madbadgalaxyman
14-09-2013, 03:51 PM
The Centaurus Galaxy Cluster
by Robert Anthony Lang
Introduction
The Centaurus Cluster, alias Abell 3526, is one of the most easily studied moderately-rich clusters of galaxies. The Centaurus Cluster is probably intermediate in richness between the Virgo Cluster and the Coma Cluster. (its distance is also intermediate between those of Virgo and Coma).
Estimates of the total (gravitating) cluster mass of the Centaurus Cluster (including dark and luminous matter) have varied wildly because of the multiple subcomponents (subclusters, subgroups) which can be found both in the apparent clumpy sky distribution and in the distribution of galaxy velocities. Galaxy clusters occupy the mass range from about 10E14 solar masses to perhaps a few times 10E15 solar masses. For comparison purposes, the largest known overdensities of matter (inhomogeneities in the universe) are of the order of 10E17 solar masses, and the gravitating mass associated with a single galaxy can be as much as 10E12 solar masses. For recent mass estimates of Centaurus and other galaxy clusters (statistically valid, but likely to be ‘indicative only’ for individual objects), see Girardi et al.(1998, ApJ, 505, 74).
The compact and galaxy-rich Core Region of this cluster [ RA(1950) 12h44m40s to 12h52m40s, and
Dec (1950) –41 deg 40’ to –40 deg 12’ ] was surveyed in detail, at high angular and morphological resolution, by Jerjen & Dressler (1997, A & AS, 124, 1), resulting in their Centaurus Cluster Catalogue ( = the CCC). However, their survey did not include the entire complex of several candidate subclusters surrounding the Core Region, which stretches some 13 by 3 degrees, elongated in the E-W direction. .
The entire Centaurus Cluster complex was surveyed by Dickens, Currie and Lucey (1986, MNRAS, 220, 679), and their paper includes a catalogue of the entire complex - usually designated the DCL catalogue. Galaxy redshifts and cluster structure and cluster kinematics were further analyzed by the same team, in Lucey et al. (1986, MNRAS, 222, 427), hereafter referred to as ‘LCD’. However, the Hubble types were binned very broadly and their survey was too shallow to detect most of the dwarf galaxies in the cluster.
The Supercluster Context
The place of the Centaurus Cluster within the wider Centaurus Supercluster of galaxies (in the constellation Centaurus) will only be briefly summarized. There are very numerous galaxies throughout the constellation Centaurus within the velocity range occupied by the Centaurus Cluster’s galaxies, indicating some type of supergiant constant-velocity “surface” of galaxies, all of them at a roughly similar distance. Particularly large numbers of widely scattered galaxies are found in the vicinity of the Centaurus Cluster near to the heliocentric recession velocity of its main component, ~3300 km/s.
In general, wherever you look in the constellation Centaurus, you will find that the galaxy velocities are strongly clustered around several specific velocities, for instance this author has found that the fields extending from NGC 4945 and Omega Centauri towards the Centaurus Cluster have heliocentric galaxy velocities in at least two distinct groups – falling either near to ~2000 km/s or in the range 2900 – 3600 km/s. (incidentally, the velocity grouping near 2000 km/s strongly argues that there is foreground structure in front of the Centaurus Cluster)
Numerous inter-cluster galaxies link Abell 3526 to several prominent galaxy clusters to its North-East, notably Abell 3565, Abell 3574, and the NGC 5419 cluster ( = ACO 4829). In general, there is a very strong degree of clustering within the Centaurus Supercluster, with very numerous galaxy groups found at similar distances (N4936, N5153, ESO 507-025, etc.), but the overall structure of the Supercluster is very poorly known.
There is also an extremely well-defined (having a small velocity range) constant-velocity surface of galaxies linking the Cen. Cluster to the prominent clusters and galaxy groups to the west of it (e.g. N3557 group, N3256 group, N3250 group, Antlia Cluster, Hydra Cluster), but this inter-cluster supergalactic structure has much fewer prominent galaxies.
The discovery of the Cen 30 and Cen 45 subclusters
LCD found that Centaurus Cluster (hereafter referred to as “Cen.” ) is composed of two apparent clusters (or subclusters) in the same line-of-sight, which are nevertheless not well separated when we view them in the sky. They discovered them by finding that each cluster or subcluster has a distinct (roughly Gaussian) velocity distribution of its galaxies. They thus split Cen. into one galaxy group, associated with NGC 4696 (the ‘dominant’ and most luminous elliptical galaxy in Cen.) and having a mean recession velocity of 3041 km/s and a velocity dispersion of 586 km/s, and another galaxy group (which is associated with NGC 4709) having a mean velocity of 4570 km/s and a velocity dispersion of 280 km/s. These two components of the Centaurus Cluster, they named Cen 30 and Cen 45, names which still have wide currency.
They estimated that the Cen 45 subcluster is 2.5 times less rich than the Cen 30 subcluster,
and estimated that the two subclusters were at approximately the same distance. Thus, the higher velocity component Cen 45 is not at 1.5 times the distance of Cen 30, which would be the conclusion if all of the velocity of Cen 45 were due to the expansion of the universe. On the assumption of approximate equality of distance for the two galaxy groups, the enormous difference in line-of-sight velocity between Cen 30 and Cen 45 is usually ascribed to Cen 45 falling into Cen 30. Thus, the (still popular) scenario was developed of a major cluster (Cen 30) attracting a smaller galaxy group (Cen 45), with the eventual result being a merger of the two galaxy aggregations.
Subsequent work has tended to confirm the dichotomy between the two major galaxy groups composing Cen., but there are some indications that some Cen. galaxies may also occupy some kind of extended filament extending in the line-of-sight. Stein et al. (1997, A & A, 327, 952) for instance found that the giant (non-dwarf) E and S0 cluster galaxies occupy a more continuous range of velocities than the more strongly clumped (in velocity space) dwarf galaxies.
This work of Stein et al., based on a deeper survey of the cluster core, also indicated a much greater degree of difference between Cen 30 and Cen 45, with Cen 30 being almost 5 times richer in galaxies.
by Robert Anthony Lang
Introduction
The Centaurus Cluster, alias Abell 3526, is one of the most easily studied moderately-rich clusters of galaxies. The Centaurus Cluster is probably intermediate in richness between the Virgo Cluster and the Coma Cluster. (its distance is also intermediate between those of Virgo and Coma).
Estimates of the total (gravitating) cluster mass of the Centaurus Cluster (including dark and luminous matter) have varied wildly because of the multiple subcomponents (subclusters, subgroups) which can be found both in the apparent clumpy sky distribution and in the distribution of galaxy velocities. Galaxy clusters occupy the mass range from about 10E14 solar masses to perhaps a few times 10E15 solar masses. For comparison purposes, the largest known overdensities of matter (inhomogeneities in the universe) are of the order of 10E17 solar masses, and the gravitating mass associated with a single galaxy can be as much as 10E12 solar masses. For recent mass estimates of Centaurus and other galaxy clusters (statistically valid, but likely to be ‘indicative only’ for individual objects), see Girardi et al.(1998, ApJ, 505, 74).
The compact and galaxy-rich Core Region of this cluster [ RA(1950) 12h44m40s to 12h52m40s, and
Dec (1950) –41 deg 40’ to –40 deg 12’ ] was surveyed in detail, at high angular and morphological resolution, by Jerjen & Dressler (1997, A & AS, 124, 1), resulting in their Centaurus Cluster Catalogue ( = the CCC). However, their survey did not include the entire complex of several candidate subclusters surrounding the Core Region, which stretches some 13 by 3 degrees, elongated in the E-W direction. .
The entire Centaurus Cluster complex was surveyed by Dickens, Currie and Lucey (1986, MNRAS, 220, 679), and their paper includes a catalogue of the entire complex - usually designated the DCL catalogue. Galaxy redshifts and cluster structure and cluster kinematics were further analyzed by the same team, in Lucey et al. (1986, MNRAS, 222, 427), hereafter referred to as ‘LCD’. However, the Hubble types were binned very broadly and their survey was too shallow to detect most of the dwarf galaxies in the cluster.
The Supercluster Context
The place of the Centaurus Cluster within the wider Centaurus Supercluster of galaxies (in the constellation Centaurus) will only be briefly summarized. There are very numerous galaxies throughout the constellation Centaurus within the velocity range occupied by the Centaurus Cluster’s galaxies, indicating some type of supergiant constant-velocity “surface” of galaxies, all of them at a roughly similar distance. Particularly large numbers of widely scattered galaxies are found in the vicinity of the Centaurus Cluster near to the heliocentric recession velocity of its main component, ~3300 km/s.
In general, wherever you look in the constellation Centaurus, you will find that the galaxy velocities are strongly clustered around several specific velocities, for instance this author has found that the fields extending from NGC 4945 and Omega Centauri towards the Centaurus Cluster have heliocentric galaxy velocities in at least two distinct groups – falling either near to ~2000 km/s or in the range 2900 – 3600 km/s. (incidentally, the velocity grouping near 2000 km/s strongly argues that there is foreground structure in front of the Centaurus Cluster)
Numerous inter-cluster galaxies link Abell 3526 to several prominent galaxy clusters to its North-East, notably Abell 3565, Abell 3574, and the NGC 5419 cluster ( = ACO 4829). In general, there is a very strong degree of clustering within the Centaurus Supercluster, with very numerous galaxy groups found at similar distances (N4936, N5153, ESO 507-025, etc.), but the overall structure of the Supercluster is very poorly known.
There is also an extremely well-defined (having a small velocity range) constant-velocity surface of galaxies linking the Cen. Cluster to the prominent clusters and galaxy groups to the west of it (e.g. N3557 group, N3256 group, N3250 group, Antlia Cluster, Hydra Cluster), but this inter-cluster supergalactic structure has much fewer prominent galaxies.
The discovery of the Cen 30 and Cen 45 subclusters
LCD found that Centaurus Cluster (hereafter referred to as “Cen.” ) is composed of two apparent clusters (or subclusters) in the same line-of-sight, which are nevertheless not well separated when we view them in the sky. They discovered them by finding that each cluster or subcluster has a distinct (roughly Gaussian) velocity distribution of its galaxies. They thus split Cen. into one galaxy group, associated with NGC 4696 (the ‘dominant’ and most luminous elliptical galaxy in Cen.) and having a mean recession velocity of 3041 km/s and a velocity dispersion of 586 km/s, and another galaxy group (which is associated with NGC 4709) having a mean velocity of 4570 km/s and a velocity dispersion of 280 km/s. These two components of the Centaurus Cluster, they named Cen 30 and Cen 45, names which still have wide currency.
They estimated that the Cen 45 subcluster is 2.5 times less rich than the Cen 30 subcluster,
and estimated that the two subclusters were at approximately the same distance. Thus, the higher velocity component Cen 45 is not at 1.5 times the distance of Cen 30, which would be the conclusion if all of the velocity of Cen 45 were due to the expansion of the universe. On the assumption of approximate equality of distance for the two galaxy groups, the enormous difference in line-of-sight velocity between Cen 30 and Cen 45 is usually ascribed to Cen 45 falling into Cen 30. Thus, the (still popular) scenario was developed of a major cluster (Cen 30) attracting a smaller galaxy group (Cen 45), with the eventual result being a merger of the two galaxy aggregations.
Subsequent work has tended to confirm the dichotomy between the two major galaxy groups composing Cen., but there are some indications that some Cen. galaxies may also occupy some kind of extended filament extending in the line-of-sight. Stein et al. (1997, A & A, 327, 952) for instance found that the giant (non-dwarf) E and S0 cluster galaxies occupy a more continuous range of velocities than the more strongly clumped (in velocity space) dwarf galaxies.
This work of Stein et al., based on a deeper survey of the cluster core, also indicated a much greater degree of difference between Cen 30 and Cen 45, with Cen 30 being almost 5 times richer in galaxies.