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4. LOCAL GROUP

The Local Group of galaxies is often arbitrarily defined by a radius Delta appeq 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

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

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) leq -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.

TABLE 1a
LOCAL GROUP

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.

TABLE 1b
LOCAL GROUP

Object µ AB µ0 Delta 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 Delta 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 Delta (Mpc) on OX, OY, OZ axes of supergalactic rectangular coordinate system.

The total mass (of galaxies) in the group MT appeq 6.5 x 1011 Msun is not much larger than the combined masses of M31 and the Galaxy (5 x 1011 Msun); 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 Msun) 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 rhobarG 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 appeq 20 (B and V). The total mass of neutral hydrogen in galaxies is MH appeq 1.0 x 1010 Msun, and the ratios of hydrogen to total mass and of hydrogen to luminosity are h = MH / MT appeq 0.015 and g appeq MH / LB appeq 0.3.

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