Eridanus Group
Encyclopedia
The Eridanus Group, sometimes called the Eridanus Cloud, is a nearby loose grouping of galaxies
at a mean distance of approximately 75 Mly (23 ± 2 Mpc) in the constellation Eridanus
. Redshift
values show that there are approximately 200 galaxies associated with the group, approximately 70% of which are spiral
and irregular
type galaxies whilst the remaining 30% are elliptical
and lenticular
types.
and −10° and −30° Declination
.
The galaxies in the Eridanus Group are distributed in several clumps and overall the group has the appearance of an intermediate system, its structure being somewhere between that of a loose group (like the Ursa Major Group) and a cluster (like the Virgo Cluster
). Dynamic studies indicate that the group is at an important stage of its evolution with the types of galaxy present already fixed. This suggests that galaxies involved in the formation of rich clusters are already highly evolved, a finding which supports aspects of Cold Dark Matter theories.
The Group is forming from the merger of a number of sub-groups. The group is predicted to be in an early stage of formation and exhibits a high number of early type galaxies. The sub-groups involved in the merger include the Fornax I Cluster
, the Eridanus Cluster
(or Fornax II Cluster) and the Dorado group. In some studies these have been broken down into further sub-groups including the Eridanus Group, the NGC 1407 Group and the NGC 1332 Group. Bright individual galaxies which are members of the group include NGC 1407 (the brightest), NGC 1332, NGC 1309
and NGC 1209.
The group's significant population of S0 type galaxies has made it a target for investigation because of an ongoing debate over the formation of this species of galaxy. Within the surveyed sub-groups of the Eridanus Group it has been found that the brightest galaxy is often the elliptical or S0 type galaxy. Present theories on galaxy evolution suggest that S0 type galaxies form as part of the evolutionary process. This would suggest that there has been time for significant evolution of the galaxies within the Group, despite the young age of the Group itself.
and two listed in the Index Catalogue
. The brightest galaxy in this sub-group is the large elliptical, NGC 1395. The sub-group is not centred on any one galaxy and studies of X-ray emissions
from the group indicate that it is dynamically young. It is involved, along with the NGC 1407 and NGC 1332 sub-groups, in the formation of the Eridanus galaxy supergroup
with the three groups predicted eventually to merge to form a single poor cluster with a combined mass of approximately 7 × 1013 solar mass
es. The NGC 1407 sub-group contains most of the early type galaxies associated with the Eridanus Group.
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...
at a mean distance of approximately 75 Mly (23 ± 2 Mpc) in the constellation Eridanus
Eridanus (constellation)
Eridanus is a constellation. It is represented as a river; its name is the Ancient Greek name for the Po River. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations. It is the sixth largest of the modern...
. 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...
values show that there are approximately 200 galaxies associated with the group, approximately 70% of which are 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...
and irregular
Irregular galaxy
An irregular galaxy is a galaxy that does not have a distinct regular shape, like a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a...
type galaxies whilst the remaining 30% are 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...
and 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...
types.
Characteristics
The concentration of galaxies in the Eridanus area was first identified by Baker in 1933. A more in depth study and discussion of the region was carried out by de Vaucouleurs in 1975. Work by Willmer et al. in 1989 suggested that the Group is condensing from the Hubble Flow and is a prominent feature in the large-scale system called the Eridanus-Fornax-Dorado Filament. The group extends over approximately 10 Mpc and on the sky is bounded by the lines of 3hr6' and 4hr30' of Right AscensionRight 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:...
and −10° and −30° Declination
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...
.
The galaxies in the Eridanus Group are distributed in several clumps and overall the group has the appearance of an intermediate system, its structure being somewhere between that of a loose group (like the Ursa Major Group) and a cluster (like the Virgo Cluster
Virgo Cluster
The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly away in the constellation Virgo. Comprising approximately 1300 member galaxies, the cluster forms the heart of the larger Local Supercluster, of which the Local Group is an outlying member...
). Dynamic studies indicate that the group is at an important stage of its evolution with the types of galaxy present already fixed. This suggests that galaxies involved in the formation of rich clusters are already highly evolved, a finding which supports aspects of Cold Dark Matter theories.
The Group is forming from the merger of a number of sub-groups. The group is predicted to be in an early stage of formation and exhibits a high number of early type galaxies. The sub-groups involved in the merger include the Fornax I 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 Eridanus Cluster
Eridanus Cluster
The center of the Eridanus Cluster is roughly 85 million light years from Earth, and contains about 73 main galaxies. It is located in the constellation Eridanus near the Fornax cluster, and is sometimes called the "Fornax II cluster."-Table of galaxies:...
(or Fornax II Cluster) and the Dorado group. In some studies these have been broken down into further sub-groups including the Eridanus Group, the NGC 1407 Group and the NGC 1332 Group. Bright individual galaxies which are members of the group include NGC 1407 (the brightest), NGC 1332, NGC 1309
NGC 1309
NGC 1309 is a spiral galaxy located approximately 120 million light-years away in the constellation Eridanus. It is about 75,000 light-years across; about 3/4s the width of our own galaxy, the Milky Way...
and NGC 1209.
The group's significant population of S0 type galaxies has made it a target for investigation because of an ongoing debate over the formation of this species of galaxy. Within the surveyed sub-groups of the Eridanus Group it has been found that the brightest galaxy is often the elliptical or S0 type galaxy. Present theories on galaxy evolution suggest that S0 type galaxies form as part of the evolutionary process. This would suggest that there has been time for significant evolution of the galaxies within the Group, despite the young age of the Group itself.
Sub-groups
The term Eridanus Group has also been applied to a sub-group of the Eridanus Cloud. This grouping of galaxies is found at a distance of 14.2 Mpc. The Eridanus Group is known to have 31 members with nine listed in the New General CatalogueNew General Catalogue
The New General Catalogue of Nebulae and Clusters of Stars is a well-known catalogue of deep sky objects in astronomy. It contains 7,840 objects, known as the NGC objects...
and two listed in the Index Catalogue
Index Catalogue
The Index Catalogue —also known as the Index Catalogue of Nebulae, the Index Catalogue of Nebulae and Clusters of Stars, IC I, or IC II— is a catalogue of galaxies, nebulae and star clusters that serves as a supplement to the New General Catalogue...
. The brightest galaxy in this sub-group is the large elliptical, NGC 1395. The sub-group is not centred on any one galaxy and studies of X-ray emissions
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
from the group indicate that it is dynamically young. It is involved, along with the NGC 1407 and NGC 1332 sub-groups, in the formation of the Eridanus galaxy supergroup
Supergroup (music)
In the late 1960s, the term supergroup was coined to describe "a rock music group whose performers are already famous from having performed individually or in other groups"....
with the three groups predicted eventually to merge to form a single poor cluster with a combined mass of approximately 7 × 1013 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es. The NGC 1407 sub-group contains most of the early type galaxies associated with the Eridanus Group.