Local Group

(astronomy) A group of at least 20 known galaxies in the vicinity of the sun; the Andromeda Spiral is the largest of the group, and the Milky Way Galaxy is the second largest.

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The small cluster of galaxies that contains the Milky Way Galaxy. Galaxies exhibit a pronounced tendency to clump together on a variety of scales. It is assumed that gravitational attraction draws galaxies together. On a very large scale, this attractive process may still be at an early stage. On a smaller scale, the process has led to collapse. Galaxies have fallen together, though with enough angular momentum that they usually orbit each other rather than collide.

Collapsed structures that contain a few dozen to a few thousand substantial galaxies are called clusters; collapsed structures that contain a few but less than 10 or 20 big galaxies are called groups. The Milky Way Galaxy is a large but not exceptional spiral galaxy and one of two dominant members of a small assemblage referred to as the Local Group.

On successively larger scales, the Local Group is a member of the Coma-Sculptor Cloud and the Local Supercluster. The closest galaxies beyond the Local Group are at a distance of 7 × 10⁶ light-years.

The Local Group has 36 known or suspected members. The only system larger than the Milky Way Galaxy is M31, the Andromeda Nebula. These two giant galaxies generate 80% of the light of the group. There are also two intermediate-scale galaxies: M33, the Triangulum Nebula near Andromeda, and the Large Magellanic Cloud, a close companion of the Milky Way and a conspicuous feature of the night sky in the Southern Hemisphere. Fainter than these are 32 small systems. The smallest among them are identified only because they are so close. Many would not be detected in even the nearest adjacent groups. The census of Local Group members may be very incomplete at the faint end and in the zones obscured by the plane of the Milky Way. See also Andromeda Galaxy; Magellanic Clouds; Milky Way Galaxy.

A member of the Local Group of galaxies, irregular galaxy Sextans A is 4.3 million light-years distant. The bright Milky Way foreground stars appear yellowish in this view. Beyond them lie the stars of Sextans A with young blue star clusters clearly visible.

Distribution of the iron content (in logarithmic scale) in four dwarf neighbouring galaxies of the Milky Way.

The Local Group is the group of galaxies that includes our galaxy, the Milky Way. The group comprises about 30 galaxies (including dwarf galaxies), with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy. The galaxies of the Local Group cover a 10 million light-year diameter (see 1 E+22 m for distance comparisons) and have a binary (dumbbell)^[1] shape. The group is estimated to have a total mass of (1.29 ± 0.14) × 10¹²M_☉.^[1] The group itself is one of many within the Virgo Supercluster (i.e. the Local Supercluster).^[2]

The two most massive members of the group are the Milky Way and the Andromeda Galaxy. These two Spiral Galaxies each have a system of satellite galaxies.

• The Milky Way's satellite system consists of Sagittarius Dwarf Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major Dwarf, Ursa Minor Dwarf, Draco Dwarf, Carina Dwarf, Sextans Dwarf, Sculptor Dwarf, Fornax Dwarf, Leo I, Leo II, and Ursa Major Dwarf.
• Andromeda's satellite system comprises M32, M110, NGC 147, NGC 185, And I, And II, And III, And IV, And V, Pegasus dSph, Cassiopeia Dwarf, And VIII, And IX, and And X.
• The Triangulum Galaxy, the third largest and only other ordinary spiral galaxy in the Local Group, may or may not be a companion to the Andromeda galaxy but probably has Pisces Dwarf as a satellite.

The other members of the group are gravitationally secluded from these large subgroups: IC10, IC1613, Phoenix Dwarf, Leo A, Tucana Dwarf, Cetus Dwarf, Pegasus Dwarf Irregular, Wolf-Lundmark-Melotte, Aquarius Dwarf, and Sagittarius Dwarf Irregular.

Contents 1 History 2 Component galaxies 2.1 Maps 2.2 Galactic bodies 3 Other notable objects 4 Diagram 5 See also 6 References 7 External links

History

The term "The Local Group" was introduced by Edwin Hubble in Chapter VI of his book The Realm of the Nebulae (Hubble 1936, pp. 124–151). There he describes it as "a typical small group of nebulae which is isolated in the general field." In that book he delineated by decreasing luminosity its members to be M31, the Milky Way, M33, the Large Magellanic Cloud, the Small Magellanic Cloud, M32, NGC 205, NGC 6822, NGC 185, IC 1613 and NGC 147. He also identified IC 10 as a possible Local Group member. In the ~70 years since his work, the number of known Local Group members has increased from his initial twelve to thirty-six as of 2003, by way of the discovery of almost two dozen low-luminosity galaxies.^[3]

Component galaxies

Maps

Local group (clickable map)

Milky Way's satellite galaxies (clickable map)

Galactic bodies

Spiral galaxies
name	type	constellation	notes
Milky Way	SBbc	n/a	Second largest, though possibly most massive galaxy in the group.[4]
Andromeda Galaxy (M31, NGC 224)	SA(s)b	Andromeda	Largest member of the group, recently (2006) discovered to also be a barred spiral. May be less massive than the Milky Way.
Triangulum Galaxy (M33, NGC 598)	SAc	Triangulum	only ordinary spiral galaxy and possible satellite of the Andromeda Galaxy
Elliptical galaxies
name	type	constellation	notes
M110 (NGC 205)	E6p	Andromeda	satellite of the Andromeda Galaxy
M32 (NGC 221)	E2	Andromeda	satellite of the Andromeda Galaxy
Irregular galaxies
name	type	constellation	notes
Wolf-Lundmark-Melotte (WLM, DDO 221)	Ir+	Cetus
IC 10	KBm or Ir+	Cassiopeia
Small Magellanic Cloud (SMC, NGC 292)	SB(s)m pec	Tucana	satellite of Milky Way
Canis Major Dwarf	Irr	Canis Major	satellite of Milky Way
Pisces Dwarf (LGS3)	Irr	Pisces	satellite of the Triangulum Galaxy?
IC 1613 (UGC 668)	IAB(s)m V	Cetus
Phoenix Dwarf	Irr	Phoenix
Large Magellanic Cloud (LMC)	Irr/SB(s)m	Dorado	satellite of Milky Way
Leo A (Leo III)	IBm V	Leo
Sextans B (UGC 5373)	Ir+IV-V	Sextans
NGC 3109	Ir+IV-V	Hydra
Sextans A (UGCA 205)	Ir+V	Sextans
Dwarf elliptical galaxies
name	type	constellation	notes
NGC 147 (DDO 3)	dE5 pec	Cassiopeia	satellite of the Andromeda Galaxy
SagDIG (Sagittarius Dwarf Irregular Galaxy)	IB(s)m V	Sagittarius	Most remote from barycenter member thought to be in the Local Group.[5]
NGC 6822 (Barnard's Galaxy)	IB(s)m IV-V	Sagittarius
Pegasus Dwarf (Pegasus Dwarf Irregular, DDO 216)	Irr	Pegasus
Dwarf spheroidal galaxies
name	type	constellation	notes
Boötes Dwarf	dSph	Boötes
Cetus Dwarf	dSph/E4	Cetus
Canes Venatici Dwarf	dSph	Canes Venatici
Andromeda III	dE2	Andromeda	satellite of the Andromeda Galaxy
NGC 185	dE3 pec	Cassiopeia	satellite of the Andromeda Galaxy
Andromeda I	dE3 pec	Andromeda	satellite of the Andromeda Galaxy
Sculptor Dwarf (E351-G30)	dE3	Sculptor	satellite of Milky Way
Andromeda V	dSph	Andromeda	satellite of the Andromeda Galaxy
Andromeda II	dE0	Andromeda	satellite of the Andromeda Galaxy
Fornax Dwarf (E356-G04)	dSph/E2	Fornax	satellite of Milky Way
Carina Dwarf (E206-G220)	dE3	Carina	satellite of Milky Way
Antlia Dwarf	dE3	Antlia
Leo I (DDO 74)	dE3	Leo	satellite of Milky Way
Sextans Dwarf	dE3	Sextans	satellite of Milky Way
Leo II (Leo B)	dE0 pec	Leo	satellite of Milky Way
Ursa Minor Dwarf	dE4	Ursa Minor	satellite of Milky Way
Draco Dwarf (DDO 208)	dE0 pec	Draco	satellite of Milky Way
SagDEG (Sagittarius Dwarf Elliptical Galaxy)	dSph/E7	Sagittarius	satellite of Milky Way
Tucana Dwarf	dE5	Tucana
Cassiopeia Dwarf (Andromeda VII)	dSph	Cassiopeia	satellite of the Andromeda Galaxy
Pegasus Dwarf Spheroidal Galaxy (Andromeda VI)	dSph	Pegasus	satellite of the Andromeda Galaxy
Ursa Major Dwarf	dSph	Ursa Major	satellite of Milky Way
Identification Unclear
name	type	constellation	notes
Virgo Stellar Stream	dSph (remnant)?	Virgo	In the process of merging with the Milky Way
Willman 1	dwarf Spherical galaxy or Globular cluster?	Ursa Major	147,000 light-years away
Andromeda IV	Irr?	Andromeda	probably not a galaxy
UGC-A 86 (0355+66)	Irr, dE or S0	Camelopardalis
UGC-A 92 (EGB0427+63)	Irr or S0	Camelopardalis
Probable non-members
name	type	constellation	notes
GR 8 (DDO 155)	Im V	Virgo
IC 5152	IAB(s)m IV	Indus
NGC 55	SB(s)m	Sculptor
Aquarius Dwarf (DDO 210)	Im V	Aquarius
NGC 404	E0 or SA(s)0-	Andromeda
NGC 1569	Irp+ III-IV	Camelopardalis
NGC 1560 (IC 2062)	Sd	Camelopardalis
Camelopardalis A	Irr	Camelopardalis
Argo Dwarf	Irr	Carina
2318-42	Irr	Grus
UKS 2323-326	Irr	Sculptor
UGC 9128 (DDO 187)	Irp+	Boötes
Palomar 12 (Capricornus Dwarf)		Capricornus	a globular cluster formerly classified as a dwarf spheroidal galaxy
Palomar 4 (originally designated Ursa Major Dwarf)		Ursa Major	a globular cluster formerly classified as a dwarf spheroidal galaxy
Sextans C

Other notable objects

• Smith's Cloud - High velocity cloud, between 32,000 and 49,000 light years from Earth,^[6] and 8000 light years from the disk of the Milky Way Galaxy.^[7]
• HVC 127-41-330 - High velocity cloud, 2.3 million light-years from Earth

Diagram

A diagram of our location in the Local Supercluster. Click to view more detail.

See also

• Groups and clusters of galaxies
• List of nearest galaxies
• List of galaxy clusters
• Local Supercluster
• Andromeda's satellite galaxies
• Milky Way's satellite galaxies

References

1. ^ ^{a b} Karachentsev, I. D.; Kashibadze, O. G. (2006). "Masses of the local group and of the M81 group estimated from distortions in the local velocity field". Astrophysics 49 (1): 3–18. doi:10.1007/s10511-006-0002-6. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006Ap.....49....3K. 
2. ^ R. B. Tully (1982). "The Local Supercluster". Astrophysical Journal 257: 389–422. doi:10.1086/159999. http://adsabs.harvard.edu/abs/1982ApJ...257..389T. 
3. ^ van den Bergh, Sidney (May 2003), "History of the Local Group", To be published in: "The Local Group as an Astrophysical Laboratory" (Cambridge University Press), http://adsabs.harvard.edu/abs/2003astro.ph..5042V 
4. ^ "Milky Way 'bigger than thought'". BBC News. 2009-01-06. http://news.bbc.co.uk/2/hi/science/nature/7813635.stm. 
5. ^ van den Bergh, Sidney (April 2000), "Updated Information on the Local Group", The Publications of the Astronomical Society of the Pacific 112 (770): 529–536, doi:10.1086/316548, http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2000PASP..112..529V 
6. ^ Wakker, B. P. (2008). "Distances to Galactic High‐Velocity Clouds. I. Cohen Stream, Complex GCP, Cloud g1". The Astrophysical Journal 672: 298. doi:10.1086/523845. 
7. ^ "Massive Gas Cloud Speeding Toward Collision With Milky Way". http://www.nrao.edu/pr/2008/smithscloud/. Retrieved 2008-06-06. 

External links

Wikimedia Commons has media related to: Local Group

• The Local Group of Galaxies, SEDS Messier pages
• A Survey of the Resolved Stellar Content of Nearby Galaxies Currently Forming Stars, Lowell Observatory
• van den Bergh, Sidney (2000). "Updated Information on the Local Group". The Publications of the Astronomical Society of the Pacific 112 (770): 529–536. doi:10.1086/316548. http://adsabs.harvard.edu/abs/2000astro.ph..1040V. 

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