The galaxies of the Local Group are our closest neighbors in the Universe. Because most of them are nearer than one megaparsec (Mpc) they are easily resolved into stars. This enables one to study these objects in much more detail than is possible for more distant galaxies. The members of the Local Group are therefore the laboratories in which individual objects, such as star clusters, planetary nebulae, supernova remnants, etc., can be studied in detail. Furthermore, important empirical laws, such as the Cepheid period–luminosity relation (Leavitt 1907), the maximum magnitude versus rate-of-decline relation for novae, and the luminosity distribution of globular clusters, can be calibrated in Local Group galaxies. For an earlier review of the properties of some of these galaxies the reader is referred to the proceedings of the symposium on The Local Group: Comparative and Global Properties (Layden, Smith & Storm 1994). Reviews of more recent work are provided in New Views of the Magellanic Clouds = IAU Symposium No. 190 (Chu, Hesser & Suntzeff 1999), in The Stellar Content of the Local Group of Galaxies = IAU Symposium No. 192 (Whitelock & Cannon 1999), and in Stellar Astrophysics for the Local Group (Aparicio, Herrero & Sánchez 1998).

Is the Local Group typical?

Inspection of the Palomar Sky Survey (Minkowski & Abell 1963) shows (van den Bergh 1962) that only a small fraction of all galaxies are isolated objects or members of rich clusters. The majority of galaxies in nearby regions of the Universe are seen to be located in small groups and clusters resembling the Local Group.

Introduction

NGC 6822 is a barred dwarf galaxy of type Ir IV–V that islocated at relatively low Galactic latitude. The first detailed study of this galaxy was by Hubble (1925c) who discovered 11 Cepheid variables. Hubble wrote “Cepheid variables, diffuse nebulae, dimensions, density, and distribution of stellar luminosities agree in defining the system as a curiously faithful copy of the [C]louds, but removed to a vastly greater distance. N.G.C. 6822 lies far outside the limits of the [G]alactic system, even as outlined by the globular clusters, and hence may serve as a stepping-stone for speculation concerning the habitants of space beyond.” From these observations Hubble (1936, p. 124) was able to establish that NGC 6822 is a relatively nearby member of the Local Group. Hubble also discovered nine nonstellar objects in this galaxy. Some of these are young star clusters embedded in nebulosity, whereas others appear to be old star clusters. Hubble's cluster VII has been studied by Cohen & Blakeslee (1998), who find that it is a typical metal-poor globular cluster with a metallicity [Fe/H]=-1.95±0.15 and an age of ∼11 Gyr. It would be interesting to determine the morphology of the horizontal branch of this cluster. Hodge (1977) gives V =16.28 for the cluster Hubble VII, from which MV ≈-8.0. Cluster Hubble VI is a much younger object with an age of ∼2 Gyr and a metallicity of [Fe/H]≈-1.0, a value similar to the present abundance of the interstellar medium in this galaxy.

Faint dwarf spheroidals are probably the most common type of galaxy in the Universe. However, they contribute only a small fraction of the total mass and luminosity of all galaxies. The number of dwarf spheroidals has been increasing slowly with time, as dwarf irregulars exhaust their supply of interstellar gas. An additional source of new Ir galaxies, which evolve into dSph galaxies, is provided by objects formed in the tidal tails produced during the interactions of giant spirals (Dottori, Mirabel & Duc 1994). However such “young” dSph galaxies will have low M/L ratios because dark matter that is pulled out of giant galaxies is stretched (Kormendy 1998). As a result fragments pulled out of more luminous galaxies will have a lower density than their progenitors. Furthermore, the initial dark matter density in giants was already lower than it is in dwarf galaxies, even before it was tidally stretched. Finally, dSph galaxies formed from gas that was pulled out of giant galaxies are expected to be more metal rich than most dwarf spheroidals.

The data in Table 2.1 show that of the 18 known Local Group galaxies fainter than MV = –14.0, 13 (72%) are dSph, two (11%) are dIr, two (11%) are of intermediate (dSph/dIr) type, and one (6%) is of unknown type. Excellent reviews on these faint galaxies have been given by Da Costa (1998) and by Mateo (1998).

The dwarf spheroidal Leo I

Leo I was discovered by Harrington (see Wilson 1955) on plates of the Palomar Sky Survey. The Leo I system is located close to the first magnitude star α Leonis, so that it is sometimes referred to as the Regulus system.

Introduction

A summary of derived properties for the probable members of the Local Group is given in Table 19.1. In the following sections these data will be used to derive various global properties of Local Group galaxies.

The motions of Local Group galaxies

Table 19.1 contains information on 35 probable Local Group members. Giving equal weight to each object, one may use these data to derive both the size of the solar motion relative to the centroid of the Local Group and the position of the apex toward which the Sun is moving. Originally Humason, Mayall & Sandage (1956) adopted a solar motion of V⊙ = 300 km s-1. More recently Yahil, Tammann & Sandage (1977) derived V⊙ = 308 km s-1 toward l = 105°, b = –7°. Their study included NGC 404, NGC 6949, IC 342, IC 5152, Maffei 1, Maffei 2, and DDO 187 (Aparicio, García-Pelayo & Moles 1988), all of which are presently considered to be nonmembers of the Local Group, and NGC 3109, Sextans A, and Sextans B, which are probable nonmembers (see Chapter 16). An unweighted least-squares solution for the 27 Local Group galaxies with known radial velocities that are listed in Table 2.1 yields a solar motion of 306±18 km s-1 toward an apex at l = 99° ± 5° and b = –4° ± 4°, in which the quoted mean errors were determined using a “bootstrap” technique (Courteau & van den Bergh 1999).

Introduction

Mayall (1935) was the first person to draw attention to this highly obscured galaxy, which is located at b = –3°.3. He pointed out that “the original negatives show, almost conclusively, that IC 10 is an extra-galactic object.” Hubble (1936, p. 147), who called it “one of the most curious objects in the sky,” first raised the possibility that IC 10 might be a member of the Local Group. This suspicion was subsequently confirmed by the Cepheid distances of Saha et al. (1996) and Wilson et al. (1996). Photographs in blue and red light of this galaxy have been published by Roberts (1962) and by de Vaucouleurs & Ables (1965). From an inspection of these images the DDO classification of this object appears to be Ir V:, the luminosity class is uncertain because of the large Galactic foreground obscuration. Roberts (1962) notes that the optical and 21-cm radio centers of IC 10 agree within their errors.

Distance and reddening

The integrated colors of IC 10 have been measured by de Vaucouleurs & Ables (1965). From these colors they estimated the reddening of this galaxy to be 〈E(B – V)〉 = 0.87. Using the colors of four Cepheids, Wilson et al. (1996) find a range in reddening of 0.6 ≲ E(B – V) ≲ 1.1 mag. From their data they adopt a mean value E(B – V) ≈ 0.80. Using this value, Wilson et al. obtain a true distance modulus of 24.57 ± 0.32, corresponding to a distance of 820 ± 80 kpc.

Introduction

Since its existence was first noted by Edwin Hubble, the Local Group has been the subject of continuing exploration. Hubble (1936, p. 125) refers to it as “a typical, small group of nebulae which is isolated in the general field.” The elongated core of the Local Group, consisting of the Andromeda galaxy, the Milky Way, and their close companions does, indeed, form a rather well-isolated group. However, the low-density outer envelope of the Local Group may mingle with the coronae of other nearby clusterings, such as the South Polar group and the Mafei/IC 342 group. As was already noted in Section 18.1 the velocity dispersion of Local Group galaxies establishes a rather well-determined radius of 1.2 Mpc for the zero-velocity surface of the Local Group. If we adopt MV (Sun) = +4.82 ± 0.02 (Hayes 1985), the total luminosity of the Local Group galaxies listed in Table 19.1 is found to be 4.2 × 1010L⊙(V). Of this amount 86% is provided by M31 and the Galaxy. The discovery of additional faint Local Group members would not increase the estimated total luminosity of the Local Group significantly. Comparison with the Local Group mass of M = (2.3±0.6)× 1012M⊙ yields M/LV = 44±12. This shows that dark matter in the Local Group outweighs visible matter by about an order of magnitude. Data on the mass-to-light ratios of the Galactic and Andromeda subgroups of the Local Group are given in Table 20.1.

Local Group calibrators

Modern research has vindicated Hubble's expectation that Local Group members (“these neighboring systems”) would furnish a small sample collection of nebulae, from which criteria might be developed to explore the remoter regions of space.

IC 1613

Introduction

The dwarf irregular galaxy IC 1613 was discovered by Wolf (1906) and is described as “F,eeL” (i.e., faint and most extremely large) in the Second Index Catalogue of Nebulae (Dreyer 1908). The true nature of IC 1613 was first recognized by Baade (1935), who determined its distance by using the period–luminosity relation for Cepheids having periods ranging from 14 days to 42 days. Baade concluded that “it is without doubt a system of low luminosity.” Subsequently it was included as a bona fide member of the Local Group by Hubble (1936, p. 145). IC 1613 is a nonbarred irregular that serves as the prototype for DDO type Ir V. The fact that IC 1613 was already known almost a century ago, even though it is quite faint (MV = –15.3), suggests that the inventory of all but the dimmest Local Group members is (excepting objects at low Galactic latitude) probably reasonably complete. A blue image taken with the Palomar 1.2-m Schmidt telescope is shown in Figure 11.1. A higher resolution photograph of IC 1613, which was obtained with the Palomar 5-m reflector, is shown in Volders & Högbom (1961). A beautiful photograph inthe light of Hα is reproduced in Sandage (1971).

Distance and reddening

The discovery of Cepheids in IC 1613 by Baade (1935) was followed up by a more detailed investigation (Baade 1963, pp. 218–226) that resulted in the identification of 25 Cepheids with periods ranging from 2.4 days to 146 days.

Introduction

While inspecting a mosaic of nine deep IIIaJ plates of M31 and its environment taken with the Palomar 1.2-m Schmidt telescope, van den Bergh (1972a) discovered three faint nebulous patches that were immediately suspected of being dwarf spheroidal companions to the Andromeda galaxy. These objects were dubbed Andromeda I, Andromeda II, and Andromeda III. Subsequently van den Bergh (1972b) used the Palomar 5-m Hale reflector to resolve And III into stars that had approximately the same V magnitude as the brightest stars in NGC 185 and NGC 205. Later van den Bergh (1974b) showed that And I and And II resolve at about the same magnitude as And III. These observations indicated that And I, And II, and And III are all located at about the same distance as M31. The suspicion that they are physically associated with M31 was strengthened by a Palomar Schmidt survey of an area of ∼700 square degrees surrounding the Andromeda galaxy, which showed that And I, II, and III are concentrated toward M31. Of a fourth candidate van den Bergh (1972a) wrote: “The object And IV is probably not a dwarf spheroidal galaxy. It is smaller and bluer than the other objects in Table 1 [i.e., And I, II & III]. Furthermore, And IV has a much higher surface brightness than do the other galaxies in this table. This suggests that And IV, which is located very close to M31, might be a relatively old star cloud in the outer Disk of M31.”

The present section will deal with the three most luminous Local Group spheroidal galaxies: NGC 205, NGC 185, and NGC 147, all of which are companions to the Andromeda galaxy. These galaxies, which are too luminous to be called dwarf spheroidals (dSph), will be designated spheroidals (Sph). In the past such objects have been classified as being of types dE, Ep, S1, or dE/dSph. It should, however, be emphasized that Sph and dSph galaxies are, respectively, the bright and faint representatives of the same morphological class of galaxies.

NGC 147 and NGC 185 are located at only 0.22 Mpc from our adopted center of the Local Group. This places them closer to the center of mass of the Local Group than any other presently known Group members.

The spheroidal galaxy NGC 205

Introduction

The luminosity of NGC 205 is similar to that of M32. However, it has a much more extended structure. Furthermore M32 rotates, but NGC 205 does not (Bender, Paquet & Nieto 1991). A good photograph of NGC 205 is shown in the Hubble Atlas of Galaxies (Sandage 1961). Images taken in the ultraviolet (Baade 1951) show it to contain a small number of young blue stars. Deep exposures (see Figure 12.1) reveal some extended patches of dust absorption. Moreover, inspection of the image of NGC 205 on the prints of the Palomar Sky Survey, or deeper plates (see, for example, Fig. 7 of Kormendy 1982), clearly show that the outer isophotes of this galaxy are distorted – presumably due to the tides exerted by M31, from which it is separated by only 37′(8.2 kpc).

Introduction

A detailed discussion of galaxies that may be located along the outer fringes of the Local Group has been given by van den Bergh (1994b). In general three criteria can be used to assess the probability that a galaxy might be associated with the Local Group: (1) The distance to that galaxy should be ≲1.5 Mpc (see Section 18.1), (2) it should lie close to the relation between radial velocity and distance from the solar apex (Vr versus cos θ diagram) for well-established Local Group members, and (3) it should not appear to be associated with a group of galaxies that is known to be located well beyond the limits of the Local Group. On the basis of these criteria van den Bergh (1994b) concluded that it was safe to exclude the following galaxies from membership in the Local Group: (1) Sculptor irregular = UKS 2323–326, (2) Maffei 1 and its companions, (3) UGC-A86 = A0355 + 66, (4) NGC 1560, (5) NGC 1569, (6) NGC 5237, and (7) DDO 187 (Hoessel, Saha & Danielson 1998). A particularly strong concentration of Local Group suspects, which includes (2), (3), (4), and (5) listed above, occurs in the IC 342/Maffei group (van den Bergh 1971, Krismer, Tully & Gioia 1995). Krismer et al. place this group at a distance of (3.6 ± 0.5) Mpc. Cassiopeia 1, which was once regarded as a Local Group suspect (Tikhonov 1996), also appears to be a member of the IC 342/Maffei group.

Introduction

Gaseous material is expected to flow into galaxies by the capture of high-velocity clouds (Oort 1966, 1970). Oort estimated that the mass of the Milky Way system might be increasing by ∼1% per Gyr as a result of such inflow. However, tidal interactions between galaxies will result in the return of some interstellar gas in galaxies to intergalactic space (Morris & van den Bergh 1994). In addition, some gas may be ejected from galaxies by supernova-induced fountains (Heiles 1987). Perhaps the best example of inflow into the Galactic Disk is provided by “Complex C” (Wakker & van Woerden 1997). From the nondetection of absorption features in stars of known distance, these authors conclude that D > 2.4 kpc and that Complex C is falling toward us with a velocity > 100 km s-1. From spectroscopic observations of S II absorption lines (which are not affected by depletion onto dust) with the Hubble Space Telescope, Wakker et al. (1999) find that Complex C has a metallicity of 0.094 ± 0.020 times solar. This very low metallicity rules out the possibility that the gas in Complex C was ejected in a Galactic fountain. Furthermore, the data on the age–metallicity relation of the Small Magellanic Cloud (Mighell, Sarajedini & French 1998) show that the gas in Complex C is so metal poor that it could only have been stripped from the SMC ≳5 Gyr ago (i.e., long before the Magellanic Stream was formed).

Introduction

The great spiral in Andromeda is the most luminous member of the Local Group. The first known record of this object is by al-Sufi (903–986). M31 was first viewed through a telescope by Marius, who described it as looking “like a candle seen through a horn,” in 1612. The spiral nature of the Andromeda galaxy was first clearly shown in photographs obtained by Roberts (1887) with a 0.5-m reflector. Ritchey (1917) referred to these arms as “great streams of nebulous stars.” It is listed as object number 31 in Messier's catalog of nebulae. An atlas of finding charts for clusters, associations, variables, etc. in M31 has been published by Hodge (1981). Van den Bergh (1991b) has written a long review paper on this object. The monograph The Andromeda Galaxy by Hodge (1992) provides a detailed historical discussion of research on this object and an annotated bibliography for the period 1885–1950. A monograph entitled Tumannost Andromedy (The Andromeda Nebula) has been published by Sharov (1982).On the sky M31 covers an area of 92′ × 197′ (Holmberg 1958). This large angular size makes the Andromeda galaxy particularly suitable for detailed studies of its structure and stellar population content. De Vaucouleurs (1959b) has shown that the distribution of surface brightness I in spiral galaxies can often be decomposed into a spheroidal component in which I ∼ exp(–R1/4) and an exponential disk in which I ∼ exp(–αR), where R is radial distance and α is a constant.

Introduction

Inspection of Table 2.1, which lists the known members of the Local Group, shows that dwarf spheroidals are the most common type of galaxy in the Local Group. Observations of rich clusters (Trentham 1998a,b) appear to show that the faint ends of their luminosity functions are steep. Moreover, the colors of these faint galaxies suggest that they are mostly dwarf spheroidals. Since dwarf spheroidals are ubiquitous in poor clusters, and even more frequent in rich clusters, it appears safe to conclude that such dwarfs are the most common type of galaxy in the Universe. The fact that they were not discovered until 1938 is entirely due to their low luminosity. Their discoverer (Shapley 1943, p. 142) writes

The suggestion that dwarf spheroidals might be distributed uniformly throughout the Local Group (Ambartsumian 1962) appears to conflict with the observation that strong clusterings of such objects are now known around the Milky Way system and the Andromeda galaxy.

In April of 1968 I gave a series of lectures on the structure, evolution, and stellar content of nearby galaxies at the University of California in Berkeley. An outline of these talks was printed as a slender volume entitled “The Galaxies of the Local Group” (van den Bergh 1968a). Since the publication of this booklet the number of known members of the Local Group has doubled. Furthermore both the quantity, and the quality, of the data that are available on the previously known Local Group members have increased enormously.

Particularly exciting developments since 1968 have been (1) the discovery of the Sagittarius dwarf, which is the nearest external galaxy, (2) the discovery of six dwarf spheroidal companions to the Andromeda nebula, (3) the application of CCD detectors to studies of stellar populations in various Local Group systems, and (4) deep high-resolution observations of various objects in the Local Group with the Hubble Space Telescope. With the presently available enlarged sample, and the improved quality of data on individual objects, we are now in a much better position to start exploring the evolutionary history of the Local Group and its constituent galaxies. Finally (5) it has become clear during the past quarter century that the masses of dark matter halos are typically an order of magnitude greater than the masses of the baryonic galaxies that are embedded within them.

Introduction

The Small Cloud is an irregular dwarf of DDO type Ir IV–V that has a low mean metallicity and a high mass fraction remaining in gaseous form. These characteristics suggest that the SMC is, from an evolutionary point of view, a more primitive and less evolved galaxy than the Large Cloud. The metallicity difference between the Galaxy and the LMC was discovered by Arp (1962), who wrote: “Taken together with the marked differences in the evolved giant branches and [C]epheid gaps in the SMC and [G]alactic clusters, there exists the inescapable implication that the chemical composition of the SMC stars is different from the chemical composition of the solar neighborhood.”

The fact that the SMC contains only a single true globular cluster may indicate that star formation started off later, or more gradually, than it did in the Large Cloud.

At the present time the SMC is forming stars less actively than is the LMC. Prima facie evidence for this is that the Small Cloud contains much smaller, and less spectacular, H II regions than does the Large Cloud. Furthermore, the LMC presently contains 110 massive Wolf–Rayet stars, whereas there are only 9 WR stars in the Small Cloud. Finally, CCD observations by Bothun & Thompson (1988) show that the SMC is redder than the LMC. They find B – V = 0.52±0.03 for the Large Cloud, versus B – V =0.61±0.03 for the integrated color of the Small Cloud.

Introduction

The compact E2 galaxy M32 is the closest companion to the Andromeda galaxy. The projected separation of these two objects on the sky is only 24′ (5.3 kpc). It was first suggested by Schwarzschild (1954) that the tides induced by M32 were responsible for the distortion of the spiral structure of M31 and the warping of its disk. Later Faber (1973) noted that CN and Mg absorption in M32 was stronger than might have been expected from its luminosity (i.e., it has the metallicity usually seen in significantly more luminous objects). She therefore proposed that M32 was initially a much more luminous galaxy had suffered severe tidal truncation by M31. The very compact object NGC 4486B, which is a companion to M87, appears to be another example of a similar type of galaxy that has suffered tidal truncation. The absence of globular clusters in M32 probably also results from its outer swarm of globulars being stripped off by tidal forces. [The innermost M32 globulars might have been sucked into its massive semi-stellar nucleus by tidal friction (Tremaine, Ostriker & Spitzer 1975).] It would be interesting (M. Mateo 1999, private communication) to study the distribution of globular clusters in NGC 4486B, which, like M32, is thought to have suffered severe tidal truncation.

Kormendy (1985) and Ziegler & Bender (1998) have pointed out that M32 is quite a unique object and that there are very few other ellipticals like it. Compared to other dwarf galaxies having similar values of MV, its central surface brightness is four orders of magnitude higher, and its core radiusrc is three orders of magnitude smaller.