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

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 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

Segue 2 02:19:16.0 +20:10:31 dSph Boo II 13:58:08.0 +12:50:54 dSph Leo V 11:31:09.6 +02:13:12 dSph Boo III 13:57:07.4 +26:46:30 dSph Psc II dSph

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: And XV 01:14.3 +38:07 And XVI 00:59.5 +32:23 And XVII 00:37.1 +44:19 dSph 16.0 1.1 x 0.9 2590 And XVIII 00:02.2 +45:05 And XIX 00:19.5 +35:03 And XX 00:07.5 +35:08 And XXI 23:54.8 +42:28 And XXII 01:27.7 +28:05 And XXIII 01:29.4 +38:43 And XXIV 01:18.5 +46:22 And XXV 00:30.1 +46:51 And XXVI 00:23.6 +47:55 And XXVII 00:37.5 +45:23 And XXVIII 22:32.7 +31:13 And XXIX 23:58.9 +30:45

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 Psc II, 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 Stream 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.)

Segue 2 MW discovered 2007 Bootes II MW discovered 2007 Leo V MW discovered 2007 Bootes III MW discovered 2009 Pisces II MW discovered 2010

And XI M31 discovered 2006, CFHT (Martin et.al.) And XII M31 discovered 2006, CFHT (Martin et.al.) And XIII M31 Psc III, discovered 2006, CFHT (Martin et.al.) And XIV M31 Psc IV, discovered 2007, KPNO (Majewski et.al.) And XV M31 discovered 2007, CFHT And XVI M31 discovered 2007, CFHT And XVII M31 discovered 2007, INT (Irwin et.al.) And XVIII M31 discovered 2008 And XIX M31 discovered 2008 And XX M31 discovered 2008 And XXI M31 discovered 2009 And XXII M31 discovered 2009 And XXIII M31 discovered 2011 And XXIV M31 discovered 2011 And XXV M31 discovered 2011 And XXVI M31 discovered 2011 And XXVII M31 discovered 2011 And XXVIII M31 discovered 2011 And XXIX M31 discovered 2011

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 (*): 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.

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.

Links

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References


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