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), as well as several smaller galaxies. These galaxies are spread in a volume of nearly 10 million light years diameter, centered somewhere between the Milky Way and M31.

Of the Local Group member galaxies, the Milky Way and M31 are by for the most massive, and therefore dominant members. Each of these to 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, and possibly the Tucana dwarf, and
• 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

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 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 South Polar group (around the South Galactic pole) containing NGC 253
• the M81 group
• the M83 group

Key:

RA, Dec:

Right Ascension and Declination for epoch 2000.0

Type:

Classification type from Tom Polakis' article

Dist:

Distance in 1000 light years

Diam:

Diameter in 1000 light years

Mass:

Mass in million solar masses

• "WLM" is "Wolf-Lundmark-Melotte"
• "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"

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

• Bill Arnett's table of Local Group galaxies
• (Lengthy) article on the Local Group by George Lake [ mirror in Korea]

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