The Local Group of Galaxies

This is "our" group of galaxies. It was first recognized by Hubble, in the time of the first distance determinations and redshift measurements (see, e.g., his The Realm of the Nebulae).

Messier objects: The Andromeda Galaxy M31 and its satellites M32 and M110, as well as the Triangulum Galaxy M33.

Other members (over 30 in all) include our Milky Way Galaxy, the Large and the Small Magellanic Cloud (LMC and SMC), which have been known before the invention of the telescope, as well as several smaller galaxies which were discovered more recently. These galaxies are spread in a volume of nearly 10 million light years diameter, centered somewhere between the Milky Way and M31. Membership is not certain for all these galaxies, and there are possible other candidate members.

Of the Local Group member galaxies, the Milky Way and M31 are by for the most massive, and therefore dominant members. Each of these two giant spirals has accumulated a system of satellite galaxies, where

The third-largest galaxy, the Triangulum spiral M33, may or may not be an outlying gravitationally bound companion of M31, but has itself probably the dwarf LGS 3 as a satellite.

The other members cannot be assigned to one of the main subgroups, and float quite alone in the gravitational field of the giant group members. The substructures of the group are probably not stable. Observations and calculations suggest that the group is highly dynamic and has changed significantly in the past: The galaxies around the large elliptical Maffei 1 have probably been once part of our galaxy group.

As this shows, the Local Group is not isolated, but in gravitional interaction, and member exchange, with the nearest surrounding groups, notably:

In the future, interaction between the member galaxies and with the cosmic neighborhood will continue to change the Local Group. Some astronomers speculate that the two large spirals, our Milky Way and the Andromeda Galaxy, may perhaps collide and merge in some distant future, to form a giant elliptical (see a summary of a possible scenario). In addition, there is evidence that our nearest big cluster of galaxies, the Virgo Cluster, will probably stop our cosmological recession away from it, accelerate the Local Group toward itself so that it will finally fall and merge into this huge cluster of galaxies, see our Virgo Cluster & Local Group page.

A table of the currently known Local Group member galaxies follows. While the positions are known very acurately, the distances are only very vaguely known for some members, and the sources even disagree for the most prominent members such as M31 and M33. If interested, look at our compilation of distances from various sources. Please note that this table is currently under review, as new data (Hipparcos distances, discovery of new members) keep our knowledge in flow.

Local Group Member Galaxies
Galaxy RA Dec Type m_v dim RV Dist
WLM 00:02.0 -15:28 IB(s) IV-V 10.9 12 x 4 - 42 4200
IC 10 00:20.4 +59:18 KBm? 10.3 7.3 x 6.4 - 83 4200:
Cet dw 00:26.1 -11:02 dSph/E4 3000:
NGC 147 00:33.2 +48:31 dE5 pec 9.5 15.0 x 9.4 + 89 2400:
And III 00:35.4 +36:31 dSph/E2 13.5p 2900:
NGC 185 00:39.0 +84:20 dE3 pec 9.2 14.5 x 12.5 + 39 2500:
M110 00:41.3 +41:41 E5 pec 8.5 19.5 x 12.5 - 1 2900
And IV 00:42.5 +40:34 Irr ? 2900:
M 32 00:42.7 +40:52 E2 (cE2) 8.1 11.0 x 7.3 + 35 2900
M 31 00:42.7 +41:16 SA(s)b I-II 3.4 185.0 x 75.0 - 59 2900
And I 00:45.7 +38:00 dSph/E3 pec ? 13.2 2900:
SMC 00:51.7 -73:14 SB(s)m pec 2.3 280 x 160 - 30 210
Scl dw 01:00.0 -33:42 dSph/E3 pec 10.5p +162 300:
LGS 3 01:03.8 +21:53 Irr 15.4p 2 3000:
IC 1613 01:05.1 +02:08 IAB(s)m V 9.2 20.0 x 18.5 -125 2900:
And V 01:10.3 +47:38 dSph 2900:
And II 01:16.4 +33:27 dSph/E0 13: 2900:
M 33 01:33.9 +30:39 SA(s)cd II-III 5.7 67.0 x 41.5 + 3 3000
Phe dw 01:51.1 -44:27 Irr 1600:
For dw 02:39.9 -34:32 dSph/E2 8.1 12.0 x 10.2 530:
UGCA 86 03:59.9 +67:08 Irr ? S0 ? +262
UGCA 92 04:27.4 +63:30 Irr ? S0 ? + 66 3000:
LMC 05:19.7 -68:57 SB(s)m 0.1 650 x 550 + 13 179
Car dw 06:14.6 -50:58 dSph/E3 20.9p 300:
Leo A 09:59.4 +30:45 IBm V 7000:
Sex B 10:00.0 +05:20 Ir+ IV-V 4000:
NGC 3109 10:03.1 -26:09 Ir+ IV-V 10. p 16.0 x 2.9 +131 4100:
Ant dw 10:04.1 -27:20 dSph/E3 14.8 4100:
Leo I 10:08.5 +12:18 dE3 9.8 880:
Sex A 10:11.1 -04:43 Ir+ V 4000:
Sex dw 10:13.2 -01:37 dSph/E3 300:
Leo II 11:13.5 +22:10 dSph/E0 pec 800:
GR 8 12:58.7 +14:13 Im V 14.5 1.2 x 1.1 +165 5000:
UMi dw 15:08.8 +67:12 dSph/E4 10.9 41.0 x 26.0 240:
Dra dw 17:20.1 +57:55 dSph/E0 pec 9.9 51.0 x 31.0 280
Milky Way 17:45.6 -28:56 SAB(s)bc I-II ? - 0 28
SagDEG 18:55 -30:30 dSph/E7 80
SagDIG 19:30.1 -17:42 IB(s)m V 15: 2000:
NGC 6822 19:44.9 -14:49 IB(s)m IV-V 9 + 66 1700:
Aqr dw 20:46.8 -12:51 Im V 13.9 2.3 x 1.2 2000:
IC 5152 22:06.1 -51:17 IAB(s)m IV 10.6 4.9 x 3.0 + 30 3000:
Tuc dw 22:41.7 -64:25 dSph/E5 3000:
And VII 23:27.8 +50:35 dSph 2900:
Peg dw 23:28.6 +14:45 Im V 6000:
And VI 23:51.7 +24:36 dSph 2900:

Below we list some nearby but probably non-member field galaxies (which are also not members of one of the neighboring groups listed above), in the same format as above:

Nearby Non-Member (?) Galaxies
Galaxy RA Dec Type m_v dim RV Dist
NGC 404 01:09.4 +35:43 E0 10.1 4.4 x 4.1 +178 8000:
Argo dw 07:05.3 -58:31 Irr 12000:
2318-42 23:20.8 -41:44 Irr
UKS2323-32623:26.5 -32:23 Irr 4500:

Key:

RA, Dec:
Right Ascension and Declination for epoch 2000.0
Type:
Classification type from Tom Polakis' article, or elsewhere
m_v
Apparent visual brightness in Magnitudes. A "p" indicates that only photographic magnitudes are available and given.
dim
Angular dimension in arc minutes
RV:
Radial velocity wrt galactic center in km/sec
Dist:
Distance in 1000 light years (kly)
A colon following a number indicates this value is uncertain.

More data of and remarks on Local Group Galaxies
Galaxy Other name R Diam Mass
WLM (*) 8 ?
IC 10
Cet dw
NGC 147 DDO 3
And III
NGC 185
M 110 NGC 205 17 10000
And IV (*)
M 32 NGC 221 8 3000
M 31 NGC 224 195 400000
And I
SMC NGC 292 25 6000
Scl dw E351-G30
LGS 3 Psc dw (*)
IC 1613 DDO 8
And V
And II
M 33 NGC 598 60 25000
Phe dw (I)
For dw E356-G04
UGCA 86 (?)
UGCA 92 EGB0427+63 (I)
LMC 30 20000
Car dw E206-G220
Leo A Leo III (?)
Sex B DDO 70 (I)
NGC 3109 (I)
Ant dw (I) 5
Leo I Regulus G.
Sex A DDO 75
Sex dw
Leo II Leo B
GR 8 DDO 155 (?)
UMi dw DDO 199
Dra dw DDO 208
Milky Way 100 750000
SagDEG Sgr dShp (*)
SagDIG Sgr dw (*)
NGC 6822 Barnard's
Aqr dw DDO 210 (?)
IC 5152 (?)
Tuc dw
And VII Cas dSph
Peg dw DDO 216
And VI Peg dSph

Key:

R:
Remarks: (*), (?) and (I); see below
Diam:
Diameter in 1000 light years (kly)
Mass:
Mass in million solar masses
Remarks (*): Galaxies marked with "(?)" may be non-members as they are not in the list of Irwin et.al. 1997 (this list has 35 Local Group members), in particular: Galaxies marked with "(I)" are newly taken into the list from Irwin's list (besides the newly discovered Antlia dwarf, these are the Phoenix dwarf, UGCA 92, Sextans B, and NGC 3109.

Various sources have listed other Local Group candidates which however have not been confirmed:

Galaxy listed Rem
1010-27 McGraw (= Ant dw ?)
DDO 187 McGraw
UMa dw LGAA
Sex C LGAA

Sources:

McGraw:
McGraw-Hill Dictionary of Astronomy, by Sybil P. Parker (ed), p. 242
LGAA:
Le Grand Atlas de L'Astronomie, Encyclopedia Universalis, 1986. German edition: "Der große JRO Atlas der Astronomie"
As our Milky Way obscures parts of the sky, there is still a steady flow of new discoveries of galaxies, in low galactic latitudes (i.e., near the equatorial plane of our galaxy, where the obscuring dust is most dense). Also, some of the galaxies are of extreme low surface brightness, and it was only recently possible to detect them. Therefore, it must be expected that more Local Group members exist, obscurred by dust, or extremely faint, and are still waiting for their detection somewhen in the future.

Jeff Bondono has compiled a comprehensive list of Local Group member and member candidate galaxies.

Links

You are invited to contribute more links which are of interest for this page; please email the maintainer.

References


This page has been used in the sci.astro FAQ


Hartmut Frommert
Christine Kronberg
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Last Modification: December 31, 1999