madbadgalaxyman
14-09-2013, 03:59 PM
Centaurus Cluster – Some assembly still Required
LCD remarked on the lack of an evenly dispersed and symmetrically arrayed galaxy population in Cen. (as compared, for instance, with the more ‘evolved’ and regular-looking Coma Cluster of galaxies). There is a strong indication that there do exist several substructures within the Centaurus Cluster, with the generally inhomogenous space distribution of its galaxies indicating that it is probably still being assembled from various galaxy groups or subgroups. Thus, Cen. is regarded as a good example of a galaxy cluster which is not yet fully evolved, and which may yet undergo successive mergers of galaxy groups in order to produce a regular-looking cluster.
Cen. is split into many apparent subgroups, with the entire complex of galaxy groups (which I will refer to as the Centaurus Cluster complex) occupying a remarkably extended linear and elongated structure with angular dimensions of about 13 degrees by 3 degrees. (though the linear alignment of galaxies does continue for >13 degrees)
Substructure is a common phenomenon in galaxy clusters (some has even been found in Coma Cluster), for example Stein (1997, A&A, 317, 670) found that 50% of the cluster cores in a sample of 12 clusters had definite substructure - as revealed by ‘clumping’ of galaxy velocities around certain specific velocities. (we say a cluster has a ‘bimodal or multi-modal velocity histogram’ if we wish to impress a professional astronomer with our mastery of their impenetrable jargon)
The Centaurus Cluster complex, which I am tempted to call ‘The Centaurus Stream’, stretches in the W to E direction in approximately p.a. = 110-120 degrees, from the NGC4373/IC3370 galaxy group in the west through to the NGC 5090 group in the east. Actually, the precise delineation of a ‘cluster border’ is particularly uncertain for such an extended and clumpy galaxy cluster!
The precise substructures within Cen. are not yet fully characterised, and the true 3D shape of the overall extended filament or ‘stream’ of galaxies is not yet known. In contrast, the subclusters ( = galaxy groups) within the Virgo Cluster of Galaxies, and the overall 3D shape of the Virgo Cluster (which is another cluster still undergoing assembly) are quite well understood due to the historical accident of Virgo being in the better-studied northern sky.
Given the enormous angular extension of the Centaurus Cluster across the sky, it would come as no surprise if it also turned out to have a substantial physical extension in the LOS.
The pioneering study of LCD isolated several apparent subclusters or galaxy groups within the complex:
(1) The galaxy-dense core of the Centaurus Cluster, a region approximately 1.5 degrees in diameter. This has been shown to be composed of galaxies from both the Cen 30 group and the Cen 45 group, arrayed in nearly the same L.O.S. This core (hereafter called the “Cen Core”) is dominated by two very luminous elliptical galaxies, NGC 4696 and NGC 4709. This core has extremely high galaxy number-density, and (presumably) a dense concentration of dark matter. Note that the apparent sparseness of the core in many amateur telescopes is due to the fact that the cluster only becomes very populous at magnitude B > 14 . The total number of Centaurus Cluster galaxies still continues to steadily increase as we adopt fainter magn. limits, continuing to do so even as we go beyond 20th magnitude! (The Cen 30 group is actually a major galaxy cluster, i.e. the ‘real Centaurus Cluster’, while the Cen 45 group is merely a loose galaxy group.)
(2) The loose NGC 4930 group which is dominated by spirals, the NGC 5011 and NGC 4767 groups which are both associated with a very luminous elliptical galaxy, and the NGC4373/IC3370 group.
Another candidate subcluster of Cen. which is mentioned in LCD is the ‘Centaurus Chain’, an elongated apparent group of galaxies which forms a major section of the overall Centaurus complex. (its galaxies include NGC 4622, NGC 4650, NGC 4650A, NGC 4603, etc. This name for this candidate galaxy group may perhaps be attributed to J.L. Sersic (1967, Z. Astrophys , 67, 306), and refers to the remarkable chain of bright galaxies starting slightly north of the dense cluster core and then extending westwards. However, the reality of the Centaurus Chain as a single galaxy group is in doubt because LCD found that the member galaxies belong to two groups of velocities which are well separated, with no overlap between them - velocity components which they ascribe to Cen 30 and Cen 45. It is not at all certain whether all of the galaxies in this apparent group are at a similar distance, given their wide range of recession velocities (2500 to 5000 km/s). For instance, NGC 4603 has one of the lowest recession velocities in the Chain (cz = 2587) and a Cepheid distance of only 109 million light years, so it is plausibly in the foreground of the Centaurus Cluster. In contrast, a number of the other spirals in the Chain are of the smooth-arm variety, which is generally the sign of a cluster environment.
LCD remarked on the lack of an evenly dispersed and symmetrically arrayed galaxy population in Cen. (as compared, for instance, with the more ‘evolved’ and regular-looking Coma Cluster of galaxies). There is a strong indication that there do exist several substructures within the Centaurus Cluster, with the generally inhomogenous space distribution of its galaxies indicating that it is probably still being assembled from various galaxy groups or subgroups. Thus, Cen. is regarded as a good example of a galaxy cluster which is not yet fully evolved, and which may yet undergo successive mergers of galaxy groups in order to produce a regular-looking cluster.
Cen. is split into many apparent subgroups, with the entire complex of galaxy groups (which I will refer to as the Centaurus Cluster complex) occupying a remarkably extended linear and elongated structure with angular dimensions of about 13 degrees by 3 degrees. (though the linear alignment of galaxies does continue for >13 degrees)
Substructure is a common phenomenon in galaxy clusters (some has even been found in Coma Cluster), for example Stein (1997, A&A, 317, 670) found that 50% of the cluster cores in a sample of 12 clusters had definite substructure - as revealed by ‘clumping’ of galaxy velocities around certain specific velocities. (we say a cluster has a ‘bimodal or multi-modal velocity histogram’ if we wish to impress a professional astronomer with our mastery of their impenetrable jargon)
The Centaurus Cluster complex, which I am tempted to call ‘The Centaurus Stream’, stretches in the W to E direction in approximately p.a. = 110-120 degrees, from the NGC4373/IC3370 galaxy group in the west through to the NGC 5090 group in the east. Actually, the precise delineation of a ‘cluster border’ is particularly uncertain for such an extended and clumpy galaxy cluster!
The precise substructures within Cen. are not yet fully characterised, and the true 3D shape of the overall extended filament or ‘stream’ of galaxies is not yet known. In contrast, the subclusters ( = galaxy groups) within the Virgo Cluster of Galaxies, and the overall 3D shape of the Virgo Cluster (which is another cluster still undergoing assembly) are quite well understood due to the historical accident of Virgo being in the better-studied northern sky.
Given the enormous angular extension of the Centaurus Cluster across the sky, it would come as no surprise if it also turned out to have a substantial physical extension in the LOS.
The pioneering study of LCD isolated several apparent subclusters or galaxy groups within the complex:
(1) The galaxy-dense core of the Centaurus Cluster, a region approximately 1.5 degrees in diameter. This has been shown to be composed of galaxies from both the Cen 30 group and the Cen 45 group, arrayed in nearly the same L.O.S. This core (hereafter called the “Cen Core”) is dominated by two very luminous elliptical galaxies, NGC 4696 and NGC 4709. This core has extremely high galaxy number-density, and (presumably) a dense concentration of dark matter. Note that the apparent sparseness of the core in many amateur telescopes is due to the fact that the cluster only becomes very populous at magnitude B > 14 . The total number of Centaurus Cluster galaxies still continues to steadily increase as we adopt fainter magn. limits, continuing to do so even as we go beyond 20th magnitude! (The Cen 30 group is actually a major galaxy cluster, i.e. the ‘real Centaurus Cluster’, while the Cen 45 group is merely a loose galaxy group.)
(2) The loose NGC 4930 group which is dominated by spirals, the NGC 5011 and NGC 4767 groups which are both associated with a very luminous elliptical galaxy, and the NGC4373/IC3370 group.
Another candidate subcluster of Cen. which is mentioned in LCD is the ‘Centaurus Chain’, an elongated apparent group of galaxies which forms a major section of the overall Centaurus complex. (its galaxies include NGC 4622, NGC 4650, NGC 4650A, NGC 4603, etc. This name for this candidate galaxy group may perhaps be attributed to J.L. Sersic (1967, Z. Astrophys , 67, 306), and refers to the remarkable chain of bright galaxies starting slightly north of the dense cluster core and then extending westwards. However, the reality of the Centaurus Chain as a single galaxy group is in doubt because LCD found that the member galaxies belong to two groups of velocities which are well separated, with no overlap between them - velocity components which they ascribe to Cen 30 and Cen 45. It is not at all certain whether all of the galaxies in this apparent group are at a similar distance, given their wide range of recession velocities (2500 to 5000 km/s). For instance, NGC 4603 has one of the lowest recession velocities in the Chain (cz = 2587) and a Cepheid distance of only 109 million light years, so it is plausibly in the foreground of the Centaurus Cluster. In contrast, a number of the other spirals in the Chain are of the smooth-arm variety, which is generally the sign of a cluster environment.