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.

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 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,
• 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).

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:

• 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

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. 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.

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

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

Remarks (*):

• "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)
• "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"

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
• Leo A, GR8, the Aquarius Dwarf and IC 5152 are listed as nearby non-member galaxies in Irwin's list.

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.

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:

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.

• Bill Arnett's table of Local Group galaxies
• (Lengthy) article on the Local Group by George Lake [ mirror in Korea]
• Mike Irwin's Local Group page and Local Group Member list
• Local Group Galaxies images and information at SEDS
• Local Group galaxy images from the Astronomical Picture of the Day archives

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

Hartmut Frommert (spider@seds.org)
Christine Kronberg (smil@lrz.uni-muenchen.de)

Last Modification: 5 Apr 1998, 18:00 MET