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 far the most massive, and therefore dominant members. Each of these two giant spirals has accumulated a system of satellite galaxies, where

the system of the Milky Way contains many (nearby) dwarf galaxies, spread all over the sky, namely Sag DEG, LMC, SMC, the recently discovered and widely disrupted Canis Major Dwarf, and the dwarf galaxies in Ursa Minor, Draco, Carina, Sextans (dwarf), Sculptor, Fornax, Leo I, Leo II, and perhaps the Phoenix Dwarf, and Leo A (which may alternatively belong to the NGC 3109 subgroup), as well as probably 10 or 11 newly discovered known dwarf spheroidal galaxies.
the system of the Andromeda galaxy is seen from outside, and thus grouped around its main galaxy M31 in Andromeda, containing bright nearby M32 and M110 as well as fainter and more far-out NGCs 147 and 185, the very faint systems And I, And II, And III and possibly And IV (but for this one see remark below), And V, And VI (Pegasus dwarf), And VII (Cassiopeia dwarf), And VIII, And IX, and And X, as well as probably the newly discovered dwarf spheroidals And XI, And XII, And XIII, and And XIV.
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. Also, IC 1613 may perhaps be a member of the M31 subgroup, as may be the doubtful members UGCA 86 and UGCA 92.
There is a third potential subgroup around irregular galaxy NGC 3109, which contains Sextans A, Sextans B, the Antlia Dwarf and perhaps Leo A (but that one may belong to the Milky Way subgroup).
The other members, mostly dwarf irregulars, cannot be assigned to one of these main subgroups, and float quite alone in the gravitational field of the giant group members. They tend to form a vast cloud, the Local Group Cloud.

Only one dwarf galaxy, GR 8, seems to be so isolated that it doesn't belong to any of these substructures.

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:

the Maffei 1 group, which besides the giant elliptical galaxy Maffei 1 also contains smaller Maffei 2, and is associated with nearby IC 342 and a number of smaller galaxies. Highly obscurred by dark dust near the Milky Way's equatorial plane.
the Sculptor Group or South Polar Group (with members situated around the South Galactic pole), dominated by NGC 253
the M81 group
the M83 group

The next more distant galaxy groups, the NGC 4244 group and the CVn I Cloud (with M94 and M64) are probably too remote to have interacted significantly with the Local Group in the past.

It is of interest that of the subgroups of the Local Group mentioned above, the NGC 3109 group tends to the general direction of the Maffei 1 group and the M81 group, while the Local Group Cloud tends to the general direction of the Sculptor Group.

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  3400
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          14.4                       2800 
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    4.5 x 3.0         2900: 
NGC 185     00:39.0 +84:20 dE3 pec           9.2    14.5 x 12.5  + 39  2300 
M110        00:41.3 +41:41 E5 pec            8.5    19.5 x 12.5  -  1  2900  
And VIII    00:42.3 +40:37 dSph pec          9.1    45   x 10    -250  2700:
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     2.5 x 2.5         2900: 
SMC         00:51.7 -73:14 SB(s)m pec        2.3     280 x 160   - 30   210  
And IX      00:52.9 +43:12 dSph             16.2         5             2900:
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 X       01:06.6 +44:48 dSph             16.1         7             2900:
And V       01:10.3 +47:38 dSph             15.9                       2900: 
And II      01:16.4 +33:27 dSph/E0          13.5     3.6 x 2.5         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              13.1     4.9 x 4.1         1600: 
For dw      02:39.9 -34:32 dSph/E2           8.1    12.0 x 10.2         500 
UGCA 92     04:32.0 +63:36 Irr ? S0 ?       13.8     2.0 x 1.0   + 66  4700 
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          20.9    23.5 x 15.5         360 
CMa dw      07:15   -28    Irr                       720                 25
Leo A       09:59.4 +30:45 IBm V            12.9     5.1 x 3.1         2500
Sex B       10:00.0 +05:20 Ir+ IV-V         11.8     5.1 x 3.5         4700  
NGC 3109    10:03.1 -26:09 Ir+ IV-V         10.4    16.0 x 2.9   +131  4500 
Ant dw      10:04.1 -27:20 dSph/E3          14.8     2.0 x 1.5         4600  
Leo I       10:08.5 +12:18 dE3               9.8     9.8 x 7.4          900 
Sex A       10:11.1 -04:43 Ir+ V            11.9     5.9 x 5.0         5200  
Sex dw      10:13.2 -01:37 dSph/E3          12.                         320  
Leo II      11:13.5 +22:10 dSph/E0 pec      12.6    12.0 x 11.0         750 
GR  8       12:58.7 +14:13 Im V             14.5     1.2 x 1.1   +165  7900: 
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                               +168    88  
SagDIG      19:30.1 -17:42 IB(s)m V         15.5     2.9 x 2.1         4200 
NGC 6822    19:44.9 -14:49 IB(s)m IV-V       9.3    15.5 x 13.5  + 66  1800 
Aqr dw      20:46.8 -12:51 Im V             13.9     2.3 x 1.2         3400
Tuc dw      22:41.7 -64:25 dSph/E5          15.7     2.9 x 1.2         3200
UKS2323-326 23:26.5 -32:23 Irr              13.9     1.5 x 1.2         4700
And VII     23:27.8 +50:35 dSph                      2.5 x 2.0         2600 
Peg dw      23:28.6 +14:45 Im V             13.2     5.0 x 2.7         3000: 
And VI      23:51.7 +24:36 dSph             11.2     3.5 x 3.5         2800

Possible further members:

UGCA 86 03:59.9 +67:08 Irr ? S0 ? 13.5 0.8 x 0.7 +262 6200 IC 5152 22:06.1 -51:17 IAB(s)m IV 10.6 4.9 x 3.0 + 30 5800

Willman 1 10:49.4 +51:03 dSph? GC ? 15.3 1.75 147 UMa I 10:34.9 +51:55 dSph 330 UMa II 08:51.5 +63:08 dSph 14.3 815 Vir Stream 12:24 -01.1 dSph 30 x 10 30 CVn I 13:28.1 +33:33 dSph 13.9 8.4 x 5.0 730 Boo dw 14:00.0 +14:30 dSph 13.6 13.0 x 8.7 200 Leo T 09:34.9 +17:03 dSph 16.0 1.4 x 1.4 1370 Com dw 12:27.0 +23:54 dSph 14.5 5.0 x 2.5 143 CVn II 12:57.2 +34:19 dSph 15.1 3.0 x 2.1 490 Her dw 16:31.0 +12:48 dSph 14.7 8.0 x 6.0 460 Leo IV 11:32.9 -00:32 dSph 15.9 3.3 x 2.5 520

And XI 00:46.3 +33:48 dSph 0.5 2900: And XII 00:47.4 +34:22 dSph 0.55 2900: And XIII 00:51.8 +33:00 dSph 0.5 2900: And XIV 00:51.6 +29:41 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 55 00:14.9 -39:11 Sc/SB(s)m 8.8 32.4 x 5.6 +129 7000 NGC 404 01:09.4 +35:43 E0 10.1 4.4 x 4.1 +178 8000: Cam A 04:25.3 +72:48 Irr 14.8 3.7 x 2.1 6500:: NGC 1569 04:30.8 +64:51 Irp+ III-IV: 11.2v 2.9 x 1.5 + 87 7500:: NGC 1560 04:32.8 +71:53 Sd 11.5v 9.8 x 2.0 +151 7500:: Argo dw 07:05.3 -58:31 Irr 14.2 3.5 x 1.7 +554 12000: UGC 9128 14:15.9 +23:03 Irp+ 14.4 1.7 x 1.3 +154

And NE 00:52.0 +44:06 Ir? 2900:

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       R   SG    Diam   Mass  Other names/Remarks
                           
WLM         (*) LGC      8       ?  UGCA 444, DDO 221
IC 10           LGC                 UGC 192
Cet dw          LGC                 
NGC 147         M31                 UGC 326, DDO 3
And III         M31                 PGC 2121
NGC 185         M31                 UGC 396
M 110           M31     17   10000  NGC 205
And VIII        M31     33
M 32            M31      8    3000  NGC 221
M 31            M31    195  400000  NGC 224
And I           M31                 PGC 2666
SMC             MW      25    6000  NGC 292, Nubecula Minor
And IX          M31
Scl dw          MW                  ESO 351- G 030, PCG 3589
LGS 3       (*) M31/33              Psc dw, PCG 3792
IC 1613         M31/LC              UGC 668, DDO 8
And X           M31
And V           M31                 
And II          M31                 PGC 4601
M 33            M31/33  60   25000  NGC 598
Phe dw      (I) MW/LGC              ESO 245- G 007, PGC 6830
For dw          MW                  ESO 356- G 004, PGC 10093
UGCA 92     (I) M31?                EGB 0427+63, PGC 15439
LMC             MW      30   20000  ESO 056- G 115, PGC 17223, Nubecula Major
Car dw          MW                  ESO 206- G 220, ESO 206- GA020, PGC 19441
CMa dw      (*) MW
Leo A           M/3109              Leo III, UGC 5364, DDO 69
Sex B       (I) N3109               UGC 5373, DDO 70
NGC 3109    (I) N3109               UGCA 194, DDO 236
Ant dw      (I) N3109    5          PGC 29194  
Leo I           MW                  UGC 5470, DDO 74, Harrington-Wilson 1, Regulus Galaxy
Sex A           N3109               UGCA 205, DDO 75
Sex dw          MW                  
Leo II          MW                  Leo B, UGC 6253, DDO 93, Harrington-Wilson 2
GR  8       (?) GR8                 UGC 8091, DDO 155, VV 558
UMi dw          MW                  UGC 9749, DDO 199
Dra dw          MW                  UGC 10822, DDO 208
Milky Way       MW     100  750000  
SagDEG      (*) MW                  Sgr dShp, Sgr I dw
SagDIG      (*) LGC                 Sgr dw, ESO 594- G 004, UKS 1927-177, PGC 63287, Kowal's Object
NGC 6822        LGC                 IC 4895, DDO 209, Barnard's Galaxy
Aqr dw          LGC                 DDO 210, PGC 65367
Tuc dw          LGC                 PCG 69519
UKS2323-326     LGC                 UGCA 438
And VII         M31                 Cas dSph, Cas dw
Peg dw          LGC                 UGC 12613, DDO 216, Peg DIG
And VI          M31                 Peg dSph, Peg II

Possible members:

UGCA 86     (?) M31/L?              PGC 14241           
Willman 1       MW                  SDSSJ1049+5103
IC 5152     (?) LGC?                ESO 237- G 027

UMa I           MW                  discovered 2005, SDSS (Willman et.al.)
UMa II          MW                  discovered 2006, SDSS (Zucker et.al.)
Virgo StStr     MW                  discovered 2005, SDSS (Duffau et.al.)
CVn dw          MW                  discovered 2006, SDSS (Zucker et.al.)
Boo dw          MW                  discovered 2006, SDSS (Belokurov et.al.)
Leo T           MW                  discovered 2007, SDSS (Irwin et.al.)
Com dw          MW                  discovered 2006, SDSS (Belokurov et.al.)
CVn II          MW                  discovered 2006, SDSS (Belokurov et.al.)
Her dw          MW                  discovered 2006, SDSS (Belokurov et.al.)
Leo IV          MW                  discovered 2006, SDSS (Belokurov et.al.)

And XI          M31                 discovered 2006, CFHT (Martin et.al.)
And XII         M31                 discovered 2006, CFHT (Martin et.al.)
And XIII        M31                 discovered 2006, CFHT (Martin et.al.)
And XIV         M31                 discovered 2007, KPNO (Majewski et.al.)

Key:

R:: Remarks: (*), (?) and (I); see below
SG:: Subgroup (after Mateo 1999). MW=Milky Way subgroup, M31: M31 subgroup, M31/33: M33 subgroup (probably part of M31 subgroup), LGC: Local Group Cloud (vast cloud of dwarf irregulars), N3109: NGC 3109 subgroup (not counted to Local Group by Van den Bergh), GR8: GR8 group (LG membership in doubt), M/N3109: Either MW or NGC 3109 group
Diam:: Diameter in 1000 light years (kly)
Mass:: Mass in million solar masses

Remarks (*):

"WLM" is "Wolf-Lundmark-Melotte"
"LGS" stands for "Local Group (suspected)" (LGS 3)
CMa dw (the Canis Major Dwarf) may be former host galaxy of M79 and 3 other globulars
"SagDEG" is the "Sagittarius Dwarf Elliptical Galaxy" (discovered 1994, containing M54)
"SagDIG" is the "Sagittarius Dwarf Irregular Galaxy"

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:

UGCA 86 may be a member of the Maffei 1 group
GR8 and IC 5152 are listed as nearby non-member galaxies in Irwin's list. They may be lonely nearby non-members near the edges of the group.
The Aquarius Dwarf is also listed by Irwin as non-member, but Van den Bergh (2000) reports its membership has now be confirmed.
Leo A was also listed in Irwin's list of non-members, but recent distant re-determination confirmed its membership.

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.

More Data on the Probable Nearby Non-Members listed above:

Galaxy Other names/Remarks

NGC 55 ESO 293- G 050 NGC 404 UGC 718 Cam A NGC 1569 UGC 3056 NGC 1560 UGC 3060 Argo dw PGC 20125 UGC 9128 DDO 187

And NE Stellar structure in M31 halo

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

"Galaxy"        listed    Rem

1010-27             McGraw   = Ant dw ?
DDO 187 (UGC 9128)  McGraw   Nearby probable non-member galaxy, see above
SDIG                KKT      Member of the Sculptor Group of Galaxies
LGS 2               Irwin    diffuse reflection nebulosity
And IV              vdB      Background galaxy, or open cluster in M31
Sex C (A 1003)      LGAA     globular cluster Palomar 3
UMa dw (A 1127)     dV,LGAA  globular cluster Palomar 4
Ser dw (A 1513)     dV       globular cluster Palomar 5
Cap dw (A 2144)     McGraw   globular cluster Palomar 12
Snickers                     Radio observations of 1975 indicated this nearby galaxy candidate
NGC 6946            Hubble   Nearby non-member galaxy
IC 342              Hubble   Member of the Maffei 1 or IC 342 Group of Galaxies
Maffei 1            Maffei   Member of the Maffei 1 or IC 342 Group of Galaxies
Maffei 2            Maffei   Member of the Maffei 1 or IC 342 Group of Galaxies
2318-42             Irwin

Sources:

dV:: G. De Vaucouleurs 1975, Nearby Groups of Galaxies, ch. 4, The Local Group. Published in "Galaxies and the Universe," ed. by A. Sandage, M. Sandage and J. Kristian.
Hubble: E.P. Hubble 1936, The Realm of the Nebulae, Yale University Press. Local group table on p. 126.
KKT:: Kraan-Korteweg & Tammann, 1979. Astronomische Nachrichten 300, p. 181
LGAA:: Le Grand Atlas de L'Astronomie, Encyclopedia Universalis, 1986. German edition: "Der große JRO Atlas der Astronomie"
McGraw:: McGraw-Hill Dictionary of Astronomy, by Sybil P. Parker (ed), p. 242
Maffei:: P. Maffei 1968, PASP 80, p. 618
vdB:: S. van den Bergh 1972, Search for Faint Companions to M31. ApJ. 171, p. L31-L33.
Andromeda IV is probably not an independent Local Group galaxy, but either a large open cluster in M31 (Jones, 1993), or a more remote background galaxy (Ferguson et.al., 2000)

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.

References

Annette M.N. Ferguson, J.S. Gallagher, and Rosemary F.G. Wyse, 2000. On the Nature of Andromeda IV. Astronomical Journal, Vol. 120, No. 2, pp. 821-832 (08/2000) [ADS: 2000AJ....120..821F]
Joseph H. Jones, 1993. CCD photometry of Andromeda IV - Dwarf irregular galaxy or M31 open cluster? Astronomical Journal, vol. 105, no. 3, pp. 933-7 (03/1993) [ADS: 1993AJ....105..933J]
Mario Mateo, 1998. Dwarf Galaxies of the Local Group. Annual Review of Astronomy and Astrophysics, Volume 36, 1998, pp. 435-506 [ADS: 1998ARA&A..36..435M]. Available online [framed version] - table of Local Group galaxies (NED Level 5)
Sidney van den Bergh, 1972. Search for Faint Companions to M31. Astrophysical Journal, Vol. 171, pp. L31-L33 [ADS: 1972ApJ...171L..31V]
Sidney van den Bergh, 1994. The Outer Fringes of the Local Group. Astronomical Journal 107, No. 4, pp. 1328-1332 (April 1994) [ADS: 1994AJ....107.1328V]
Sidney van den Bergh, 2000. Updated Information on the Local Group. Publications of the Astronomical Society of the Pacific, Vol. 112, No. 770, pp. 529-536 (April 2000) [ADS: 2000PASP..112..529V]
Tom Polakis has published an interesting article for observers of local group galaxies, Observing the Local Group, in the Deep Sky magazine (#36), which was reprinted in David Eicher's book Galaxies and the Universe. (was also available online but disappeared)
Paul W. Hodge, Galaxies. Harvard University Press, 1986. In particular Chapter 7: The Local Group.
Edwin P. Hubble, The Realm of the Nebulae, Yale University Press, 1936 (in particular, pp. 124, 145)

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Last Modification: June 5, 2007

The Local Group of Galaxies

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