Clusters of Galaxies
Several classification schemes have been developed and correlate reasonably well with one another. One of these is the Rood and Sastry (RS) classification, which is based on the projected distribution of the brightest 10 members. They recognize these types:
- cD - single dominant cD (elliptical) galaxy (A2029, A2199)
- B - dominant binary, like Coma
- L - linear array of galaxies (Perseus)
- C - single core of galaxies
- F - flattened (IRAS 09104+4109)
- I - irregular distribution (Hercules)
Numerous collisions and close encounters are believed to have taken place within clusters of the cD type, with the giant ellipticals having grown by absorbing smaller ones in successive acts of "galactic cannibalism." Figure 04-05b shows a comet-like tail of glowing gas, 200,000 light-years long, streams from galaxy C153 as it plunges through galaxy cluster Abell 2125 at
||These appear to have affinities in a tuning-fork pattern with concentration generally increasing leftward as shown in Figure 04-05a. It conveys a strong impression that the different types are the same kind of system at different stages of development. The evolution sequence seems to be running from right to left.
It is highly improbable that clusters of galaxies have been built up by chance encounters of galaxies in the general field. It just takes too long (longer than the age of the universe) to complete the process. Therefore, either that clusters are systems whose member galaxies became gravitationally bound at more or less the same time, or that the clusters represent condensations from pregalaxian material and that subcondensations within them became galaxies.
Figure 04-05a Cluster of Galaxies Classification
||nearly 8 million kilometers per hour. Itself a member of the giant cluster of galaxies, C153 may once have been a spiral galaxy like the Milky Way. In this series of images, false-color composites of x-ray and optical data, zooms in on this galaxy's fate. A passage through the hot intracluster gas in the central regions of Abell 2125 is seen to be stripping C153 of its own star forming material and distorting its shape. As other galaxies in the cluster suffer a similar fate, the hot gas collecting in the cluster's core should become enriched in heavy elements. The violent spectacle was taking place about 3 billion lys from Earth and is thought to illustrate a common process in the cosmic evolution of large clusters of galaxies. |
Figure 04-05b Cluster of Galaxies Evolution [view large image]