Eridanus Group
A nearby loose galaxy group with intermediate structure and many S0 galaxies.
The Eridanus Group, also referred to as the Eridanus Cloud, is a nearby, loosely bound collection of galaxies located roughly 75 million light-years away in the constellation Eridanus. It contains about 200 galaxies, with around 70% being spiral or irregular types and the remaining 30% elliptical or lenticular.
First noted by Baker in 1933, the group was studied in greater detail by de Vaucouleurs in 1975. Later work in 1989 suggested it is condensing from the Hubble Flow and forms a prominent part of the large-scale Eridanus-Fornax-Dorado Filament. Spanning about 10 megaparsecs, the group occupies a sky region bounded by right ascension 3h6' to 4h30' and declination −10° to −30°.
The group’s galaxies are distributed in several clumps, giving it an intermediate structure between a loose group like the Ursa Major Group and a richer cluster like the Virgo Cluster. Dynamic studies indicate it is at a key evolutionary stage, with galaxy types already fixed. This suggests that galaxies forming in rich clusters are already highly evolved, supporting aspects of Cold Dark Matter theories.
The Eridanus Group is forming from the merger of several sub-groups, including the Fornax I Cluster, the Eridanus Cluster (or Fornax II Cluster), and the Dorado Group. Some studies further break these into the Eridanus Group, NGC 1407 Group, and NGC 1332 Group. Bright member galaxies include NGC 1407 (the brightest), NGC 1332, NGC 1309, and NGC 1209.
Its large population of S0 galaxies makes it a focus of debate over how this galaxy type forms. Within the surveyed sub-groups, the brightest galaxy is often an elliptical or S0 type. Current theories suggest S0 galaxies form as part of the evolutionary process, implying significant evolution has occurred within the group despite its apparent youth.
The term "Eridanus Group" also refers to a sub-group of the Eridanus Cloud, located at a distance of 14.2 Mpc. This sub-group has 31 known members, nine listed in the New General Catalogue and two in the Index Catalogue. Its brightest galaxy is the large elliptical NGC 1395. Not centered on any single galaxy, X-ray studies show it is dynamically young. Along with the NGC 1407 and NGC 1332 sub-groups, it is merging to form the Eridanus galaxy supergroup, which is predicted to eventually become a single poor cluster with a combined mass of about 7 × 10¹³ solar masses.
- Mean distance
- 75 Mly (23 ± 2 Mpc)
- Number of galaxies
- approximately 200
- Spiral and irregular galaxies
- approximately 70%
- Elliptical and lenticular galaxies
- approximately 30%
- Angular extent on sky
- 3hr6' to 4hr30' Right Ascension, −10° to −30° Declination
- Linear extent
- approximately 10 Mpc
- Brightest galaxy
- NGC 1407
Lore & Background
The concentration of galaxies in the Eridanus area was first identified by Baker in 1933, with a more in-depth study by de Vaucouleurs in 1975. Work by Willmer et al. in 1989 suggested the group is condensing from the Hubble Flow and is a prominent feature in the large-scale Eridanus-Fornax-Dorado Filament. The group extends over approximately 10 Mpc and is bounded on the sky by specific right ascension and declination lines.
The galaxies are distributed in several clumps, giving the group an appearance intermediate between a loose group (like the Ursa Major Group) and a cluster (like the Virgo Cluster). Dynamic studies indicate the group is at an important stage of its evolution, with galaxy types already fixed, supporting aspects of Cold Dark Matter theories. The group is forming from the merger of sub-groups including the Fornax I Cluster, the Eridanus Cluster (or Fornax II Cluster), and the Dorado Group, with further sub-groups such as the NGC 1407 Group and the NGC 1332 Group.
The group's significant population of S0 type galaxies has made it a target for investigation due to an ongoing debate over S0 formation. Within surveyed sub-groups, the brightest galaxy is often elliptical or S0. Present theories suggest S0 galaxies form as part of the evolutionary process, implying significant evolution has occurred despite the group's young age.
Reader's Guide
The Eridanus Group holds significance as an intermediate system whose structure lies between that of a loose group and a rich cluster, offering insights into galaxy group evolution. Its study supports aspects of Cold Dark Matter theories by showing that galaxies involved in rich cluster formation are already highly evolved. The group's substantial population of S0 type galaxies provides a key testing ground for the ongoing debate over how these galaxies form, with evidence suggesting they arise through evolutionary processes. The group is predicted to be in an early stage of formation, yet it already exhibits a high number of early type galaxies, indicating that significant galaxy evolution can occur before a group fully assembles. The sub-group known as the Eridanus Group (a sub-group of the Eridanus Cloud) is dynamically young, not centered on any one galaxy, and is predicted to merge with the NGC 1407 and NGC 1332 sub-groups to form a single poor cluster with a combined mass of approximately 7 × 10^13 solar masses. The NGC 1407 sub-group contains most of the early type galaxies associated with the larger Eridanus Group.
Did You Know?
- The Eridanus Group is also called the Eridanus Cloud.
- Approximately 70% of its galaxies are spiral and irregular types, while 30% are elliptical and lenticular.
- The group is predicted to merge from sub-groups including the Fornax I Cluster, the Eridanus Cluster, and the Dorado Group.
- The brightest galaxy in the overall group is NGC 1407.
More in Galaxy Clusters and Groups 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced