Local Group

The Local Group is the galaxy group that includes the Milky Way. The Local Group comprises more than 54 galaxies, most of them dwarf galaxies. Its gravitational center is located somewhere between the Milky Way and the Andromeda Galaxy. The Local Group covers a diameter of 10 Mly (3.1 Mpc) (about 10²³ meters) and has a binary (dumbbell)^[1] distribution. The group itself is a part of the larger Virgo Supercluster, which in turn may be a part of the Laniakea Supercluster.

The three largest members of the group (in descending order) are the Andromeda galaxy, the Milky Way^[2] and the Triangulum Galaxy. The larger two of these spiral galaxies each have their own system of satellite galaxies.

The Andromeda Galaxy's satellite system consists of Messier 32 (M32), Messier 110 (M110), NGC 147, NGC 185, Andromeda I (And I), And II, And III, And IV, And V, Pegasus dSph (aka And VI), Cassiopeia Dwarf (aka And VII), And VIII, And IX, And X, And XI, And XIX, And XXI and And XXII, plus several additional ultra-faint dwarf spheroidal galaxies.^{[citation needed]}
Milky Way's satellite galaxies system comprises 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 I Dwarf and Ursa Major II Dwarf, plus several additional ultra-faint dwarf spheroidal galaxies.^[3]
The Triangulum Galaxy may or may not be a companion to the Andromeda Galaxy. Pisces Dwarf is equidistant from the Andromeda Galaxy and the Triangulum Galaxy, so it may be a satellite of either.^[4]
The membership of NGC 3109, with its companions Sextans A and the Antlia Dwarf, is uncertain due to extreme distances from the center of the Local Group.^{[citation needed]}
The other members of the group are gravitationally secluded from these large subgroups: IC 10, IC 1613, Phoenix Dwarf, Leo A, Tucana Dwarf, Cetus Dwarf, Pegasus Dwarf Irregular, Wolf–Lundmark–Melotte, Aquarius Dwarf, and Sagittarius Dwarf Irregular.^{[citation needed]}

History

The term "The Local Group" was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae.^[5] There, he described it as "a typical small group of nebulae which is isolated in the general field" and delineated, by decreasing luminosity, its members to be M31, Milky Way, M33, Large Magellanic Cloud, 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.

By 2003, the number of known Local Group members has increased from his initial 12 to 36 by the discovery of almost two dozen^[clarify] low-luminosity galaxies.^[6]

Component galaxies

Map

List of galactic bodies

Spiral galaxies
name	type	constellation	notes
Andromeda Galaxy (M31, NGC 224)	SA(s)b	Andromeda	~220 kly in diameter, it is the largest and most massive galaxy in the group.
Milky Way	SBbc	Sagittarius (centre)	Where we reside; the second largest galaxy in the group.^[7]
Triangulum Galaxy (M33, NGC 598)	SA(s)cd	Triangulum	Third largest, only unbarred spiral galaxy and possible satellite of the Andromeda Galaxy.
Elliptical galaxies
name	type	constellation	notes
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	Fourth largest member of the group, 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
M110 (NGC 205)	dE6p	Andromeda	satellite of the Andromeda Galaxy
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.^[8]
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 I	dSph	Boötes
Cetus Dwarf	dSph/E4	Cetus
Canes Venatici I Dwarf and Canes Venatici II Dwarf	dSph	Canes Venatici
KKs 3	dSph
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/dSph/Irr?	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
SagDSG (Sagittarius Dwarf Spheroidal 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 I Dwarf and Ursa Major II Dwarf	dSph	Ursa Major	satellite of Milky Way
Leo IV	dSph	Leo	satellite of the Milky Way
Leo V	dSph	Leo	satellite of the Milky Way
Leo T	dSph/Irr	Leo	satellite of the Milky Way
Boötes II	dSph	Boötes	satellite of the Milky Way
Boötes III	dSph	Boötes	satellite of the Milky Way
Coma Berenices	dSph	Coma Berenices	satellite of the Milky Way
Segue 2	dSph	Aries	satellite of the Milky Way
Hercules	dSph	Hercules	satellite of the Milky Way
Pisces II	dSph	Pisces	satellite of the Milky Way
Reticulum II	dSph	Reticulum	satellite of the Milky Way
Eridanus II	dSph	Eridanus	satellite of the Milky Way
Grus	dSph	Grus	satellite of the Milky Way
Tucana II	dSph	Tucana	satellite of the 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
UGCA 86 (0355+66)	Irr, dE or S0	Camelopardalis
UGCA 92 (EGB0427+63)	Irr or S0	Camelopardalis
Horologium	dSph or Globular Cluster	Horologium	satellite of the Milky Way. Not to be confused with the Horologium Supercluster.
Pictoris	dSph or Globular Cluster	Pictor	satellite of the Milky Way
Phoenix II	dSph or Globular Cluster	Phoenix	satellite of the Milky Way
Indus	dSph or Globular Cluster	Indus	satellite of the Milky Way
Eridanus III	dSph or Globular Cluster	Eridanus	satellite of the Milky Way
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
ESO 347-8 (2318–42)	Irr	Grus
UKS 2323-326	Irr	Sculptor
UGC 9128 (DDO 187)	Irp+	Boötes
Sextans C
Objects in the Local Group no longer recognised as galaxies
name	type	constellation	notes
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

Other notable objects

Smith's Cloud, a high-velocity cloud, between 32,000 and 49,000 light years from Earth^[9] and 8,000 light years from the disk of the Milky Way galaxy^[10]
HVC 127-41-330, a high-velocity cloud, 2.3 million light-years from Earth
Monoceros Ring, a ring of stars around the Milky Way that is proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy

Diagram

A diagram of our location in the observable universe. (Click here for smaller image.)

References

^ 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. Bibcode:2006Ap.....49....3K. doi:10.1007/s10511-006-0002-6.
^ "The Local Group". NASA's High Energy Astrophysics Science Archive Research Center (HEASARC). NASA. Retrieved 2015-05-05.
^ Sergey E. Koposov; Vasily Belokurov; Gabriel Torrealba; N. Wyn Evans (10 March 2015). "Beasts of the Southern Wild. Discovery of a large number of Ultra Faint satellites in the vicinity of the Magellanic Clouds". The Astrophysical Journal. arXiv:1503.02079. Bibcode:2015arXiv150302079K.
^ Miller, Bryan W.; et al. (December 2001), "The Star Formation History of LGS 3", The Astrophysical Journal, 562 (2): 713–726, arXiv:astro-ph/0108408, Bibcode:2001ApJ...562..713M, doi:10.1086/323853
^ Hubble, E.P. (1936). The realm of the nebulae (PDF). Mrs. Hepsa Ely Silliman memorial lectures, 25. New Haven: Yale University Press. ISBN 9780300025002. OCLC 611263346. Archived from the original on 2012-09-29.(pp. 124–151)
^ 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: 5042. arXiv:astro-ph/0305042. Bibcode:2003astro.ph..5042V.
^ http://www.atlasoftheuniverse.com/galaxies.html
^ 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. arXiv:astro-ph/0001040. Bibcode:2000PASP..112..529V. doi:10.1086/316548.
^ Wakker, B. P.; York, D. G.; Wilhelm, R.; Barentine, J. C.; Richter, P.; Beers, T. C.; Ivezić, Ž.; Howk, J. C. (2008). "Distances to Galactic High‐Velocity Clouds. I. Cohen Stream, Complex GCP, Cloud g1". The Astrophysical Journal. 672 (1): 298–319. arXiv:0709.1926. Bibcode:2008ApJ...672..298W. doi:10.1086/523845.
^ "Massive Gas Cloud Speeding Toward Collision With Milky Way". Retrieved 2008-06-06.

External links

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. arXiv:astro-ph/0001040. Bibcode:2000PASP..112..529V. doi:10.1086/316548.

[Karachentsevetal2006-1] 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. Bibcode:2006Ap.....49....3K. doi:10.1007/s10511-006-0002-6.

[2] "The Local Group". NASA's High Energy Astrophysics Science Archive Research Center (HEASARC). NASA. Retrieved 2015-05-05.

[Koposov2015-3] Sergey E. Koposov; Vasily Belokurov; Gabriel Torrealba; N. Wyn Evans (10 March 2015). "Beasts of the Southern Wild. Discovery of a large number of Ultra Faint satellites in the vicinity of the Magellanic Clouds". The Astrophysical Journal. arXiv:1503.02079. Bibcode:2015arXiv150302079K.

[apj562_2-4] Miller, Bryan W.; et al. (December 2001), "The Star Formation History of LGS 3", The Astrophysical Journal, 562 (2): 713–726, arXiv:astro-ph/0108408, Bibcode:2001ApJ...562..713M, doi:10.1086/323853

[Hubble-1936-5] Hubble, E.P. (1936). The realm of the nebulae (PDF). Mrs. Hepsa Ely Silliman memorial lectures, 25. New Haven: Yale University Press. ISBN 9780300025002. OCLC 611263346. Archived from the original on 2012-09-29.(pp. 124–151)

[Bergh2003-6] 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: 5042. arXiv:astro-ph/0305042. Bibcode:2003astro.ph..5042V.

[7] ttp://www.atlasoftheuniverse.com/galaxies.html

[Bergh2000-8] 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. arXiv:astro-ph/0001040. Bibcode:2000PASP..112..529V. doi:10.1086/316548.

[wakker-9] Wakker, B. P.; York, D. G.; Wilhelm, R.; Barentine, J. C.; Richter, P.; Beers, T. C.; Ivezić, Ž.; Howk, J. C. (2008). "Distances to Galactic High‐Velocity Clouds. I. Cohen Stream, Complex GCP, Cloud g1". The Astrophysical Journal. 672 (1): 298–319. arXiv:0709.1926. Bibcode:2008ApJ...672..298W. doi:10.1086/523845.

[10] "Massive Gas Cloud Speeding Toward Collision With Milky Way". Retrieved 2008-06-06.

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v t e Andromeda Galaxy
Location	Andromeda Galaxy → Andromeda subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Local Hole → Observable universe → Universe Each → may be read as "within" or "part of".	Andromeda Galaxy
Satellite galaxies	M32 M110 NGC 185 NGC 147 Andromeda I Andromeda II Andromeda III Andromeda IV* Andromeda V Andromeda VI Andromeda VII Andromeda VIII Andromeda IX Andromeda X Andromeda XI Andromeda XII Andromeda XIII Andromeda XIV Andromeda XV Andromeda XVI Andromeda XVII Andromeda XVIII Andromeda XIX Andromeda XX Andromeda XXI Andromeda XXII Andromeda XXIII Andromeda XXIV Andromeda XXV Andromeda XXVI Andromeda XXVII Andromeda XXVIII Andromeda XXIX Triangulum subgroup* *It is uncertain whether these are companion galaxies of the Andromeda Galaxy
Catalogued stars	AE Andromedae AF Andromedae M31-RV
Other	Andromeda–Milky Way collision S Andromedae Mayall II M32p NGC 206 3XMM J004232.1+411314
Category

v t e Triangulum Galaxy
Location	Triangulum Galaxy → Triangulum subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Local Hole → Observable universe → Universe Each arrow (→) may be read as "within" or "part of".	Triangulum Galaxy
H II regions	NGC 588 NGC 595 NGC 604
Suspected satellite galaxies	Pisces Dwarf Andromeda II Andromeda XXII

v t e Location of Earth
Included	Earth → Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Local Hole → Observable universe → Universe Each arrow (→) may be read as "within" or "part of".
Related	Cosmic View (1957 book) To the Moon and Beyond (1964 film) Cosmic Zoom (1968 film) Powers of Ten (1968 and 1977 films) Cosmic Voyage (1996 documentary) Cosmic Eye (2012) History of the center of the Universe Order of magnitude Pisces–Cetus Supercluster Complex
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