The Local Group of galaxies is often arbitrarily defined by a radius
1.0 Mpc from the Galaxy or, possibly, from the center of gravity
of the Galaxy and M31, the two giant members. The Local Group includes
two average spirals M33 and the Large Magellanic Cloud, over a
half-dozen dwarf Magellanic irregulars (Small Cloud, NGC 6822, IC 1613, IC 10, A2359, A0956, A0957,
A1009), and at least a dozen dwarf
ellipticals (NGC 147, NGC 185, NGC 205, NGC 221, A0058, A0237, A1003,
A1006, A1111, A1127, A1719, and other poorly known
globular-like
systems). Detailed information on the Local Group members is given in
Tables 1a and 1b.
Additional S, I members hidden by galactic absorption might be discovered by their 21-cm emission. An apparent obscuration patch in Microscopium, suggested as a possible intergalactic dark cloud within the Local Group (Hoffmeister 1962), has not been confirmed.
The distance moduli in Table 1b based on primary and secondary criteria (de Vaucouleurs 1955; van den Bergh 1960c; Sandage 1961; and chap. 12 by van den Bergh) define the distance scale used in the present chapter.
Figure 1 shows the apparent distribution of Local Group members in supergalactic coordinates. There is only a slight concentration toward the supergalactic equator, as might be expected for very close objects.
|
Figure 1. Apparent distribution of Local Group members in supergalactic coordinates. |
Figure 2 is a map of the Local Group projected onto the supergalactic plane; note the strong ``subclustering'' tendency around two dominant multiplets (Galaxy-LMC-SMC triplet + dE satellites; M31-M33 pair + dE satellites). The overall diameter of the group is about 2.0 x 1.5 Mpc in the SG plane and 1.0 Mpc at right angles to it (0.5 Mpc if A0956, A1009 are not members).
|
Figure 2. Map of Local Group projected onto supergalactic plane. |
The Local Group is a typical loose group without central condensation. Except for the satellite dE systems it includes only spirals of type Sb and later and Magellanic irregulars. The distribution of total absolute magnitudes (assuming MB = -18.8 for the Galaxy, after Gyllenberg 1937) is as follows:
MT(B)  | -20 | -19 | -18 | -17 | -16 | -15 | -14 | |
| n | 1 | 0 | 3 | 1 | 1 | 2 | 4 | |
| N | 1 | 1 | 4 | 5 | 6 | 8 | 12 |
for a total of 12 members brighter than MT(B) = - 14; another dozen or more with less precisely known magnitudes are in the range - 14 to - 10. For further discussion of dwarfs in the Local Group see Section 8.
| lII | SGL | Type | B(0) | log D | log D(0) | D(0) | |
| Object | bII | SGB | L | V0 | log R | (') | (kpc) |
| (1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) |
| The Galaxy | Sbc? | (12) | |||||
| NGC 0147, D3 | 119.8 | 344.0 | E5p | 11.48 | 0.78 | 0.70 | 1.0 |
| -14.3 | 15.3 | 0.22 | |||||
| NGC 0185 | 120.8 | 344.0 | E+3p | 10.92 | 0.73 | 0.70 | 1.0 |
| -14.5 | 14.3 | -10 | 0.07 | ||||
| NGC 0205 | 120.7 | 337.2 | E+5p | 9.71 | 1.00 | 0.88 | 1.5 |
| -21.1 | 13.1 | -6 | 0.29 | ||||
| NGC 0221 | 121.2 | 336.5 | E2 | 9.39 | 0.56 | 0.51 | 0.6 |
| -22.0 | 12.5 | +17 | 0.12 | ||||
| NGC 0224, M31 | 121.2 | 336.9 | SA(s)b | 4.61 | 2.20 | 2.00 | 20.0 |
| -21.6 | 12.6 | 2 | -68 | 0.49 | |||
| NGC 0598, M33 | 133.6 | 329.2 | SA(s)cd | 6.47 | 1.79 | 1.70 | 10.5 |
| -31.3 | -0.1 | 4 | -11 | 0.21 | |||
| NGC 6822, D209 | 25.4 | 229.8 | IB(s)m | 9.49 | 1.22 | 1.16 | 2.1 |
| -18.4 | 57.1 | 8 | +73 | 0.13 | |||
| IC 0010 | 119.0 | 355.1 | SB(s)m | 12.5: | 0.60 | 0.54 | 1.3 |
| -3.3 | 17.9 | 7 | -92 | 0.15 | |||
| IC 1613, D8 | 129.9 | 299.9 | Im | 11.02 | 1.05 | 1.02 | 2.0 |
| -60.6 | -1.8 | -129 | 0.07 | ||||
| SMC, A0051 | 302.8 | 224.9 | IB(s)m | 3.1: | 2.4: | 2.3: | 2.9 |
| -44.3 | -14.8 | -13 | 0.30 | ||||
| Scl, A0058 | 287.8 | 264.7 | dE | ||||
| -83.2 | -9.6 | ||||||
| For, A0237 | 237.3 | 266.0 | dE | ||||
| -65.7 | -30.2 | -70 | |||||
| LMC, A0524 | 280.5 | 216.5 | SB(s)m | 1.2: | 2.7: | 2.65: | 6.5 |
| -32.9 | -34.1 | +16 | 0.07 | ||||
| Leo A, A0956, D69 | 196.9 | 70.6 | Im | ||||
| 52.4 | -25.8 | ||||||
| Sex B, A0957, D70 | 233.2 | 96.1 | Im | ||||
| 43.8 | -39.6 | ||||||
| Sex C, A1003 | 240.1 | 103.2 | dE | ||||
| 41.9 | -40.4 | ||||||
| Leo I, A1006, D74 | 226.0 | 89.6 | E4 | ||||
| 49.1 | -34.6 | ||||||
| Sex A, A1009, D75 | 246.2 | 109.8 | IBm | 0.73 | 0.70 | 1.5 | |
| 39.9 | -40.6 | +118 | 0.09 | ||||
| Leo II, A1111, D93 | 220.1 | 87.8 | dE | ||||
| 67.2 | -16.3 | ||||||
| UMa, A1127 | 202.3 | 83.0 | dE | ||||
| 71.8 | -10.2 | ||||||
| UMi, A1508, D199 | 105.1 | 48.4 | dE | ||||
| 44.8 | 27.1 | ||||||
| Ser, A1513 | 0.9 | 128.1 | dE | ||||
| 45.9 | 33.7 | ||||||
| Dra, A1719, D208 | 86.4 | 44.5 | dE | ||||
| 34.7 | 44.2 | ||||||
| Cap, A2144* | 30.5 | 257.1 | dE | 12.37 | 0.68 | 0.68 | |
| -47.7 | 34.0 | 0.00 | |||||
| Peg, A2304 | 87.1 | 303.2 | dE | ||||
| -42.7 | 29.1 | ||||||
| WLM, A2359, D221 | 75.7 | 278.5 | Im | 0.8: | 0.7: | 1.3 | |
| -73.6 | 8.1 | +2 | 0.3: | ||||
EXPLANATIONS OF COLUMNS.- Col. (1), identification in NGC, IC, BGC, DDO, or Anon. Col. (2), new galactic coordinates lII, bIII. Col. (3), supergalactic coordinates SGL, SGB (cf. BGC). Col. (4), revised type and DDO luminosity class coded as in BGC. Col. (5), B(0) = B mag within standard ``face-on'' diameter D(0) (cf. BGC). V0 = corrected redshift. Col. (6), log D = log major diameter in standard system (D in min of arc). log R = log D/d = log axis ratio. Col. (7), log D(0) = log ``face-on'' major diameter (min of arc). Col. (8), D(0) = ``face-on'' diameter in kpc.
* May be a globular cluster.
| Object | µ | AB | µ0 | | BT | -MT | X | Y | Z |
| (1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9) | (10) |
| Galaxy | 0.01 | (18.8) | 0 | 0 | 0 | ||||
| NGC 0147 | 25.0: | 0.8 | 24.2: | 0.69 | 10.6 | -14.4 | +0.64 | -0.18 | +0.18 |
| NGC 0185 | 25.0: | 0.8 | 24.2: | 0.69 | 10.3 | -14.7 | +0.64 | -0.18 | +0.17 |
| NGC 0205 | 24.7 | 0.5 | 24.2 | 0.69 | 8.9 | -15.8 | +0.62 | -0.26 | +0.16 |
| NGC 0221 | 24.7 | 0.5 | 24.2 | 0.69 | 9.1 | -15.6 | +0.62 | -0.27 | +0.15 |
| NGC 0224 | 24.7 | 0.5 | 24.2 | 0.69 | 4.4 | -20.3 | +0.62 | -0.26 | +0.15 |
| NGC 0598 | 24.6 | 0.3 | 24.3: | 0.72 | 6.3 | -18.3 | +0.62 | -0.37 | 0.00 |
| NGC 6822 | 24.1 | 0.6 | 23.5 | 0.50 | 9.3 | -14.8 | -0.18 | -0.21 | +0.42 |
| IC 0010 | 29.0 | 3.5: | 25.5: | 1.26 | 11.7 | -17.3 | +1.20 | -0.10 | +0.39 |
| IC 1613 | 24.3 | 0.2 | 24.1 | 0.66 | 10.1 | -14.2 | +0.33 | -0.57 | -0.02 |
| SMC | 18.8 | 0.3 | 18.5 | 0.05 | 2.8 | -16.0 | -0.03 | -0.03 | -0.01 |
| Sculptor | 20.4: | 0.2 | 20.2: | 0.11 | 9.2 | -11.2 | -0.01 | -0.11 | -0.02 |
| Fornax | 22.0: | 0.2 | 21.8: | 0.23 | 9.1 | -12.9 | -0.01 | -0.20 | -0.12 |
| LMC | 18.7 | 0.4 | 18.3 | 0.05 | 0.6 | -18.1 | -0.03 | -0.02 | -0.03 |
| Leo A | 25.4: | 0.2 | 25.2: | 1.10 | 13.1 | -12.3 | +0.33 | +0.93 | -0.48 |
| Sex B | 0.3 | 12.0 | |||||||
| Sex C | 21.0: | 0.3 | 20.7 | 0.14 | -0.02 | +0.10 | -0.09 | ||
| Leo I | 22.0: | 0.2 | 21.8: | 0.23 | 11.3 | -10.7 | 0.00 | +0.19 | -0.13 |
| Sex A | 25.3: | 0.3 | 25.0: | 1.00 | 11.7 | -13.6 | -0.26 | +0.71 | -0.63 |
| Leo II | 22.0: | 0.2 | 21.8: | 0.23 | 12.9 | -09.1 | +0.01 | +0.22 | -0.06 |
| UMa | 20.5: | 0.2 | 20.3: | 0.12 | +0.01 | +0.12 | -0.02 | ||
| UMi | 19.6: | 0.3 | 19.3: | 0.08 | +0.05 | +0.05 | +0.04 | ||
| Ser | 0.3 | ||||||||
| Dra | 19.4: | 0.4 | 19.0 | 0.06 | +0.03 | +0.03 | +0.04 | ||
| Cap | 0.3 | ||||||||
| Peg | 21.5: | 0.3 | 21.2: | 0.17 | +0.08 | -0.12 | +0.08 | ||
| WLM | 24.9: | 0.2 | 24.7: | 0.87 | 11.2 | -13.7 | +0.13 | -0.85 | +0.12 |
EXPLANATIONS OF COLUMNS.-
Col. (1), identification.
Col. (2), apparent modulus µ in B system.
Col. (3), Galactic absorption AB in B system.
Col. (4), corrected (geometric) apparent modulus µ0.
Col. (5), distance in megaparsecs.
Col. (6), apparent total (asymptotic) magnitude BT in
B system.
Col. (7), absolute magnitude -MT in B system.
Cols. (8), (9), (10), projection of (Mpc) on OX, OY, OZ axes of supergalactic
rectangular coordinate system.
The total mass (of galaxies) in the group MT 6.5 x 1011
M
is not
much larger than the combined masses of M31 and the Galaxy (5 x 1011
M); M33 and the
Magellanic irregulars LMC, SMC, NGC 6822, IC 10, etc.,
add only 30 percent, and the combined mass of the ellipticals is
probably negligible (< 1010 M) even if their space density is
as high as 100 per Mpc3. For a volume of the order of 2
Mpc3, the smoothed
mean density is 
G 0.5 x 10-28 g
cm-3. The total absolute magnitude of
the group is MT - 20.9 (B) or - 21.6 (V)
with some uncertainty due
to the indirect estimate for our Galaxy; the average mass-luminosity
ratio is f = MT / L
20 (B and
V). The total mass of neutral hydrogen
in galaxies is MH
1.0 x 1010
M, and the
ratios of hydrogen to total
mass and of hydrogen to luminosity are h = MH /
MT
0.015 and g
MH / LB
0.3.