The Local Group of galaxies. The
Milky Way and
Andromeda are the most massive galaxies by far. (
Click to enlarge)
Distribution of the
iron content (in logarithmic scale) in four dwarf neighbouring galaxies of the Milky Way
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 ) and has a binary (dumbbell)[ 1] distribution. The group itself is a part of the larger Laniakea Supercluster (i.e., the Local Supercluster).[ 2] The local supercluster was previously thought to be the Virgo Supercluster , now an appendage to the newly defined local supercluster.[ 3]
The two most massive members of the group are the Milky Way and Andromeda galaxies.[ 4] These two spiral galaxies have each a system of satellite galaxies.
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 .[citation needed ]
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 ]
The Triangulum Galaxy , the third largest and only unbarred spiral galaxy in the Local Group, 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.[ 5]
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 book The Realm of the Nebulae .[ 6] There, he described it as "a typical small group of nebulae which is isolated in the general field". He 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 way of the discovery of almost two dozen [clarify ] low-luminosity galaxies.[ 7]
Component galaxies
Map
Local Group (clickable map)
List of galactic bodies
Spiral galaxies
name
type
constellation
notes
Milky Way
SBbc
Sagittarius (centre)
Where we reside; the second largest galaxy in the group.[ 8]
Andromeda Galaxy (M31, NGC 224)
SA(s)b
Andromeda
With ~220 kly in diameter, it is the largest and most massive galaxy in the group.
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.[ 9]
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
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 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
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 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
Diagram
See also
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 .
^ Gillman, Ollie. "The road map to the Universe" . Daily Mail . Daily Mail. Retrieved 24 April 2015 .
^ Gibney, Elizabeth. "Earth's new address: 'Solar System, Milky Way, Laniakea' " . Nature . Nature. Retrieved 24 April 2015 .
^ "The Local Group" . NASA's High Energy Astrophysics Science Archive Research Center (HEASARC) . Retrieved 2015-05-05 .
^ 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
Location
Satellite galaxies
* It is uncertain whether the Triangulum Galaxy is a companion galaxy of the Andromeda Galaxy
Other