The Dorado Group is a loose concentration of galaxies containing both

spiral In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point. Helices Two major definitions of "spiral" in the American Heritage Dictionary are:elliptical Elliptical may mean: * having the shape of an ellipse, or more broadly, any oval shape ** in botany, having an elliptic leaf shape ** of aircraft wings, having an elliptical planform * characterised by ellipsis (the omission of words), or by conc ...

s. It is generally considered a 'galaxy group' but may approach the size of a 'galaxy cluster'. It lies primarily in the southern constellation

Dorado Dorado () is a constellation in the southern sky. It was named in the late 16th century and is now one of the 88 modern constellations. Its name refers to the dolphinfish (''Coryphaena hippurus''), which is known as ''dorado'' in Spanish, altho ...

and is one of the richest galaxy groups of the Southern Hemisphere.

Gérard de Vaucouleurs Gérard Henri de Vaucouleurs (25 April 1918 – 7 October 1995) was a French astronomer. Life and career Born in Paris, he had an early interest in amateur astronomy and received his undergraduate degree in 1939 at the Sorbonne in that city. ...

was the first to identify it in 1975 as a large complex nebulae II in the Dorado region, designating it as G16.

Characteristics

A rough distance estimate from NGC 1549 (using the

Hubble constant Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving ...

as 70) put the cluster at 18.4 megaparsecs (Mpc). The

Cepheid A Cepheid variable () is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. A strong direct relationship between a Cepheid varia ...

distance estimate from Freedman et al. 2001 is 15.3 Mpc. Based upon the 2001 work of Tonry et al. the

surface brightness fluctuation Surface brightness fluctuation (SBF) is a secondary distance indicator used to estimate distances to galaxies. It is useful to 100 Mpc (parsec). The method measures the variance in a galaxy's light distribution arising from fluctuations in the n ...

(SBF) of six member galaxies was averaged and adjusted to estimate the group's distance at in 2007. At the center of the cluster lie

interacting galaxies Interacting galaxies (''colliding galaxies'') are galaxies whose gravitational fields result in a disturbance of one another. An example of a minor interaction is a satellite galaxy disturbing the primary galaxy's spiral arms. An example of a ...

NGC 1549 and NGC 1553. The dominant group members, ordered by luminosity, are: spiral

NGC 1566 NGC 1566, sometimes known as the Spanish Dancer, is an intermediate spiral galaxy in the constellation Dorado, positioned about 3.5° to the south of the star Gamma Doradus. It was discovered on May 28, 1826 by Scottish astronomer James Dunlop. ...

, lenticular NGC 1553, and elliptical NGC 1549. The group spans an area of the sky 10° square, corresponding to an actual area of around 3 Mpc square. The group exhibits a relatively small

harmonic mean In mathematics, the harmonic mean is one of several kinds of average, and in particular, one of the Pythagorean means. It is sometimes appropriate for situations when the average rate is desired. The harmonic mean can be expressed as the recipro ...

radius (230 ± 40 kpc) due to the concentration at its core of more luminous galaxies. All together, the group has an overall luminosity of 7.8 ± 1.6 ''L''_⊙. The Dorado Group contains three dominant smaller groups within itself, NGC 1672 Group, NGC 1566 Group and the NGC 1433 Group, as evidenced by the H I distribution of the region. The Dorado Group is in the

Fornax Wall The Fornax Wall is a superstructure known as a galaxy filament or galaxy wall. It is a long filament of galaxies with a major axis longer than its minor one. The filament contains not only Dorado Group but also the Fornax cluster The Fornax Clus ...

that connects these three groups. Due to its location in the Fornax Wall, the group is at a similar distance as the

Fornax Cluster The Fornax Cluster is a cluster of galaxies lying at a distance of 19 megaparsecs (62 million light-years). It has an estimated mass of solar masses, making it the second richest galaxy cluster within 100 million light-years, after the consider ...

. The Dorado Group is richer than the

Local Group The Local Group is the galaxy group that includes the Milky Way. It has a total diameter of roughly , and a total mass of the order of . It consists of two collections of galaxies in a "dumbbell" shape: the Milky Way and its satellites form ...

, while still being dominated by disk types of galaxies (i.e. its two brightest members are spiral NGC 1566 and lenticular NGC 1553) and its member galaxies have H I masses on par with non-interacting galaxies of the same morphological type. With the group's apparent crossing time being 12.6 ± 0.6 % of the universe's age, recent analyses deduce that the group is unvirialized, and thus this may explain the abundance of spirals and H I.

Members

The table below lists eighteen galaxies that were identified in 1982 as associated with the Dorado Group by John Peter Huchra and Margaret J. Geller with the ones later dropped struck out. In 1989, the list of members was expanded to 46 by Maia, da Costa, & Latham. In 1990–1991, Henry C. Ferguson and

Allan Sandage Allan Rex Sandage (June 18, 1926 – November 13, 2010) was an American astronomer. He was Staff Member Emeritus with the Carnegie Observatories in Pasadena, California. He determined the first reasonably accurate values for the Hubble con ...

identified 34 other possible candidate members of the group with magnitudes greater than 19 and eliminated one member from Maia et al. putting the list at 79 galaxies. Kilborn et al. 2005 confirmed 26 members from the list. In 2006, the list was refined again by Firth et al. Using

redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in f ...

data, they excluded eleven (as being background galaxies or interloper) from the Ferguson et al. list, confirmed the membership of twenty on the list, and left 48 unconfirmed. NGC 1487 HST

The Maia et al. 1989 thirty-four added members were: IC 2049, NGC 1536, IC 2058, IC 2032, NGC 1602, NGC 1581, IC 2085, NGC 1522, PGC 15149, NGC 1556, NGC 1527, NGC 1494, NGC 1493, PGC 14416, IC 2000, NGC 1483,

NGC 1433 NGC 1433 (also known as PGC 13586) is a barred spiral galaxy with a double ring structure located in the constellation of Horologium. It is at a distance of 46 million light-years from Earth. It is a Seyfert galaxy with an active galactic nucleu ...

, PGC 14078, NGC 1495,

NGC 1510 NGC 1510 is a dwarf lenticular galaxy approximately 38 million light-years away from Earth in the constellation of Horologium. It was discovered by John Herschel on December 4, 1836. Gravitational interaction with NGC 1512 NGC 1510 is under ...

NGC 1512 NGC 1512 is a barred spiral galaxy approximately 38 million light-years away from Earth in the constellation Horologium. The galaxy displays a double ring structure, with a (nuclear) ring around the galactic nucleus and an (inner) further out ...

, IC 1959, IC 1986,

NGC 1448 NGC 1448 or NGC 1457 is an unbarred spiral galaxy seen nearly edge-on in the constellation Horologium. It is at a distance of 55 million light years from Earth. It was discovered by John Herschel in 1835. Five supernovae have been discovered ...

, NGC 1487, IC 1933,

NGC 1311 NGC commonly refers to: * New General Catalogue of Nebulae and Clusters of Stars, a catalogue of deep sky objects in astronomy NGC may also refer to: Companies * NGC Corporation, name of US electric company Dynegy, Inc. from 1995 to 1998 * Na ...

, IC 1954, IC 1914, NGC 1411, IC 1970, PGC 13818, NGC 1249, and PGC 11139. And the six dropped from the 1982 list were:

NGC 2082 NGC 2082 is an intermediate spiral galaxy in the Dorado constellation. It was originally thought to be part of the Dorado Group of galaxies, but was later removed. It was discovered on November 30, 1834 by John Herschel. Supernova SN 1992ba, a ...

, NGC 1947, NGC 1796, NGC 1688, NGC 1672, and

NGC 1559 NGC 1559 is a barred spiral galaxy in the constellation Reticulum. It is also a Seyfert galaxy. Although it was originally thought to be a member of the Dorado Group, subsequent observations have shown that it is in fact not a member of any gala ...

. In 2007, a study of ultracompact dwarfs (UCD) identified one definite and two possible UCD members of the group. The thirty-four added by Ferguson et al. 1990 included IC 2038 and IC 2039. The NGC 1566 Group of Dorado contains H I with ''M''_HI = 3.5 ''M''_⊙ of which 40% alone comes from the NGC 1566 galaxy. More than half of its members are outside its virial radius of 580 kpc which suggests this group is a young non-virialized group. The 2005 Kilborn et al. set of confirmed NGC 1566 Group members (within the Dorado Group) is:

External links

References

{{DEFAULTSORT:Dorado Group Dorado (constellation) Southern Supercluster