The Antlia-Sextans Group is a small grouping of galaxies in the constellations Hydra, Sextans, Antlia and Leo. It is generally considered to be at the very edge of the Local Group and thus part of it. However, other researchers indicate it is an independent group, and thus the nearest group to the Local Group. It is, on average, approximately 4.3 million light-years away from the Milky Way.
The Antlia-Sextans Group consists of the galaxies NGC 3109, Sextans A, Sextans B, Antlia Dwarf, Leo P and Antlia B. Leo A might also belong to the group, but this is considered unlikely.
NGC 3109 is the largest and dominant member of this group, with a diameter of 41,700 light-years, almost half the diameter of the Milky Way. [1] It was the first discovered member of the group, discovered in 1835. It is also second closest to Earth, at a distance of 4.348 million light-years away. It was thought to be an irregular galaxy, but is now theorized to possibly be a barred spiral. It seems to be a galaxy with no central core. Based on spectroscopy of blue supergiants in NGC 3109, it is known that the galaxy has a low metallicity, similar to that to the Small Magellanic Cloud. [2] It is one of the most metal-poor galaxies in the Local group, if it is included. [3] NGC 3109 seems to contain an unusually large number of planetary nebulae for its luminosity. [4] It also contains a substantial amount of dark matter. [5]
From measurements of the neutral atomic hydrogen in the galaxy, it has been found that the disk of NGC 3109 is warped. [6]
Antlia Dwarf is the smallest and closest galaxy in the group, only 2,610 light-years in diameter at a distance of 4.305 million light-years. [7] The gas in the Antlia Dwarf galaxy has the same radial velocity as a warp in the disk of NGC 3109, indicating that the two galaxies had a close encounter approximately one billion years ago. [6]
Sextans A is 7,990 light-years in diameter, and square-shaped, and contains numerous star clusters, located at the distance of about 4.658 million light-years away. [8] Sextans A has a peculiar square shape. Massive short-lived stars exploded in supernovae that caused more star formation, triggering yet more supernovae, ultimately resulting in an expanding shell. Young blue stars now highlight areas and shell edges high in current star formation, which from the perspective of observers on Earth appears roughly square. [9] The 10.4m telescope Gran Telescopio Canarias recently observed the OB-type stars that power the giant HII regions. [10] Sextans A have formed a pair with the most remote galaxy in the group, Sextans B.
Sextans B is the second largest galaxy in the group, with a diameter of 8,900 light-years. [11] Sextans B is the most distant from Earth in the group, at 5.101 million light-years away. [11] Sextans B has a uniform stellar population, [12] but the interstellar medium in it may be inhomogeneous. [13] Its mass is estimated to be about 2 × 108 times the mass of the Sun, of which 5.5 × 107 is in the form of atomic hydrogen. [12] Star formation in the galaxy seems to have proceeded in distinct periods of low intensity, separated by shorter periods of no activity. [12] The existence of Cepheid variables in the galaxy implies that Sextans B contains at least some young stars. [14] The metallicity of Sextans B is rather low, with a value of approximately Z = 0.001. [12] Sextans B is receding from the Milky Way with a speed of approximately 300 kilometres per second (190 mi/s), [13] and probably lies just outside the edge of the Local Group, so as its neighbour Sextans A. [15]
Five planetary nebulae have been identified in Sextans B, which is one of the smallest galaxies where planetary nebulae have been observed. These appear point-like and can be identified by their spectral emission lines. [16] It also contains a massive globular cluster. [17]
Leo P (AGC 208583 [18] ) is a small irregular galaxy discovered in 2013. It is only 0.4 Mpc from the Sextans B, so it is considered as a member of this grouping. It is the most distant member of all, with a distance of 5.3 million light years.
Antlia B is recently discovered small galaxy. It is known to be a satellite of NGC 3109, and is similar to the Antlia Dwarf in many ways, and is transitioning from an irregular galaxy to a dwarf spheroidal.
| Name | Type [20] | R.A. (J2000) [20] | Dec. (J2000) [20] | Redshift (km/s) [20] | Apparent Magnitude [20] |
|---|---|---|---|---|---|
| NGC 3109 | SB(s)m | 10h 03m 06.9s | −26° 09′ 34″ | 403 ± 1 | 10.4 |
| Sextans A | IBm | 10h 11m 00.8s | −04° 41′ 34″ | 324 ± 2 | 11.9 |
| Sextans B | ImIV-V | 10h 00m 00.1s | +05° 19′ 56″ | 300 ± 0 | 11.9 |
| Antlia Dwarf | dSph/Irr | 10h 04m 03.9s | −27° 19′ 55″ | 362 ± 0 | 15.67 ± 0.02 |
| Leo P | Irr | 10h 21m 45.1s | +18° 05' 17" | N/A | 16.89 |
| Antlia B | N/A | N/A | N/A | N/A | N/A |
The Local Group is the galaxy group that includes the Milky Way. It has a total diameter of roughly 3 megaparsecs (10 million light-years; 9×1019 kilometres), and a total mass of the order of 2×1012 solar masses (4×1042 kg). 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 800 kiloparsecs (3×106 ly; 2×1019 km) and are moving toward one another with a velocity of 123 km/s. 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 Triangulum Galaxy is a spiral galaxy 2.73 million light-years (ly) from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC (New General Catalogue) 598. With the D25 isophotal diameter of 18.74 kiloparsecs (61,100 light-years), the Triangulum Galaxy is the third-largest member of the Local Group of galaxies, behind the Andromeda Galaxy and the Milky Way.
Centaurus A is a galaxy in the constellation of Centaurus. It was discovered in 1826 by Scottish astronomer James Dunlop from his home in Parramatta, in New South Wales, Australia. There is considerable debate in the literature regarding the galaxy's fundamental properties such as its Hubble type and distance. NGC 5128 is one of the closest radio galaxies to Earth, so its active galactic nucleus has been extensively studied by professional astronomers. The galaxy is also the fifth-brightest in the sky, making it an ideal amateur astronomy target. It is only visible from the southern hemisphere and low northern latitudes.
The Sombrero Galaxy (also known as Messier Object 104, M104 or NGC 4594) is a peculiar galaxy of unclear classification in the constellation borders of Virgo and Corvus, being about 9.55 megaparsecs (31.1 million light-years) from the Milky Way galaxy. It is a member of the Virgo II Groups, a series of galaxies and galaxy clusters strung out from the southern edge of the Virgo Supercluster. It has a D25 isophotal diameter of approximately 29.09 kiloparsecs (94,900 light-years), making it slightly bigger in size than the Milky Way.
Messier 105 or M105, also known as NGC 3379, is an elliptical galaxy 36.6 million light-years away in the equatorial constellation of Leo. It is the biggest elliptical galaxy in the Messier catalogue that is not in the Virgo cluster. It was discovered by Pierre Méchain in 1781, just a few days after he discovered the nearby galaxies Messier 95 and Messier 96. This galaxy is one of a few not object-verified by Messier so omitted in the editions of his Catalogue of his era. It was appended when Helen S. Hogg found a letter by Méchain locating and describing this object which matched those aspects under its first-published name, NGC 3379.
Sextans A is a small dwarf irregular galaxy. It spans about 5000 light-years across, and is located at 4.3 million light-years away, in the outskirts of the Local Group of galaxies, which includes the Milky Way galaxy, and to which Sextans A may or may not belong.
NGC 3109 is a small barred Magellanic type spiral or irregular galaxy around 4.35 Mly away in the direction of the constellation of Hydra. NGC 3109 is believed to be tidally interacting with the dwarf elliptical galaxy Antlia Dwarf. It was discovered by John Herschel on March 24, 1835 while he was in what is now South Africa.
The Sextans Dwarf Spheroidal is a dwarf spheroidal galaxy that was discovered in 1990 by Mike Irwin as the 8th satellite of the Milky Way, located in the constellation of Sextans. It is also an elliptical galaxy, and displays a redshift because it is receding from the Sun at 224 km/s. The distance to the galaxy is 320,000 light-years and the diameter is 8,400 light-years along its major axis.
NGC 2683 is a field spiral galaxy in the northern constellation of Lynx. It was nicknamed the "UFO Galaxy" by the Astronaut Memorial Planetarium and Observatory. It was discovered by the astronomer William Herschel on February 5, 1788.
NGC 520 is a pair of colliding spiral galaxies about 105 million light-years away in the constellation Pisces. They were discovered by astronomer William Herschel on 13 December 1784.
Sextans B is an irregular galaxy that may be part of the Local Group, or lie just beyond it. Sextans B is 4.44 million light-years away from Earth and thus is one of the most distant members of the Local Group, if it is indeed a member. It forms a pair with its neighbouring galaxy Sextans A. It is a type Ir IV–V galaxy according to the galaxy morphological classification scheme. Sextans B may also be gravitationally associated with the galaxies NGC 3109 and the Antlia Dwarf.
The Antlia Dwarf is a dwarf spheroidal/irregular galaxy. It lies about 1.3 Mpc from Earth in the constellation Antlia. It is the fourth and faintest member of the nearby Antlia-Sextans Group of galaxies. The galaxy contains stars of all ages, contains significant amounts of gas, and has experienced recent star formation. The Antlia Dwarf is believed to be tidally interacting with the small barred spiral galaxy NGC 3109.
NGC 4605 is a dwarf barred spiral galaxy in the constellation Ursa Major, located at a distance of 18.1 ± 0.3 megalight-years from the Milky Way. Physically it is similar in size and in B-band absolute magnitude to the Large Magellanic Cloud. It is a member of the M81 Galaxy Group, along with Messier 81 and Messier 101.
NGC 4449, also known as Caldwell 21, is an irregular Magellanic type galaxy in the constellation Canes Venatici, being located about 13 million light-years away. It is part of the M94 Group or Canes Venatici I Group that is relatively close to the Local Group hosting our Milky Way galaxy.
The Antlia Cluster is a cluster of galaxies located in the Hydra–Centaurus Supercluster. The Antlia Cluster is the third-nearest to the Local Group after the Virgo Cluster and Fornax Cluster. Antlia's distance from Earth is 40.5 megaparsecs to 40.9 Mpc (133.4 Mly) and can be viewed from Earth in the constellation Antlia. The Antlia Cluster should not be confused with the Antlia Dwarf galaxy.
NGC 3169 is a spiral galaxy about 75 million light years away in the constellation Sextans. It has the morphological classification SA(s)a pec, which indicates this is a pure, unbarred spiral galaxy with tightly-wound arms and peculiar features. There is an asymmetrical spiral arm and an extended halo around the galaxy. It is a member of the NGC 3166 Group of galaxies, which is a member of the Leo II Groups, a series of galaxies and galaxy clusters strung out from the right edge of the Virgo Supercluster.
NGC 3271 is a barred lenticular galaxy in the constellation Antlia. At magnitude 11.7, it is the brightest galaxy in the Antlia Cluster, which lies about 40.7 megaparsecs away. It was discovered on May 1, 1834 by the astronomer John Herschel.
NGC 3260 is an elliptical galaxy in the constellation Antlia. It is a member of the Antlia Cluster, which lies about 40.7 megaparsecs away. It was discovered on May 2, 1834 by the astronomer John Herschel.
NGC 3311 is a supergiant elliptical galaxy located about 190 million light-years away in the constellation Hydra. The galaxy was discovered by astronomer John Herschel on March 30, 1835. NGC 3311 is the brightest member of the Hydra Cluster and forms a pair with NGC 3309 which along with NGC 3311, dominate the central region of the Hydra Cluster.
NGC 4306 is a dwarf barred lenticular galaxy located about 100 million light-years away in the constellation Virgo. The galaxy was discovered by astronomer Heinrich d'Arrest on April 16, 1865. Although considered to be a member of the Virgo Cluster, its high radial velocity and similar distance as NGC 4305 suggest that NGC 4306 is a background galaxy. NGC 4306 is a companion of NGC 4305 and appears to be interacting with it.
