Annu. Rev. Astron. Astrophys. 1998. 36: 435-506 Copyright © 1998 by Annual Reviews. All rights reserved

1. INTRODUCTION

While observing the Andromeda Nebula with a fine 18-ft telescope ... I saw another small nebula about one minute in diameter which appeared to throw out two small rays; one to the right and the other to the left.

      G-J-H-J-B Le Gentil de la Galazière, October 29, 1749

        Remarks on the Nebulous Stars (1759)

To the rest of the Universe, the Local Group is an ordinary collection of dwarf galaxies dominated by two giant spirals. But to Earthbound astronomers interested in galaxy evolution, the Local Group is particularly special. The dwarfs of the Local Group provide a uniquely well-studied and statistically useful sample of low-luminosity galaxies. Indeed, virtually all currently known dwarfs less luminous than M_V ~ -11.0 are found in the Local Group (Whiting et al 1997). Dwarf galaxies represent the dominant population, by number, of the present-day Universe (Marzke & Da Costa 1997), and they were almost certainly much more numerous at past epochs (Ellis 1997). Studies of nearby clusters (Phillips et al 1998 and references therein) suggest that the summed optical luminosity of all dwarfs may rival that of the giant high-surface brightness galaxies in these environments. If low-luminosity galaxies are universally dominated by dark matter (DM) to the extent Local Group dwarfs may be (Section 6), they could account for a large fraction of the mass of galaxy clusters and perhaps the entire Universe. The dwarf galaxies of the Local Group offer the best opportunity to study a representative sample of these important but, by nature, inconspicuous galaxies in detail.

Dwarf galaxies are known to exist in large numbers in other environments, particularly nearby groups (Karachentseva et al 1985, Miller 1996, Côté et al 1997), and clusters (Sandage & Binggeli 1984, Sandage et al 1985b, Ferguson & Sandage 1991, Phillips et al 1998). But there are fundamental reasons why the dwarfs of the Local Group remain especially important:

• What is the relationship, if any, between dwarf irregular (dIrr) and dwarf spheroidal/dwarf elliptical (dSph/dE) galaxies? The Local Group contains a unique mixture of low-luminosity galaxies of both types that provide some ways to address this question (e.g. Bothun et al 1986, Binggeli 1994, Skillman & Bender 1995).

• Low-luminosity dwarfs tend to be metal poor (Section 5); thus, the low-luminosity dwarfs in the Local Group represent a sample of galaxies that is still largely composed of nearly primordial material. Dwarf galaxy abundances are typically determined from their HII regions, but only in the Local Group can we also obtain reliable abundances from resolved stellar populations. The large luminosity range of Local Group dwarfs makes them excellent labs to study how other fundamental parameters vary with luminosity, such as DM content, the interstellar medium (ISM) properties, and star-formation history.

• Dwarfs are the simplest galactic systems known. However, Local Group dwarfs are plainly telling us that "simple" is a relative term. The star- formation and chemical-enrichment histories of these galaxies are complex, varied, and in most cases triggered and sustained by as-yet unknown mechanisms (Section 6). The Hubble Space Telescope (HST) is capable of reaching the main-sequence turnoff of the oldest stars in galaxies throughout the Local Group (Gregg & Minniti 1997), so these galaxies will likely remain for some time the only well-defined sample for which we can derive complete star-formation histories.

• Dwarf galaxies may be among the darkest single galaxies known (Section 7). They play an important role in addressing the DM problem, having already placed interesting constraints on the nature and distribution of DM, and even whether the DM paradigm is valid for these systems (Ashman 1992, Mateo 1997). The Local Group currently provides our only opportunity to measure the internal kinematics of ultra-low-surface brightness dwarfs.

• There is considerable evidence of ongoing, past, and future interactions between Local Group dwarfs and larger galaxies (Section 8), which may have helped assemble the larger Local Group galaxies over time. Recent measurements of the star-formation histories, space motions, three-dimensional shapes, and detailed internal kinematics of some of these interacting dwarfs in the Local Group offer new constraints on the dwarf/giant relationship and interaction models (Mateo 1996, Unavane et al 1996).

I have two principle goals for this review. First, I present here a set of tables that aim to provide a summary of basic observational data for all of the currently known Local Group dwarf galaxies. These tables also highlight areas where fundamental observations are lacking or remain of poor quality. My second goal is to illustrate ways in which studies of Local Group dwarfs offer unique opportunities to understand galaxy evolution, DM, galaxy interactions, and the relation between stellar populations and the ISM. At the start of most sections, I cite specialized reviews pertinent to the topics that follow. Some recent general reviews relevant to Local Group dwarf galaxies include those by Hodge (1989), Gallagher & Wyse (1994), Ferguson & Binggeli (1994), Binggeli (1994), Skillman & Bender (1995), Da Costa (1994a, b, 1998), Grebel (1997). I have also found the proceedings of three recent meetings to be particularly helpful: the CTIO/ESO workshop on the Local Group held in La Serena, Chile, in 1994 (Layden et al 1994); the ESO/OHP workshop on dwarf galaxies in France in 1993 (Meylan & Prugniel 1994); and the Tucson workshop on the Galactic halo and in honor of George Preston (Morrison & Sarajedini 1996).

SOME WORDS ABOUT THE TABLES AND NOMENCLATURE     The tables are meant to be self-contained with complete notes and references. Although the data are presented in a uniform format, the tables are all based on a large, inhomogeneous set of independent studies. Many entries are subjectively weighted mean values of the results from independent sources. Galaxies are listed in order of increasing right ascension in all the tables. I try to provide realistic estimates of the 1 errors. Where possible, the errors are taken from the original source, or they reflect the scatter of independent results. In several tables and one figure, errors were omitted for photometric results if 0.04 mag. ASCII files containing most of the information in the tables are available via anonymous ftp at ftp://ra.astro.lsa.umich.edu/pub/mateo/get/LGDtables.dat .

"Transition" galaxies are the five objects listed in Table 1 as dIrr/dSph systems. Sandage & Hoffman (1991) referred to such galaxies as "mixed-morphology" systems. The non-dIrr galaxies of the Local Group may or may not belong to a single family; hence, when I wish the discussion to include M32 (a dE system) and the five transition galaxies, I use "early-type" galaxies.

Table 1. Local Group Galaxies

Galaxy	Other
namea	nameb	2000c	2000d	le	bf	Typeg	Subgrouph	Imagesi
WLM	DDO 221	00 01 58	-15 27.8	75.9	-73.6	IrrIV-V	LGC	1
NGC 55		00 15 08	-39 13.2	332.7	-75.7	IrrIV	LGC	3, 4, 51
IC 10	UGC 192	00 20 25	+59 17.5	119.0	-3.3	dIrr	M31	5, 6
NGC 147	DDO 3	00 33 12	+48 30.5	119.8	-14.3	dSph/dE5	M31	7, 51
And III		00 35 17	+36 30.5	119.3	-26.2	dSph	M31	8
NGC 185	UGC 396	00 38 58	+48 20.2	120.8	-14.5	dSph/	M31	7, 51
						dE3p
NGC 205j	M110	00 40 22	+41 41.4	120.7	-21.1	E5p/	M31	7, 51
						dSph-N
M32	NGC 221	00 42 42	+40 51.9	121.2	-22.0	E2	M31	9, 10, 51
M31	NGC 224	00 42 44	+41 16.1	121.2	-21.6	SbI-II	M31	10, 50, 51
And I		00 45 43	+38 00.4	121.7	-24.9	dSph	M31	8
SMC	NGC 292	00 52 44	-72 49.7	302.8	-44.3	IrrIV-V	MW	11, 12, 51
Sculptor		01 00 09	-33 42.5	287.5	-83.2	dSph	MW	13, 49
LGS 3	Pisces	01 03 53	+21 53.1	126.8	-40.9	dIrr/dSph	M31	14
IC 1613	DDO 8	01 04 54	+02 08.0	129.8	-60.6	IrrV	M31/LGC	15–17
And IIk		01 16 27	+33 25.7	128.9	-29.2	dSph	M31	8
M33	NGC 598	01 33 51	+30 39.6	133.6	-31.3	ScII-III	M31	17, 18, 51
Phoenix		01 51 06	-44 26.7	272.2	-68.9	dIrr/dSph	MW/LGC	19, 20
Fornax		02 39 59	-34 27.0	237.1	-65.7	dSph	MW	21
EGB0427+63	UGCA 92	04 32 01	+63 36.4	144.7	+10.5	dIrr	M31	23
LMC		05 23 34	-69 45.4	280.5	-32.9	IrrIII-IV	MW	11, 12, 51
Carina		06 41 37	-50 58.0	260.1	-22.2	dSph	MW	24
Leo A	DDO 69	09 59 24	+30 44.7	196.9	+52.4	dIrr	MWN3109	29, 51
Sextans B	DDO 70	10 00 00	+05 19.7	233.2	+43.8	dIrr	N3109	27, 28, 51
NGC 3109	DDO 236	10 03 07	-26 09.5	262.1	+23.1	IrrIV-V	N3109	29, 30, 51
Antlial		10 04 04	-27 19.8	263.1	+22.3	dIrr/dSph	N3109	31
Leo I	DDO 74	10 08 27	+12 18.5	226.0	+49.1	dSph	MW	32
Sextans A	DDO 75	10 11 06	-04 42.5	246.2	+39.9	dIrr	N3109	33, 51
Sextans		10 13 03	-01 36.9	243.5	+42.3	dSph	MW	34
Leo II	DDO 93	11 13 29	+22 09.2	220.2	+67.2	dSph	MW	52
GR 8	DDO 155	12 58 40	+14 13.0	310.7	+77.0	dIrr	GR8	36, 37, 54
Ursa Minor	DDO 199	15 09 11	+67 12.9	105.0	+44.8	dSph	MW	38
Draco	DDO 208	17 20 19	+57 54.8	86.4	+34.7	dSph	MW	39
Milky Way		17 45 40	-20 00.5	0.0	0.0	Sbc	MW
Sagittariusj		18 55 03	-30 28.7	5.6	-14.1	dSph-N	MW	40
SagDIG	UKS1927-177	19 29 59	-17 40.7	21.1	-16.3	dIrr	LGC	41, 53
NGC 6822	DDO 209	19 44 56	-14 48.1	25.3	-18.4	IrrIV-V	LGC	42, 43, 51
DDO 210m	Aquarius	20 46 46	-12 51.0	34.0	-31.3	dIrr/dSph	LGC	36, 44, 54
IC 5152		22 02 42	-51 17.7	343.9	-50.2	dIrr	LGC	51
Tucanan		22 41 50	-64 25.2	322.9	-47.4	dSph	LGC	47, 48
UKS2323-326	UGCA 438	23 26 27	-32 23.3	11.9	-70.9	dIrr	LGC	41
Pegasus	DDO 216	23 28 34	+14 44.8	94.8	-43.5	dIrr/dSph	LGC	25, 44, 51
a Entries in italics refer to the five giant Local Group galaxies that are not discussed in this review in any detail.
b A common alternative name.
c,d Right ascension and declination for epoch J2000.0, respectively.
e,f Galactic longitude and latitude, respectively.
g Galaxy type following van den Bergh (1994a).
h Subgroup membership within the Local Group.
i References to optical and 21-cm images (if available): 1, Sandage & Carlson 1985b; 2, Lautsen et al 1977; 3, Hummel et al 1986; 4, Puche et al 1991; 5, de Vaucouleurs & Ables 1965; 6, Shostak & Skillman 1989; 7, Young & Lo 1997a; 8, Caldwell et al 1992; 9, Kent 1987; 10, Hodge 1981; 11, de Vaucouleurs & Freeman 1972; 12, Mathewson & Ford 1984; 13, Carignan et al 1998; 14, Young & Lo 1997b; 15, Ables 1971; 16, Lake & Skillman 1989; 17, Sandage 1961; 18, Corbelli et al 1989; 19, van de Rydt et al 1991; 20, Carignan et al 1991; 21, Hodge 1971; 22, Saha & Hoessel 1991; 23, Hoessel et al 1988; 24, Smecker-Hane et al 1994; 25, Sandage 1986b; 26, Young & Lo 1996a; 27, Sandage & Carlson 1985a; 28, Skillman et al 1988; 29, Sandage & Carlson 1988; 30, Jobin & Carignan 1990; 31, Whiting et al 1997; 32, Hodge 1963a; 33, Sandage & Carlson 1982; 34, Mateo et al 1995a; 35, deleted in proof; 36, Fisher & Tully 1979; 37, Carignan et al 1990; 38, van Agt 1967; 39, Baade & Swope 1961; 40, Ibata et al 1997; 41, Longmore et al 1978; 42, Hodge 1978; 43, Gottesman & Weliachew 1977; 44, Lo et al 1993; 45, deleted in proof; 46, deleted in proof; 47, Lavery & Mighell 1992; 48, Oosterloo et al 1996; 49, van Agt 1978; 50, Hodge 1992a; 51, Sandage & Bedke 1994; 52, Vogt et al 1995; 53, Cesarsky et al 1977; 54, Hopp & Schulte-Ladbeck 1995.
j The N suffix has been added to indicate that these systems may be nucleated.
k The position listed here is based on an independent measurement by Paul Hodge (private communication).
l Also known as PGC 29194 (Fouqué et al 1990) before it was rediscovered as a probable Local Group member by Whiting et al (1997). A thorough discussion of "pre-discovery" observations of Antlia is provided by Aparicio et al (1997a).
m Marconi et al (1990) claimed that the original position of DDO 210 (Fisher & Tully 1975) was significantly in error. However, Lo et al (1993) noted that the original position seemed to be correct, and Marconi et al (1995) subsequently agreed. The position listed here corresponds to the original one from Fisher & Tully (1975).
n Tucana was known prior to its rediscovery by Lavery & Mighell (1992), who thoroughly document all listings of the galaxy in earlier catalogs. Lavery & Mighell (1992) did first claim Tucana to be a possible Local Group member.