Dorado Group
Encyclopedia
The Dorado Group is a loose concentration of galaxies
containing both spiral
s and elliptical
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
and is one of the richest galaxy groups of the Southern Hemisphere
. Gérard de Vaucouleurs
was the first to identify it in 1975 as a large complex nebulae II in the Dorado region, designating it as G16.
distance estimate from Freedman et al. 2001 is 15.3 Mpc. Based upon the 2001 work of Tonry et al. the surface brightness fluctuation
(SBF) of six member galaxies was averaged and adjusted to estimate the group's distance at 19.1 ± 0.8 Mpc in 2007.
At the center of the cluster lie interacting galaxies
NGC 1549 and NGC 1553
. The dominant group members, ordered by luminosity, are: spiral NGC 1566
, lenticular
NGC 1553, and elliptical NGC 1549. The group spans an area of the sky 10° square which represents around 3 Mpc square. The group exhibits a relatively small harmonic mean
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 that connects these three groups. Because it is in the Fornax Wall, it is at a similar distance as the Fornax Cluster
. The Dorado Group is interesting for being richer than the Local Group
, 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.
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
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.
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, PGC 14078 , NGC 1495, NGC 1510, NGC 1512
, IC 1959, IC 1986, NGC 1448, NGC 1487, IC 1933, NGC 1311, 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 1947, NGC 1796, NGC 1688, NGC 1672
, and NGC 1559
. 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 MHI = 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:
Galaxy groups and clusters
Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. They form the densest part of the large scale structure of the universe...
containing both spiral
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
s and elliptical
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
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
Dorado is a constellation in the southern sky. It was created in the late 16th century and is now one of the 88 modern constellations. Its name refers to the dolphinfish , which is known as dorado in Spanish, although it has also been depicted as a swordfish...
and is one of the richest galaxy groups of the Southern Hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
. Gérard de Vaucouleurs
Gérard de Vaucouleurs
-External links:* * http://nedwww.ipac.caltech.edu/level5/normal_galaxies.html-Other resources:...
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 as 70) puts the cluster at 18.4 megaparsecs (Mpc). The CepheidCepheid variable
A Cepheid is a member of a class of very luminous variable stars. The strong direct relationship between a Cepheid variable's luminosity and pulsation period, secures for Cepheids their status as important standard candles for establishing the Galactic and extragalactic distance scales.Cepheid...
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
Surface brightness fluctuation is a secondary distance indicator used to estimate distances to galaxies. The technique uses the fact that galaxies are made up of a finite number of stars. The number of stars in any small patch of the galaxy will vary from point to point, creating a noise-like...
(SBF) of six member galaxies was averaged and adjusted to estimate the group's distance at 19.1 ± 0.8 Mpc in 2007.
At the center of the cluster lie interacting galaxies
Interacting galaxy
Interacting galaxies are galaxies whose gravitational fields result in a disturbance of one another. An example of a minor interaction is a satellite galaxy's disturbing the primary galaxy's spiral arms. An example of a major interaction is a galactic collision.-Satellite interaction:A giant...
NGC 1549 and NGC 1553
NGC 1553
NGC 1553 is a prototypical lenticular galaxy in the constellation Dorado. It is the second brightest member of the Dorado Group of galaxies. British astronomer John Herschel discovered NGC 1553 on December 5, 1834 using an 18.7 inch reflector....
. The dominant group members, ordered by luminosity, are: spiral NGC 1566
NGC 1566
NGC 1566 is an intermediate spiral galaxy in the constellation Dorado. It is the dominant member of the Dorado Group and also its brightest member. It is the second brightest Seyfert galaxy after NGC 1068. Its absolute luminosity is 3.7 L☉...
, lenticular
Lenticular galaxy
A lenticular galaxy is a type of galaxy which is intermediate between an elliptical galaxy and a spiral galaxy in galaxy morphological classification schemes. Lenticular galaxies are disk galaxies which have used up or lost most of their interstellar matter and therefore have very little ongoing...
NGC 1553, and elliptical NGC 1549. The group spans an area of the sky 10° square which represents around 3 Mpc square. The group exhibits a relatively small harmonic mean
Harmonic mean
In mathematics, the harmonic mean is one of several kinds of average. Typically, it is appropriate for situations when the average of rates is desired....
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
H I region
An H I region is an interstellar cloud composed of neutral atomic hydrogen , in addition to the local abundance of helium and other elements. These regions are non-luminous, save for emission of the 21-cm region spectral line. This line has a very low transition probability, so requires large...
distribution of the region. The Dorado Group is in the Fornax Wall that connects these three groups. Because it is in the Fornax Wall, it is at a similar distance as the Fornax Cluster
Fornax Cluster
At a distance of approximately 62.0 Mly , the Fornax Cluster is the second richest cluster of galaxies within 100 million light-years, although it is much smaller than the Virgo Cluster. It lies primarily in the constellation Fornax, and may be associated with the nearby Eridanus Group...
. The Dorado Group is interesting for being richer than the Local Group
Local Group
The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...
, 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
Interacting galaxy
Interacting galaxies are galaxies whose gravitational fields result in a disturbance of one another. An example of a minor interaction is a satellite galaxy's disturbing the primary galaxy's spiral arms. An example of a major interaction is a galactic collision.-Satellite interaction:A giant...
of the same morphological type. With the group's apparent crossing time being 12.6 ± 0.6 % of the universe's age
Age of the universe
The age of the universe is the time elapsed since the Big Bang posited by the most widely accepted scientific model of cosmology. The best current estimate of the age of the universe is 13.75 ± 0.13 billion years within the Lambda-CDM concordance model...
, 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 stuck out. In 1989, the list of members was expanded to 46 by Maia, da Costa, & Latham. Lastly, in 1990/1991, Henry C. Ferguson & Allan SandageAllan Sandage
Allan Rex Sandage was an American astronomer. He was Staff Member Emeritus with the Carnegie Observatories in Pasadena, California. He is best known for determining the first reasonably accurate value for the Hubble constant and the age of the universe.-Career:Sandage was one of the most...
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
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
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.
| Name | Type | R.A. Right ascension Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:... (J2000) |
Dec. Declination In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and... (J2000) |
Redshift Redshift In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum... (km/s Second The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock.... ) |
Apparent Magnitude Apparent magnitude The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere... |
|---|---|---|---|---|---|
| |
SAB(rs+)c | 1184 ± 6 | 12.6 | ||
| |
S0- pec | 1100 ± 24 | 11.7 | ||
| |
(R)SB(r)dm: | 1014 ± 9 | 12.9 | ||
| |
SB(rs)dm | 1228 ± 6 | 12.6 | ||
| NGC 1672 NGC 1672 is a barred spiral galaxy located in the constellation Dorado. It was originally thought to be a member of the Dorado Group, however, this membership was later rejected. NGC 1672 has a large bar which is estimated to measure around 20 kpc. It has very strong radio emissions emanating... |
(R'_1:)SB(r)bc Sy2 | 1331 ± 3 | 10.3 | ||
| IC 2056 | SAB(r)b | 1133 ± 10 | 12.5 | ||
| NGC 1559 NGC 1559 is a barred spiral galaxy in the constellation Reticulum. It is also a Seyfert galaxy. In 2005, a supernova was observed. Two other supernovae discovered in NGC 1559 were SN 1984J and SN 1986L... |
SB(s)cd | 1304 ± 4 | 11.0 | ||
| NGC 1543 | (R)SB(l)00 | 1176 ± 7 | 11.5 | ||
| NGC 1574 | SA0- | 1050 ± 25 | 11.4 | ||
| NGC 1533 NGC 1533 NGC 1533 is a barred lenticular galaxy with faint spiral structure in the constellation Dorado and the seventh brightest member of the Dorado Group and 1° off the group's center. It is surrounded by a vast arc or ring of H I which is connected to IC 2038 and IC 2039. The ring orbits around 32... |
(L)SB(rs)00 | 790 ± 5 | 11.7 | ||
| NGC 1546 | SA0+? | 1284 ± 14 | 11.8 | ||
| NGC 1553 NGC 1553 NGC 1553 is a prototypical lenticular galaxy in the constellation Dorado. It is the second brightest member of the Dorado Group of galaxies. British astronomer John Herschel discovered NGC 1553 on December 5, 1834 using an 18.7 inch reflector.... |
SA(rl)00 | 1080 ± 11 | 10.3 | ||
| NGC 1549 | E0-1 | 1220 ± 15 | 10.7 | ||
| NGC 1566 NGC 1566 NGC 1566 is an intermediate spiral galaxy in the constellation Dorado. It is the dominant member of the Dorado Group and also its brightest member. It is the second brightest Seyfert galaxy after NGC 1068. Its absolute luminosity is 3.7 L☉... |
(R'_1)SAB(rs)bcSy1 | 1504 ± 2 | 10.3 | ||
| NGC 1617 | (R')SAB(rs)a | 1063 ± 21 | 11.4 | ||
| NGC 1515 | SAB(s)bc | 1175 ± 7 | 12.1 | ||
| NGC 1705 NGC 1705 NGC 1705 is a peculiar lenticular galaxy in the constellation Pictor. It is estimated to be approximately 17 million light-years from the Earth. It is a member of the Dorado Group.-External links:*... |
SA0- pec | 633 ± 6 | 12.8 | ||
| NGC 1596 | SA0: sp | 1510 ± 8 | 12.1 | ||
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
IC 2000
The IC 2000 is a double-deck train in Switzerland and is run by Swiss Federal Railways as part of its InterCity service. Top speed is 200 km/h. The train set can be composed of up to 10 coaches and provides close to 1,000 seats, according to SBB...
, NGC 1483, NGC 1433, PGC 14078 , NGC 1495, NGC 1510, NGC 1512
NGC 1512
NGC 1512 is a barred spiral galaxy approximately 30 million light-years away from Earth in the constellation Horologium. It is a member of the Dorado Group.-External links:* *...
, IC 1959, IC 1986, NGC 1448, NGC 1487, IC 1933, NGC 1311, 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 1947, NGC 1796, NGC 1688, NGC 1672
NGC 1672
NGC 1672 is a barred spiral galaxy located in the constellation Dorado. It was originally thought to be a member of the Dorado Group, however, this membership was later rejected. NGC 1672 has a large bar which is estimated to measure around 20 kpc. It has very strong radio emissions emanating...
, and NGC 1559
NGC 1559
NGC 1559 is a barred spiral galaxy in the constellation Reticulum. It is also a Seyfert galaxy. In 2005, a supernova was observed. Two other supernovae discovered in NGC 1559 were SN 1984J and SN 1986L...
. 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
H I region
An H I region is an interstellar cloud composed of neutral atomic hydrogen , in addition to the local abundance of helium and other elements. These regions are non-luminous, save for emission of the 21-cm region spectral line. This line has a very low transition probability, so requires large...
with MHI = 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:
| Name | Type | R.A. Right ascension Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:... (J2000) |
Dec. Declination In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and... (J2000) |
Redshift Redshift In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum... (km/s Second The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock.... ) |
Apparent Magnitude Apparent magnitude The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere... |
|---|---|---|---|---|---|
| IC 2049 | SAB(s)d? | 1469 ± 7 | 14.5 | ||
| NGC 1536 | SB(s)c pec | 1217 ± 13 | 13.2 | ||
| NGC 1543 | (R)SB(l)00 | 1176 ± 7 | 11.5 | ||
| LSBG F157-081 | Irregular | 1215 ± 7 | 16.7 | ||
| NGC 1533 NGC 1533 NGC 1533 is a barred lenticular galaxy with faint spiral structure in the constellation Dorado and the seventh brightest member of the Dorado Group and 1° off the group's center. It is surrounded by a vast arc or ring of H I which is connected to IC 2038 and IC 2039. The ring orbits around 32... |
(L)SB(rs)00 | 790 ± 5 | 11.7 | ||
| IC 2038 | Sd pec | 712 ± 52 | 15.5 | ||
| APMBGC 157+016+068 | Irregular | 1350 ± 4 | 16.3 | ||
| NGC 1546 | SA0+? | 1284 ± 14 | 11.8 | ||
| IC 2058 | Sc | 1379 ± 1 | 13.9 | ||
| IC 2032 | IAB(s)m pec | 1068 ± 7 | 14.7 | ||
| NGC 1566 NGC 1566 NGC 1566 is an intermediate spiral galaxy in the constellation Dorado. It is the dominant member of the Dorado Group and also its brightest member. It is the second brightest Seyfert galaxy after NGC 1068. Its absolute luminosity is 3.7 L☉... |
(R'_1)SAB(rs)bcSy1 | 1504 ± 2 | 10.3 | ||
| NGC 1596 | SA0: sp | 1510 ± 8 | 12.1 | ||
| NGC 1602 | IB(s)m pec | 1568 ± 8 | 13.3 | ||
| NGC 1515 | SAB(s)bc | 1175 ± 7 | 12.1 | ||
| NGC 1522 | (R')S00: pec | 898 ± 7 | 13.9 | ||
| ESO 118-019 | S00 pec | 1239 | 14.9 | ||
| ESO 157-030 | E4 | 1471 ± 28 | 14.7 | ||
| ESO 157-047 | S0/a? pec sp | 1655 ± 10 | 15.5 | ||
| ESO 157-049 | S? | 1678 ± 5 | 14.3 | ||
| IC 2085 | S00 pec sp | 982 ± 10 | 13.9 | ||
| NGC 1549 | E0-1 | 1220 ± 15 | 10.7 | ||
| NGC 1553 NGC 1553 NGC 1553 is a prototypical lenticular galaxy in the constellation Dorado. It is the second brightest member of the Dorado Group of galaxies. British astronomer John Herschel discovered NGC 1553 on December 5, 1834 using an 18.7 inch reflector.... |
SA(rl)00 | 1080 ± 11 | 10.3 | ||
| NGC 1574 | SA0- | 1050 ± 25 | 11.4 | ||
| NGC 1581 | S0- | 1600 ± 27 | 13.6 | ||
| NGC 1617 | (R')SAB(rs)a | 1063 ± 21 | 11.4 | ||
| Abell 3202 | Irregular | 1135 ± 40 | 16.9 | ||