Copyright © 1998 by Annual Reviews. All rights reserved
|
| Annu. Rev. Astron. Astrophys. 1998. 36:
435-506 Copyright © 1998 by Annual Reviews. All rights reserved |
Measurement of the integrated photometric and structural properties of Local Group dwarfs is challenging. Because they are so close, many Local Group dwarfs are quite extended, ranging from under 10 arcmin in diameter to over 40°. Few telescope/detector combinations can survey the entire extent of the larger systems in one or even several exposures (though see Kent 1987, Bothun & Thompson 1988). Nearly all Local Group dwarfs have very low surface brightnesses, which not only makes it difficult to discover these galaxies but greatly complicates obtaining reliable follow-up photometry. Nonetheless, there have been many attempts over the past 35 years to study the integrated properties and structural parameters of Local Group dwarfs. Hodge, de Vaucouleurs, and Ables pioneered these studies, and in many cases their results remain the only ones available (e.g. Hodge 1963a, b, 1973;, de Vaucouleurs & Ables 1965, 1968, 1970;, Ables 1971;, Ables & Ables 1977).
Table 3 lists the integrated V-band magnitudes and, when
available, the integrated colors of Local Group dwarfs. In some cases, these
values are based on observations of only a small fraction of the
galaxy. For example, less than 1% of the surface area of Sagittarius
has been measured photometrically
(Mateo et al 1995c,
1996;,
Fahlman et al 1996),
though a large fraction has been mapped photographically
(Ibata et al 1997).
Combined with the distance and
reddening values in Table 2, the
photometry in Table 3 can be used to
derive integrated absolute magnitudes and luminosities
(Table 4)
and the luminosity function (LF) of the Local Group
(Figure 4). For
MB 
-14,
the Local Group LF matches that of the "poor" groups studied by
Ferguson & Sandage
(1991).
The best-fitting
Schechter (1976)
LF for the poor groups of
Ferguson & Sandage
(1991)
is also shown. Note that the analytic expression, if extrapolated as
in Figure 4, implies that the Local Group
contains many galaxies less luminous than MB ~ -12
that have yet to be discovered.
|
| ||||||||||
| Major-axis | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Galaxy | VTa | (B-V)Tb | Other colorsc |
 0d |
re | Rf | position angleg | rexpi | ||
| name | (mag) | (mag) | [mag (bands)] | [mag (arcsec-2)] | (arcmin) | (arcmin) | (deg) | eh | (arcmin) | Refj |
|
| ||||||||||
| WLM | 10.42 ± 0.2 | 0.62 ± 0.12 | -0.21 [1] | 20.36 ± 0.05 | 2.2 | 5.5 | 175 ± 5 | 0.59 ± 0.04 | 3.3 ± 0.2 | 57, 61 |
| NGC 55 | 7.95 ± 0.15 | 0.50 ± 0.04 | 0.87 [3]; 0.77 [4]; | 20.8 ± 0.5 | 4.6 | 20.2 | 105 ± 5 | 0.77 | 3.4 ± 0.2 | 1, 2, 4, 60, 73 |
| 0.42 [5]; | ||||||||||
| -0.05 ± 0.2 [6] | ||||||||||
| IC 10 | 11.55 ± 0.3 | 1.37 | 0.28 [1] | 22.1 ± 0.4 | — | 5.0 ± 2.0 | 140 ± 20 | 0.3 ± 0.1 | 1.9 ± 0.3 | 1, 67, 70, 71, |
| 75 | ||||||||||
| NGC 147 | 9.35 ± 0.15 | 0.92 ± 0.07 | 0.55 [2]; 1.12 [3] | 21.6 ± 0.2 | 1.1 | 20 | 34 ± 4 | 0.46 ± 0.03 | 2.1 ± 0.3 | 1, 5–8, 11, |
| 82 | ||||||||||
| And III | 14.21 ± 0.15 | — | — | 24.49 ± 0.05 | 1.27 ± 0.17 | 6.2 ± 1.0 | 135 ± 5 | 0.6 | 0.75 ± 0.01 | 5, 8 |
| NGC 185 | 9.09 ± 0.15 | 0.96 ± 0.06 | 0.30 ± 0.05 [1]; | 20.1 ± 0.4 | 1.0 ± 0.3 | 16 ± 2 | 41 ± 5 | 0.26 | 1.7 ± 0.3 | 1, 5, 6, 8–11, |
| 0.60 [2]; | 74, 82 | |||||||||
| 1.25 [3]; 2.06 [6] | 74, 82 | |||||||||
| NGC 205k,l | 8.05 ± 0.15 | 0.70 ± 0.10 | 0.34 [1]; 0.52 [2]; | 20.4 ± 0.4 | 1.5 ± 0.5 | 6.2 ± 2.0 | 165 ± 15 | 0.46 ± 0.03 | 1.7 | 1, 5, 6, 8, 63–65, |
| 1.23 [3]; 3.08 [5] | 79, 82 | |||||||||
| M32l,m | 8.10 ± 0.15 | 0.99 ± 0.05 | 0.64 [1]; 0.42 [2]; | <11.6 | — | 9 ± 2 | 165 ± 5 | 0.18 ± 0.03 | — | 1, 6, 12–14, 65, |
| 1.07 [3]; 3.06 [5]; | 76 | |||||||||
| 0.64 [6]; 0.85 [7] | ||||||||||
| And I | 12.75 ± 0.2 | 0.75 ± 0.06 | 0.29 [1] | 24.37 ± 0.01 | 1.58 ± 0.08 | 13.4 ± 1.4 | — | 0.0 | 1.46 ± 0.06 | 1, 5, 47 |
| Sculptor | 8.5 ± 0.3 | — | — | 23.7 ± 0.4 | 5.8 ± 1.6 | 76.5 ± 5.0 | 99 ± 1 | 0.32 ± 0.03 | 6.7 ± 0.2 | 5, 8, 20, 77 |
| LGS 3 | 14.26 ± 0.15 | 0.74 ± 0.06 | 0.47 [2]; 1.00 [3] | 24.7 ± 0.2 | 0.82 ± 0.05 | 14.5 ± 4.5 | 175 ± 5 | 0.26 ± 0.06 | 0.78 | 16–18, 78 |
| IC 1613 | 9.59 ± 0.15 | 0.60 ± 0.10 | -0.25 [1] | 22.8 ± 0.3 | 3.3 ± 1.0 | 11 ± 3 | 83 ± 6 | 0.24 ± 0.06 | 5.4 ± 3.0 | 48, 49, 75 |
| And II | 12.7 ± 0.2 | — | — | 24.47 ± 0.05 | 1.64 ± 0.08 | 17.2 ± 1.0 | — | 0.3 | 1.57 ± 0.03 | 5, 8 |
| Phoenix | 13.2 ± 0.2 | 0.61 ± 0.05 | -0.21 [1] | — | — | >8.6 | 160 ± 10 | 0.3 ± 0.1 | — | 1, 19 |
| Fornax | 7.6 ± 0.3 | 0.63 ± 0.05 | 0.08 [1]; 0.45 [2]; | 23.4 ± 0.3 | 13.8 ± 0.8 | 71 ± 4 | 48 ± 6 | 0.31 ± 0.03 | 10.2 | 5, 20–22, 24, |
| 1.02 [3] | 50 | |||||||||
| EGB 0427+63n | 13.88 ± 0.12 | 1.34 ± 0.05 | — | 23.9 ± 0.3 | 0.5 | 1.0 | 73 ± 5 | 0.55 ± 0.05 | — | 16, 72 |
| Carina | 10.85 ± 0.25 | — | — | 25.5 ± 0.4 | 8.8 ± 1.2 | 28.8 ± 3.6 | 65 ± 5 | 0.33 ± 0.05 | 5.5 | 5, 20, 26, 83 |
| Leo A | 12.8 ± 0.2 | 0.15 ± 0.2 | -0.2 ± 0.1 [1] | — | 2.3 | 3.5 | 94 ± 5 | 0.36 | — | 1, 2, 57, 59 |
| Sextans B | 11.43 ± 0.15 | 0.48 | -0.16 [1] | — | 3.0 | 3.9 | 130 ± 15 | 0.23 | — | 2, 42, 57, 58 |
| NGC 3109 | 9.88 ± 0.15 | 0.52 | -0.12 [1] | 23.6 ± 0.2 | 2.8 | 13.3 | 92 ± 1 | 0.80 | 3.1 | 1, 53, 84 |
| Antliao | 14.8 ± 0.2 | — | 0.5 ± 0.1 [2]; | 24.3 ± 0.2 | 0.80 ± 0.05 | 5.2 ± 0.2 | 145 ± 5 | 0.35 ± 0.03 | 1.1, 0.3 | 28, 29 |
| 1.2 ± 0.2 [3] | ||||||||||
| Leo I | 10.1 ± 0.3 | 0.8 ± 0.2 | 0.15 [1] | 22.4 ± 0.3 | 3.3 ± 0.3 | 12.6 ± 1.5 | 79 ± 3 | 0.21 ± 0.03 | 1.8 ± 0.2 | 5, 8, 20 |
| Sextans A | 11.30 ± 0.15 | 0.38 ± 0.07 | -0.32 [1] | 23.5 ± 0.3 | 3.2 | 4.0 | 52 ± 5 | 0.21 | 1.5 ± 0.3 | 1, 2, 75 |
| Sextans | 10.3 ± 0.3 | — | — | 26.2 ± 0.5 | 16.6 ± 1.2 | 160 ± 50 | 56 ± 5 | 0.35 ± 0.05 | 12.3 ± 3.0 | 5, 8, 20, 30 |
| Leo II | 12.0 ± 0.2 | 0.65 ± 0.15 | — | 24.0 ± 0.3 | 2.9 ± 0.6 | 8.7 ± 0.9 | 12 ± 10 | 0.13 ± 0.05 | 1.5 | 1, 20, 32, 33 |
| GR 8 | 14.40 ± 0.15 | 0.37 ± 0.05 | -0.51 [1]; 0.39 [2] | 22.3 ± 0.2 | 0.7 | 1.0 | 47 ± 7 | 0.31 ± 0.04 | 0.24 ± 0.02 | 1, 8, 35, 55–57 |
| Ursa Minor | 10.3 ± 0.4 | 1.3 ± 0.3 | -0.1 ± 0.3 [1] | 25.5 ± 0.5 | 15.8 ± 1.2 | 50.6 ± 3.6 | 53 ± 5 | 0.56 ± 0.05 | 8.0 ± 2.5 | 1, 5, 20, 42, 64 |
| Draco | 10.9 ± 0.3 | 0.95 ± 0.2 | 0.1 ± 0.3 [1] | 25.3 ± 0.5 | 9.0 ± 0.7 | 28.3 ± 2.4 | 82 ± 1 | 0.29 ± 0.01 | 4.5 | 20, 42 |
| Sagittarius | 4.0 ± 0.5 | — | — | 25.4 ± 0.3 | — | >10° | 120 ± 10 | 0.80 ± 0.15 | — | 38–40, 68, 69 |
| SagDIG | 13.5 ± 0.4 | 0.4 ± 0.2 | — | 24.4 ± 0.3 | 0.9 | 1.7 | 90 ± 10 | 0.47 ± 0.10 | — | 41, 54 |
| NGC 6822n | 9.1 ± 0.2 | 0.73 ± 0.05 | 0.04 ± 0.20 [1] | 21.4 ± 0.2 | 2.5 ± 1.0 | 40 ± 10 | 10 ± 5 | 0.47 ± 0.03 | 2.4 ± 0.4 | 1, 80–82 |
| DDO 210 | 14.71 ± 0.15 | 0.15 | -0.18 [1]; 1.11 [4] | — | 0.9 | 1.6 | 100 ± 10 | 0.44 | — | 1, 2, 57, 58 |
| IC 5152 | 11.2 ± 0.2 | 0.34 ± 0.1 | -0.16 [1] | — | 2.3 | 2.8 | 95 ± 10 | 0.18 | — | 1, 42, 62 |
| Tucana | 15.15 ± 0.2 | 0.7 ± 0.1 | 0.55 [2] | 25.05 ± 0.06 | 0.70 ± 0.10 | 3.7 ± 1.2 | 97 ± 2 | 0.48 ± 0.03 | 0.49 ± 0.08 | 43, 44 |
| UKS2323-326 | 13.8 ± 0.4 | 0.4 ± 0.2 | — | 24.6 ± 0.5 | 1.1 | 1.2 | 135 ± 25 | 0.05 ± 0.03 | — | 41, 54 |
| Pegasus | 12.04 ± 0.15 | 0.61 | 0.06 ± 0.06 [1] | — | 2.3 | 3.9 | 125 ± 15 | 0.40 | — | 1, 2, 57, 58 |
|
| ||||||||||
| a Integrated apparent V-band magnitude. | ||||||||||
| b Integrated B-V color. | ||||||||||
| c Other integrated colors. [1]=U–B; [2]=V–R; [3]=V–I; [4]=B–R; [5]=V–K; [6]=J–H; [7]=H–K. | ||||||||||
| d V-band central surface brightness; values in italics are B-band results. | ||||||||||
| e Roman type: core radius in arcmin;
italic type: semi-minor axis dimension to the Holmberg limit of
0,B = 26.5
mag arcsec-2.
| ||||||||||
| f Roman type: tidal radius in arcmin; italic type: semi-major axis dimension to the Holmberg limit as for core radius. | ||||||||||
| g Major-axis position angle, with N = 0° and E = 90°. | ||||||||||
| h The ellipticity of the outer parts of the galaxy defined as e = (1 -b/a), where b = minor axis and a = major axis. | ||||||||||
| i The exponential scale length of the surface-brightness distribution, typically along the major axis. | ||||||||||
| j References: 1, Longo & de Vaucouleurs 1983; 2, de Vaucouleurs et al 1991; 3, deleted in proof; 4, Pierce & Tully 1992; 5, Caldwell et al 1992; 6, Kent 1987; 7, Hodge 1976; 8, de Vaucouleurs et al 1981; 9, Price 1985; 10, Hodge 1963b; 11, Buta & Williams 1995; 12, Lugger et al 1992; 13, Michard & Nieto 1991; 14, Silva & Elston 1994; 15, deleted in proof; 16, Karachentseva et al 1996; 17, Lee 1995a; 18, Schild 1980; 19, van de Rydt et al 1991; 20, Irwin & Hatzidimitriou 1995; 21, Hodge & Smith 1974; 22, de Vaucouleurs & Ables 1968; 23, deleted in proof; 24, Poulain & Nieto 1994; 26, Mateo et al 1993; 27, deleted in proof; 28, Aparicio et al 1997a; 29, Whiting et al 1997; 30, Mateo et al 1991a; 31, deleted in proof; 32, Hodge 1982; 33, Vogt et al 1995; 34, deleted in proof; 35, Hopp & Schulte-Ladbeck 1995; 36, deleted in proof; 37, deleted in proof; 38, Ibata et al 1994; 39, Mateo et al 1995c; 40, Ibata et al 1997; 41, Longmore et al 1982; 42, Longo & de Vaucouleurs 1985; 43, Saviane et al 1996; 44, Lavery & Mighell 1992; 45, deleted in proof; 46, deleted in proof; 47, Mould & Kristian 1990; 48, Hodge et al 1991a; 49, Hodge 1978; 50, Demers et al 1994b; 51, deleted in proof; 52, deleted in proof; 53, Carignan 1985; 54, Longmore et al 1978; 55, Carignan et al 1990; 56, de Vaucouleurs & Moss 1983; 57, Fisher & Tully 1975; 58, Fisher & Tully 1979; 59, Allsopp 1978; 60, Puche et al 1991; 61, Ables & Ables 1977; 62, Sérsic & Cerruti 1979; 63, Hodge 1973; 64, Lee 1996; 65, Peletier 1993; 67, de Vaucouleurs & Freeman 1972; 68, Mateo et al 1996; 69, Fahlman et al 1996; 70, Shostak 1974; 71, de Vaucouleurs & Ables 1965; 72, Hoessel et al 1988; 73, Fitzgibbons 1990; 74, Lee 1993; 75, Ables 1971; 76, Burstein et al 1987; 77, Hodge 1966; 78, Tikhonov & Makarova 1996; 79, Price & Grasdalen 1983; 80, Hodge 1977; 81, Hodge et al 1991b; 82, Kodaira et al 1990; 83, Demers et al 1983; 84, Jobin & Carignan 1990; 85, Hodge 1964. | ||||||||||
| k The major axis position angle varies significantly outward from the galaxy center. | ||||||||||
| l
0 is determined
from the extrapolation of
the outer-surface-brightness profile to r = 0. For NGC 205,
rc is
estimated from surface-brightness profile as the location where the
central surface brightness drops a factor of two below the level
listed in this table. Both M32 and NGC 205 exhibit composite-surface-brightness
profiles; no single King model, exponential profile, or power-law
profile can fit the observed profiles at all radii.
| ||||||||||
| m The outer isophotes of M32 are not truncated according to Kent (1987). | ||||||||||
| n The position angle and e values refer to the inner bar structure. | ||||||||||
| o Aparicio et al (1997a) found that the radial–surface-brightness profile is best fit with two exponential components. The values of rexp refer to the inner and outer components, respectively. The two exponential components meet at a radial distance of 40 arcsec from the center of the galaxy. | ||||||||||
|
| ||||||||||
|
| ||||||||||||
| Galaxy | MVa | MBb | LVc | Rcd |
 0e |
I0f | Total massg | (M/L)0,Vh | (M/L)tot,Vi | MHI/Mtotj | MHI/LBk | (vr /
 )*l |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| name | (mag) | (mag) | (106
L ) |
(pc) |
(M pc-3) |
(L pc-3) |
(106
M) |
(solar) | (solar) |
(M/L) |
||
|
| ||||||||||||
| WLMm,n | -14.5 | -13.9 | 50.2 | 710 | (0.46) | 0.19 | 150 | — | 3.0 | 0.40 | 1.2 | (2.8) |
| NGC 55m-o | -18.0 | -17.5 | 1290 | 875 | (0.26) | 0.10 | 15600 | — | 12 | 0.09 | 0.94 | (10.8) |
| IC 10 | -15.7 | -15.2 | 160 | 475 | 0.047 | 0.63 | 1580 | 0.1 | 9.9 | 0.10 | 0.86 | 5.8 |
| NGC 147 | -15.5 | -14.8 | 131 | 170 | 2.8 | 0.39 | 110 | 7.1 | 0.8 | <0.001 | <0.001 | 0.32 |
| And IIIn | -10.3 | -9.7 | 1.13 | 180 | (0.044) | 0.018 | — | — | — | — | <0.07 | — |
| NGC 185 | -15.5 | -14.7 | 125 | 155 | 4.3 | 1.76 | 130 | 2.5 | 1.0 | 0.001 | 0.001 | 0.08 |
| NGC 205 | -16.6 | -16.0 | 366 | 260 | 5.1 | 0.43 | 740 | 12 | 2.0 | 0.001 | 0.001 | 0.04 |
| M32p | -16.7 | -15.8 | 383 | 635 | 1.0 | 786 | 2120 | 0.0 | 5.6 | <0.001 | <0.009 | 0.51 |
| And In | -11.9 | -11.2 | 4.71 | 375 | (0.023) | 0.009 | — | — | — | — | <0.02 | — |
| Sculptor | -11.1 | -10.4 | 2.15 | 110 | 0.60 | 0.055 | 6.4 | 11 | 3.0 | 0.004 | 0.01 | — |
| LGS 3 | -10.5 | -9.9 | 1.33 | 160 | 0.37 | 0.018 | 13 | 21 | 9.7 | 0.03 | 0.33 | — |
| IC 1613 | -14.7 | -14.2 | 63.6 | 585 | 0.035 | 0.025 | 795 | 1.4 | 12 | 0.07 | 0.81 | 4.3 |
| And IIn | -11.1 | -10.5 | 2.35 | 205 | (0.043) | 0.017 | — | — | — | — | — | — |
| Phoenix | -10.1 | -9.5 | 0.90 | 310 | 0.14 | — | 33 | — | 37 | 0.006 | 0.21 | — |
| Fornax | -13.2 | -12.6 | 15.5 | 460 | 0.086 | 0.018 | 68 | 4.8 | 4.4 | <0.001 | <0.001 | — |
| EGB 0427+63m,n | -12.6 | -11.6 | 9.12 | 85 | (0.33) | 0.13 | — | — | — | — | 2.6 | (3.7) |
| Carina | -9.3 | -8.6 | 0.43 | 210 | 0.17 | 0.006 | 13 | 30 | 31 | <0.001 | <0.002 | — |
| Leo A | -11.4 | -11.3 | 3.03 | 185 | 0.20 | — | 11 | — | 3.5 | 0.72 | 1.6 | — |
| Sextans B | -14.2 | -13.8 | 40.7 | 445 | 0.27 | — | 885 | — | 22 | 0.05 | 0.96 | 2.1 |
| NGC 3109 | -15.7 | -15.2 | 160 | 630 | 0.042 | 0.018 | 6550 | 2.4 | 41 | 0.11 | 3.8 | 6.8 |
| Antlia | -10.8 | -10.2 | 1.73 | 230 | 0.12 | 0.016 | 12 | 7.4 | 7.1 | 0.08 | 0.58 | — |
| Leo I | -11.9 | -11.1 | 4.79 | 215 | 0.28 | 0.092 | 22 | 3.1 | 4.6 | <0.001 | <0.007 | — |
| Sextans A | -14.6 | -14.2 | 55.7 | 700 | 0.022 | 0.011 | 395 | 2.0 | 7.1 | 0.20 | 1.1 | 4.1 |
| Sextans | -9.5 | -8.8 | 0.50 | 335 | 0.065 | 0.002 | 19 | 34 | 39 | <0.001 | <0.001 | — |
| Leo II | -9.6 | -9.0 | 0.58 | 160 | 0.29 | 0.029 | 9.7 | 10 | 17 | <0.001 | <0.02 | — |
| GR 8 | -11.6 | -11.2 | 3.43 | 110 | 1.7 | 0.20 | 7.6 | 8.3 | 2.2 | 0.59 | 1.0 | 1.1 |
| Ursa Minor | -8.9 | -7.6 | 0.29 | 200 | 0.35 | 0.006 | 23 | 60 | 79 | <0.002 | <0.25 | 0.48 |
| Draco | -8.8 | -7.8 | 0.26 | 180 | 0.46 | 0.008 | 22 | 58 | 84 | <0.001 | <0.02 | — |
| Sagittariusr | -13.4 | -12.8 | 18.1 | 550 | 0.030 | — | — | 22 | 52 | <0.001 | <0.001 | <0.18 |
| SagDIGq | -12.3 | -12.1 | 6.85 | 125 | 0.58 | 0.044 | 9.6 | 13 | 1.4 | 9.2 | 8.6 | — |
| NGC 6822m,n | -15.2 | -14.7 | 94.4 | 260 | (0.97) | 0.39 | 1640 | — | 17 | 0.08 | 1.2 | (6.4) |
| DDO 210 | -10.0 | -9.9 | 0.81 | 95 | 0.84 | — | 5.4 | — | 6.7 | 0.35 | 1.4 | — |
| IC 5152m | -14.8 | -14.5 | 70.3 | 390 | 0.000 | — | 400 | — | 5.7 | 0.15 | 0.64 | (4.7) |
| Tucanan | -9.6 | -8.9 | 0.55 | 130 | (0.032) | 0.013 | — | — | — | — | <0.18 | — |
| UKS2323-326n | -12.0 | -11.7 | 5.25 | 150 | (0.051) | 0.020 | — | — | — | — | 0.90 | — |
| Pegasus | -12.9 | -12.3 | 12.0 | 280 | 0.16 | — | 58 | — | 4.8 | 0.09 | 0.44 | 1.7 |
|
| ||||||||||||
| a Integrated V-band absolute magnitude. | ||||||||||||
| b Integrated B-band absolute magnitude. | ||||||||||||
| c Visual luminosity in units of
106 L.
| ||||||||||||
| d "Core" radius in parsecs, corresponding to the observed core radius, Rc, when available, or 1.25 xrexp if Rc is not measured but the exponential scale length is measured (compare with Bender et al 1991; the average of Rc/rexp for the 18 galaxies in Table 3 with both radii is 1.27 ± 0.12), or a/3 if neither Rc nor rexp is available, where a is the observed Holmberg semi-major axis as defined in Table 3 | ||||||||||||
| e The central mass density in
M pc-3,
here approximated as
0 = 166
02 /
Rc, where
0 is the central
velocity dispersion in kms-1 and Rc is in
parsecs; this is computed only for
systems that are pressure supported (see Mateo et al 1991b for
details). For rotating LG dwarfs, the central mass density
is dominated by the visible material and has been approximated as 2.5
I0, where I0 is defined in footnote
f (also see footnote n).
| ||||||||||||
| f The central luminosity density in
L pc-3,
taken as I0 = S0 / 2
Rc, where S0 is the central surface
brightness expressed in units of
L pc-2,
and Rc is in pc
(see Mateo et al 1991b for details).
| ||||||||||||
| g The total mass; if a central velocity
dispersion is known and exceeds the rotational
velocity, then total mass Mtot = 167 ß
Rc
02,
where ß is a scaling factor for King profiles taken here to be 8.0,
appropriate for low-concentration King models. If
vrot >
0, then
Mtot = Rrot
vrot2 / G, where
vrot is the rotational
velocity - corrected for the galaxy inclination - at the projected
distance Rrot from the Galaxy center.
| ||||||||||||
| h The central V-band mass-to-light
ratio, defined as
0 /
I0, in solar units.
| ||||||||||||
| i The integrated V-band mass-to-light ratio defined as Mtot / LV, in solar units. | ||||||||||||
| j The ratio of integrated HI mass to the total mass, Mtot (see footnote g). | ||||||||||||
| k The ratio of the integrated HI mass and the blue luminosity, in solar B-band units. | ||||||||||||
| l The parameter (vrot /
)* is simply
(vrot /
0) for dIrr
systems or (vrot /
0)(e /
(1 - e))-1/2, where e is the
ellipticity, for ellipsoidal systems (Bender et al 1991). See footnote m
for details about values in parentheses.
| ||||||||||||
| m The interstellar medium (ISM) velocity dispersion of these galaxies is assumed to be 8 km s-1 (see Table 7). | ||||||||||||
| n The central mass density has not been
determined
kinematically for these galaxies. Instead, we assume
0 = 2.5
I0.
| ||||||||||||
| o The results given here for NGC 55 are especially uncertain because of internal reddening that affects the inferred total luminosity and central luminosity density. See Puche et al (1991) for a complete discussion and for more precise estimates of the luminosity and mass of this galaxy. Note that these authors assumed a distance of 1.6 Mpc for NGC 55, in contrast to the distance of 1.48 ± 0.15 Mpc used here (Table 2). | ||||||||||||
| p Owing to the possible presence of a massive central black hole, the nuclear kinematics are too complex for the simple analysis used here. See Kormendy & Richstone (1995) for details. | ||||||||||||
q MHI /
Mtot 
1.0. This indicates an error
in the distance (MHI / Mtot
 D, where D is the
distance), a misinterpretation of or error in the kinematic data, an
error in the HI flux measurement, or some combination of these.
| ||||||||||||
| r Sagittarius probably violates the
assumption of
equilibrium that is implicitly adopted in the analysis used here and
its structural parameters remain highly uncertain. The mass and
M / L ratios given here are taken directly from Ibata et al (1997).
Rc and
0 are
also taken from this source.
| ||||||||||||
|
| ||||||||||||
|
Figure 4. The differential luminosity
function of galaxies in the Local Group based on the data in
Table 4. The upper and lower
panels show the V- and B-band luminosity functions (LFs),
respectively. The lower panel also shows a best-fitting
Schechter (1976) function
( |
Figure 5 is the color-magnitude diagram of the Local Group based on the integrated V-band absolute magnitudes (MV0) and (B-V)0 colors. The giant/dIrr galaxies are segregated from the early-type galaxies (denoted as a dotted line in Figure 5). EGB 0427+63 is the only dIrr that lies redward of this boundary, but its photometric properties and reddening are poorly known (Karachentseva et al 1996). The transition galaxies - so named in Table 1 solely on the basis of their morphological properties - are located close to but on both sides of the dIrr-early-type dividing line. NGC 205 is a luminous dSph system that contains a bright central region of recent star formation and has a luminous blue nucleus (Hodge 1973, Price 1985); it is located on the dIrr side of the dividing line in Figure 5. The smaller region of young stars in NGC 185 (Hodge 1963b) has a negligible effect on that galaxy's integrated colors (Price 1985).
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Figure 5. The MV0-(B-V)0 color-magnitude diagram of Local Group galaxies based on the data from Tables 2 and 3. The diagonal dashed line separates galaxies classified as Spirals, or Irregular systems (filled squares; Table 1), and dSph or Elliptical systems (open circles). Five "transition" objects, Antlia, LGS 3, Phoenix, Pegasus, and DDO 210, are plotted as filled triangles; NGC 205 is plotted as a filled circle. |
In the optical, the structure of Local Group dIrr galaxies is dominated by star-forming complexes and OB associations with typical diameters of 200-300 pc (Fisher & Tully 1979, Hodge et al 1991a, b; see the "Images" references in Table 1). These clumps are usually not found near the optical center of symmetry of the galaxies. NGC 3109 - one of the most luminous dIrr galaxies in the sample - shows clear evidence for spiral structure underlying a patchy morphology (Demers et al 1985, Sandage & Carlson 1988). In all suitably studied Local Group dIrr systems, the clumpy young stellar populations are superimposed on a more extended, smoother, and symmetric distribution of older stars (Hodge et al 1991a, b;, Minniti & Zijlstra 1996). Either dynamical effects smooth out the structures with time, or else the star-formation regions migrate through individual galaxies, eventually forming a more symmetric sheet of old stars (Skillman & Bender 1995, Hunter & Plummer 1996, Dohm-Palmer et al 1997).
The early-type dwarfs of the Local Group are dominated by a symmetric spheroidal component (Hodge 1971, Irwin & Hatzidimitriou 1995), with occasional instances of superimposed concentrations of relatively young stars (NGC 185 and NGC 205: Hodge 1963b, 1973;, Price & Grasdalen 1983;, Price 1985;, Lee et al 1993b). Of interest, the star-forming regions in NGC 185 and NGC 205 are similar in size to those seen in dIrr galaxies, but these young stars are found near, though slightly offset from, the centers of the galaxies. Demers et al (1994b, 1995) carefully searched for substructure in a number of dSph systems but found weak evidence for such structure only in Ursa Minor (Olszewski & Aaronson 1985).
Only three Local Group dwarfs contain nuclei: NGC 205, Sagittarius, and M32. The latter is now widely believed to contain a massive central black hole (Kormendy & Richstone 1995). The nucleus of NGC 205 is extremely blue (Price & Grasdalen 1983, Lee 1996), dynamically colder than the surrounding galaxy envelope (Carter & Sadler 1990), and has a spectrum dominated by young stars (Bica et al 1990, Jones et al 1996). The existence of a nucleus of Sagittarius - the globular cluster M54 - is somewhat controversial. Da Costa & Armandroff (1995) argued that the velocity dispersion and metallicity of M54 are incompatible with its identification as a normal dSph nucleus. However, M54 is the second most luminous globular cluster in the entire Milky Way, nearly as luminous as the nucleus of NGC 205 (Peterson 1993); exhibits an internal abundance dispersion (Sarajedini & Layden 1995); and is located close to the center of symmetry of Sagittarius (Ibata et al 1994, 1997). Such an unusual object seems unlikely to have been merely an isolated globular cluster in a dSph galaxy such as Sagittarius.
The observed structural parameters of Local Group dwarfs are listed in Table 3, including ellipticity, major-axis position angle, King core and tidal radii, Holmberg radii, and exponential scale lengths. The corresponding derived structural parameters are listed in Table 4. Historically, the surface-brightness profiles for the dIrr galaxies are fit with exponential profiles, whereas for early-type systems, King profiles are preferred. Many authors have noted that both profiles produce acceptable fits to the red populations of dIrr and dSph systems (Eskridge 1988a, c;, Hodge et al 1991a, b;, Irwin & Hatzidimitriou 1995). Aparicio et al (1997a) found that the best fit to the surface-brightness profile of Antlia requires two exponential profiles (Table 3). Sérsic (1968) profiles may be better suited to describing these varied types of surface-brightness profiles with only a single additional parameter (Prugniel & Simien 1997). Sagittarius and NGC 205 show highly elongated or otherwise disturbed outer structures, indicative of strong interactions with the Milky Way and M31, respectively. All galaxies with exponential scale lengths > 500 pc are dIrr systems, while 90% with smaller scale lengths are early-type systems (James 1991).
= -1.16,
M*,B = -21.42) and the empirical LF that