The Local Group of Galaxies

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 system of the Milky Way contains many (nearby) dwarf galaxies, spread all over the sky, namely Sag DEG, LMC, SMC, and the dwarf galaxies in Ursa Minor, Draco, Carina, Sextans (dwarf), Sculptor, Fornax, Leo I and Leo II,
• 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 And VII (Cassiopeia dwarf).

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:

• the Maffei 1 group, which besides the giant elliptical galaxy Maffei 1 also contains smaller Maffei 2, and is associated with nearby IC 342. 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

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.

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:

Key:

RA, Dec:

Right Ascension and Declination for epoch 2000.0 (* indicates epoch 1950.0 coordinates)

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)

Key:

R:

Remarks: (*), (?) and (I); see below

Diam:

Diameter in 1000 light years (kly)

Mass:

Mass in million solar masses

• "WLM" is "Wolf-Lundmark-Melotte"
• Andromeda IV is probably a large open cluster in M31, not an independent galaxy (ApJ 105:933-7, 3/1993), or a more remote background galaxy (ApJ 120:821-832, 8/2000)
• "LGS" stands for "Local Group (suspected)" (LGS 3)
• "SagDEG" is the "Sagittarius Dwarf Elliptical Galaxy" (discovered 1994, containing M54)
• "SagDIG" is the "Sagittarius Dwarf Irregular Galaxy"

• UGCA 86 may be a member of the Maffei 1 group
• Leo A, GR8, the Aquarius Dwarf and IC 5152 are listed as nearby non-member galaxies in Irwin's list.

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

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"

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

Links

• Bill Arnett's table of Local Group galaxies
• (Lengthy) article on the Local Group by George Lake
• Mike Irwin's Local Group page and Local Group Member list
• Brian Skiff's list of Local Group galaxies (based on Sidney van den Bergh's article in PASP May 2000 issue)
• Local Group Galaxies table from NED (from Mateo 1999)
• Local Group Galaxies images and information at SEDS
• Local Group galaxy images from the Astronomical Picture of the Day archives
• 0bserving the Local Group by Tom Polakis; from Deep Sky Magazine #36.
• Local Group Galaxies, derived from Mateo 1999 (NED Level 5)

References

• 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. Also available online.
• 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: 31 Dec 1999, 17:00 MET