Local Group

The Local Group is the group of galaxies that includes our galaxy, the Milky Way. The group comprises over 35 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 E22 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 barred spirals 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 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: IC1613, Phoenix Dwarf, Leo A, Tucana Dwarf, Cetus Dwarf, Pegasus Dwarf Irregular, WLM, Aquarius Dwarf, and Sagittarius Dwarf Irregular.

History

Edwin Hubble was the first to identify the Local Group in Chapter VI of his book The Realm of 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 12 to 36 in 2003. This was by way of almost two dozen low-luminosity galaxies.[3]

Component galaxies

Map

Map of the Local Group of Galaxies (*)

Galactic bodies

Spiral galaxies
name type constellation notes
Milky Way SBbc n/a Second largest, though possibly most massive galaxy in the group.
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.[4]
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 (And 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, 8000 light-years from Earth

* HVC 127-41-330 - High velocity cloud, 2.3 million light-years from Earth

Links

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. 
  2. ^ R. B. Tully (1982). "The Local Supercluster". Astrophysical Journal 257: 389–422. doi:10.1086/159999. 
  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. ^ 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, <http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2000PASP..112..529V> 

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