elongated significantly along our line of sight, resembling its close passage by M31). The Magellanic Clouds reveal a cigar whose long axis is roughly pointed towards us. considerable evidence that they too have interacted in This elongation reflects the stretching of the galaxy as it is the past. For example, the MAGELLANIC STREAM is a long literally pulled apart by its abusive neighbor. Detailed arc of neutral gas that was probably ejected from one or models of the dynamical evolution of the Magellanic both of the Clouds as they passed close to one another Clouds indicate that they will both fall into the Milky Way and passed the Milky Way. Because the SMC is the least in the next few billion years. Although the Clouds will be

Copyright © Nature Publishing Group 2001 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2001 Dirac House, Temple Back, Bristol, BS1 6BE, UK 7 Local Group E NCYCLOPEDIA OF A STRONOMY AND A STROPHYSICS completely disrupted in the process, even the much-larger On an even larger scale, the Local Group itself Milky Way will be affected as its disk is puffed up by the seems to interact significantly with other nearby groups of energy injected by the infalling dwarfs. galaxies. One of these, the Sculptor Group, appears to be Perhaps the most spectacular example of an interac- strongly elongated along a line projecting back to the Local tion within the Local Group involves the Sagittarius dSph Group. Because our group is considerably more massive, galaxy. Discovered only in 1994, the SAGITTARIUS DWARF it is probable that the gravitational force of the Local GALAXY is now known to be the closest galaxy to the Milky Group has significantly distorted the Sculptor Group. One Way. Sadly for Sagittarius, this is too close, and the dwarf result of this interaction is the lack of a clear boundary is being severely torn apart by the gravitational tidal forces between the Sculptor Group and our own. Some galaxies exerted on it by our much more massive Galaxy. A clear traditionally associated with Sculptor are occasionally indication that this process has already begun is the fact catalogued with the Local Group and vice versa. Unlike that Sagittarius has been stretched along a long arc that the Local Group, Sculptor lacks any giant galaxies, but currently is known to extend nearly all of the way around does contain a number of lower-luminosity spirals and the sky. Like the Magellanic Clouds, Sagittarius does not many dwarf galaxies. On roughly the opposite side of have long to live as a separate galaxy before it too is com- the sky the M81–Maffei Group represents another nearby pletely disrupted and merges into the main body of the concentration of galaxies that may have a significant Milky Way. effect on the evolution and internal motions of the Local Group. Unlike the Sculptor Group, the M81–Maffei Group Cosmological implications of the Local Group contains some giant galaxies, including the large spiral M81 and the elliptical galaxy Maffei 1. As with the Although most studies of the origin of the universe focus Sculptor Group, some of the galaxies assigned to the M81– on radiation coming from extremely distant galaxies, the Maffei Group have been considered at times to be Local Local Group also plays a critical role in cosmological Group members and vice versa. On top of the interactions studies. For example, with the advent of large ground- with neighboring groups, the Local Group is also ‘falling’ based and space-based telescopes and the use of high- into the nearby VIRGO CLUSTER of galaxies. The attempt sensitivity electronic detectors, it has become possible to understand the details and implications of these local to measure the complete fossil record of star formation interactions and large scale motions remains a very active in many of the galaxies of the Local Group. These field of modern research. investigations provide a critical point of comparison with studies aimed at understanding how galaxies formed in Bibliography the first place by studying objects detectable at the edge of Hodge P 200 An Atlas of Local Group Galaxies (Dordrecht: the visible universe. So far, observations of Local Group Kluwer) (an atlas of photographs of all Local Group systems reveal that normal galaxies have formed stars in galaxies) unexpectedly complex and varied ways. Some nearby Mateo M 1998 Dwarf galaxies of the Local Group Ann. dwarf spheroidal galaxies are composed almost entirely Rev. Astron. Astrophys. 36 435–506 (a recent technical of ancient stars formed soon after the big bang, but most review of the Local Group) of these galaxies formed large numbers of stars over long Sandage A and Bedke J 1994 The Carnegie Atlas of Galaxies periods of time extending as recently as the past few (Washington, DC: Carnegie Institution) (stunning hundred million years. The dwarf irregulars continue to photographs of many galaxies including most of the form stars today, in some cases perhaps for the first time. brighter members of the Local Group) Astronomers are just now beginning to try to reconcile van den Bergh S 2000 The Galaxies of the Local Group these detailed observations of Local Group galaxies with (Cambridge: Cambridge University Press) (a galaxy- studies of larger numbers of extremely remote galaxies. by-galaxy description of the Local Group; many The Local Group also offers insights into the question historical comments included) of whether small galaxies have merged over time to form larger systems such as M31, the Milky Way or the All of these contain many additional references. Magellanic Clouds. Some of the mergers can be studied Mario L Mateo 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.