Editing Local Group (section)

{{short description|Group of galaxies that includes the Milky Way}}
{{Use dmy dates|date=April 2020}}
{{Redirect|Local group}}
{{Infobox galaxy cluster
| name                = Local Group
| image               = Local Group and nearest galaxies.jpg
| image_scale         = 1.5
| alt                 = Local Group of galaxies, including the massive members [[Messier 31]] (Andromeda Galaxy) and Milky Way, as well as other nearby galaxies.
| caption             = Local Group of galaxies, including the massive members [[Messier 31]] (Andromeda Galaxy) and Milky Way, as well as other nearby galaxies.
| epoch               = [[Epoch (astronomy)#Julian years and J2000|J2000]]
| constellation       = 
| ra                  = 
| dec                 = 
| brightest_member    = 
| member_no           = 
| parent              = [[Virgo Supercluster]]
| major_axis_mpc      = 
| minor_axis_mpc      = 
| richness            = 
| bmtype              = 
| velocity_dispersion = 
| redshift            = 
| distance            = 
| temperature         = 
| mass                = 
| luminosity          = 
| flux                = 
| notes               = 
| other_names         = 
| names               = 
| references          =SIMBAD
}}
[[Image:VLT Shows Milky Way's Neighbouring Galaxies Have Different History.jpg|thumb|Distribution of the iron content (in logarithmic scale) in four neighbouring dwarf galaxies of the Milky Way]]

The '''Local Group''' is the [[galaxy group]] that includes the [[Milky Way]], where [[Earth]] is located. It has a total diameter of roughly {{convert|3|Mpc|e6ly km|lk=on|abbr=off|sigfig=1}},<ref>{{cite journal | last = Redd | first = Nola Taylor | title = Astronomers track dwarf galaxies to better understand the Milky Way's make-up and evolution | journal = Proceedings of the National Academy of Sciences | date = 18 December 2017 | volume = 115 | issue = 51 | pages = 12836–12838 | doi = 10.1073/pnas.1817136115 | pmid = 30568025 | pmc = 6304947 | s2cid = 58645004 | doi-access = free}}</ref> and a total mass of the order of {{convert|2e12|solar mass|kg|lk=in|sigfig=1}}.<ref>The mass of the Local Group is essentially accounted for by the mass of the Milky Way and the Andromeda Galaxy.
Estimates for the mass of each galaxy are compatible with {{val|e=12|u={{solar mass}}}}, 
and Peñarrubia et al. (2014) estimate {{val|2.3|0.7|e=12|u={{solar mass}}}} for the Local Group,
but Karachentsev and Kashibadze (2006) estimate the somewhat lower value of  {{val|1.29|0.14|e=12|u={{solar mass}}}}.</ref>
It consists of two collections of galaxies in a "[[dumbbell]]" shape; the Milky Way and its satellites form one lobe, and the [[Andromeda Galaxy]] and its satellites constitute the other. The two collections are separated by about {{convert|800|kpc|e6ly km|sigfig=1}} and are moving toward one another with a velocity of {{val|123|u=km/s}}.<ref name="Karachentsevetal2006">{{Cite journal |last1=Karachentsev |first1=I. D. |last2=Kashibadze |first2=O. G. |year=2006 |title=Masses of the local group and of the M81 group estimated from distortions in the local velocity field |journal=Astrophysics |volume=49 |issue=1 |pages=3–18 |bibcode=2006Ap.....49....3K |doi=10.1007/s10511-006-0002-6|s2cid=120973010 }}</ref> The group itself is a part of the larger [[Virgo Supercluster]], which may be a part of the [[Laniakea Supercluster]].
The exact number of galaxies in the Local Group is unknown as some are occluded by the Milky Way; however, at least 80 members are known, most of which are [[dwarf galaxies]].

The two largest members, the Andromeda and the Milky Way galaxies, are both [[spiral galaxies]] with masses of about {{val|e=12}} solar masses each. Each has its own system of [[satellite galaxies]]:
* The [[List of Andromeda's satellite galaxies|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 V]], [[And VI]] (also known as the Pegasus Dwarf Spheroidal Galaxy, or Pegasus dSph), [[And VII]] (a.k.a. the Cassiopeia Dwarf Galaxy), [[And VIII]], [[And IX]], [[And X]], [[Andromeda XI|And XI]], [[And XIX]], [[And XXI]] and [[And XXII]], plus several additional [[dwarf galaxy#Ultra-faint dwarf galaxies|ultra-faint]] [[dwarf spheroidal galaxies]].<ref>{{Cite journal |last1=Kalirai |first1=Jason S. |last2=Beaton |first2=Rachael L. |last3=Geha |first3=Marla C.|author3-link= Marla Geha |last4=Gilbert |first4=Karoline M. |last5=Guhathakurta |first5=Puragra |last6=Kirby |first6=Evan N. |last7=Majewski |first7=Steven R. |last8=Ostheimer |first8=James C. |last9=Patterson |first9=Richard J. |date=17 February 2010 |title=The Splash Survey: Internal Kinematics, Chemical Abundances, and Masses of the Andromeda I, Ii, III, Vii, X, and Xiv Dwarf Spheroidal Galaxies |journal=The Astrophysical Journal  |volume=711 |issue=2 |pages=671–692 |arxiv=0911.1998 |bibcode=2010ApJ...711..671K |doi=10.1088/0004-637X/711/2/671 |s2cid=43188686 |issn=0004-637X}}</ref>
* The [[Milky Way's satellite galaxies]] system comprises the [[Sagittarius Dwarf Spheroidal Galaxy|Sagittarius Dwarf Galaxy]], [[Large Magellanic Cloud]], [[Small Magellanic Cloud]], [[Canis Major Dwarf Galaxy]] (disputed, considered by some not a galaxy), [[Ursa Minor Dwarf]] Galaxy, [[Draco Dwarf]] Galaxy, [[Carina Dwarf]] Galaxy, [[Sextans Dwarf Galaxy]], [[Sculptor Dwarf Galaxy]], [[Fornax Dwarf]] Galaxy, [[Leo I (dwarf galaxy)|Leo I]] (a dwarf galaxy), [[Leo II (dwarf galaxy)|Leo II]] (a dwarf galaxy), [[Ursa Major I Dwarf]] Galaxy and [[Ursa Major II Dwarf]] Galaxy, plus several additional ultra-faint dwarf spheroidal galaxies.<ref name="Koposov2015">{{Cite journal |last1=Sergey E. Koposov |last2=Vasily Belokurov |last3=Gabriel Torrealba |last4=N. Wyn Evans |date=10 March 2015 |title=Beasts of the Southern Wild. Discovery of a large number of Ultra Faint satellites in the vicinity of the Magellanic Clouds |journal=The Astrophysical Journal |volume=805 |issue=2 |page=130 |arxiv=1503.02079 |bibcode=2015ApJ...805..130K |doi=10.1088/0004-637X/805/2/130|s2cid=118267222 }}</ref>

[[File:Local Group Galaxies Comparison.png|thumb|right|350px|Visual size comparison of the six largest Local Group galaxies, with details]]

The [[Triangulum Galaxy]] (M33) is the third-largest member of the Local Group, with a mass of approximately {{convert|5e10|solar mass|kg|sigfig=1|abbr=on}}, and is the third spiral galaxy.<ref>{{Cite web |url=https://heasarc.gsfc.nasa.gov/docs/cosmic/local_group_info.html |title=The Local Group |website=NASA's High Energy Astrophysics Science Archive Research Center (HEASARC) |publisher=[[NASA]] |access-date=5 May 2015}}</ref> It is unclear whether the Triangulum Galaxy is a companion of the Andromeda Galaxy; the two galaxies are 750,000 light years apart,<ref>{{Cite web|title=Messier Object 33|url=https://www.messier.seds.org/m/m033.html|access-date=2021-05-21|website=www.messier.seds.org}}</ref> and experienced a close passage 2–4&nbsp;billion years ago which triggered star formation across Andromeda's disk. The [[Pisces Dwarf]] Galaxy is equidistant from the Andromeda Galaxy and the Triangulum Galaxy, so it may be a satellite of either.<ref name="apj562_2">{{Cite journal |last1=Miller |first1=Bryan W. |last2=Dolphin |first2=Andrew E. |last3=Lee |first3=Myung Gyoon |last4=Kim |first4=Sang Chul |last5=Hodge |first5=Paul |display-authors=1 |date=December 2001 |title=The Star Formation History of LGS 3 |url=http://stacks.iop.org/0004-637X/562/i=2/a=713 |journal=The Astrophysical Journal  |volume=562 |issue=2 |pages=713–726 |arxiv=astro-ph/0108408 |bibcode=2001ApJ...562..713M |doi=10.1086/323853 |s2cid=119089499 |issn=0004-637X}}</ref>

The other members of the group are likely gravitationally secluded from these large subgroups: [[IC 10]], [[IC 1613]], [[Phoenix Dwarf]] Galaxy, [[Leo A]], [[Tucana Dwarf]] Galaxy, [[Cetus Dwarf]] Galaxy, [[Pegasus Dwarf Irregular Galaxy]], [[Wolf–Lundmark–Melotte]], [[Aquarius Dwarf]] Galaxy, and [[Sagittarius Dwarf Irregular Galaxy]].<ref name=McConnachie>{{Cite journal |last1=McConnachie |first1=Alan W. |year=2012 |title=The observed properties of dwarf galaxies in and around the Local Group |journal=The Astronomical Journal |volume=144 |issue=1 |pages=4 |arxiv=1204.1562 |doi=10.1088/0004-6256/144/1/4|bibcode=2012AJ....144....4M |s2cid=118515618 |url=http://www.astro.uvic.ca/~alan/Nearby_Dwarf_Database.html }}</ref>

The membership of [[NGC 3109]], with its companions [[Sextans A]] and the [[Antlia Dwarf]] Galaxy as well as [[Sextans B]], [[Leo P]], [[Antlia B]] and possibly [[Leo A]], is uncertain due to extreme distances from the center of the Local Group.<ref name="Karachentsevetal2006" /> The [[Antlia-Sextans Group]] is unlikely to be gravitationally bound to the Local Group due to probably lying outside the Local Group's [[Zero-velocity surface]]—which would make it a true galaxy group of its own rather than a subgroup within the Local Group.<ref>{{cite journal |last1=van den Bergh |first1=Sidney |date=June 1999 |title=The Nearest Group of Galaxies |journal=The Astrophysical Journal |volume=517 |issue=2 |pages=L97–L99 |doi=10.1086/312044 |arxiv=astro-ph/9904425 |bibcode=1999ApJ...517L..97V }}</ref> This possible independence may, however, disappear as the Milky Way continues coalescing with Andromeda due to the increased mass, and density thereof, plausibly widening the radius of the zero-velocity surface of the Local Group.