10 arcmin x 10 arcmin field showing luminosity (top) and temperature (bottom) maps of Abell 3888 before (left) and after (right) point source removal. The color scale in the luminosity map is set so that white corresponds to the maximum cluster flux. The point source is 100 times brighter than this level. The scale in the temperature map ranges from 2 to 10 keV. Credit: Andersson, K. et al., 2009
Studying substructures of galaxy clusters can reveal important information about the morphology and evolution processes of these gravity-bound groups of galaxies. Optical spectroscopy helps unravel the history of large-scale structure formation in the universe. That’s why a team of NZ astronomers conducted a series of spectroscopic observations to peek into the galaxy cluster Abell 3888, unveiling that this cluster is dynamically young and may be an indicator of an ongoing or past merger event.
They used the AAOmega spectrograph on the 3.9-m Anglo-Australian Telescope (AAT) situated at the Siding Spring Observatory in Australia. Thanks to the spectrograph’s 400 fibres covering a 2-degree field when projected on sky, it is an excellent instrument for examining the optical substructure in nearby southern clusters.
They were initially targeting nearly 800 galaxies up to 30′ radius from the centre of the cluster. They detected 254 new redshifts in this region and in combination with previous findings, they determined Abell 3888 has 71 member galaxies. Importantly, they identified substructures in the field that were very helpful in unraveling the merging nature of this galaxy cluster. “The combination of pieces of evidence from the optical analysis, the elongated optical galaxy distribution, and our substructure test which showed that Abell 3888 is bimodal strongly suggests that this cluster has had dynamical interactions and is highly likely to be a young cluster in an active merging state,” the paper reads.
Galaxies and galaxy groups come together and merge to form larger units such as galaxy clusters. Cluster merging is believed to be a key parameter in formation and evolution of galaxy clusters. The process is very common and has great impact on cluster characteristics eg velocity dispersion, temperature and mass. It often generates clumps of galaxies within the cluster volume. This change in galaxy volumetric density is known as “substructure.” …”Substructures may be formed through the infall of individual galaxies or galaxy groups into a relaxed cluster or during the merging of 2 or more entire galaxy clusters”
They also detected 6 galaxy over-densities in the field, with 3 of classified as new galaxy clusters. They stressed the need for single slit spectroscopy or more usefully observations with an integral field unit to increase the spectroscopic coverage in the cluster core. “This would allow a more detailed probe of the cluster core and better statistics on the merging populations,” the scientists noted.
http://phys.org/news/2016-02-peek-merging-galaxy-cluster-abell.htmljCp http://arxiv.org/abs/1602.03756
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