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Hunting for new galaxy clusters
Hunting for new galaxy clusters
Counting galaxies that are within a given radius of a point just reveals how many lie in what amounts to a volume of sky in the shape of a cylinder. Without some method of knowing the distance to each galaxy it is impossible to sort out genuine clusters from line of sight arrangements of no astronomical significance. The problems that this creates is best considered via a worked example. Consider galaxies identified as lying close to 233.3831d +31.1464d. Roughly speaking this corresponds to Abell 2092 listed with a radius of 30 arc mins and part of the Corona Borealis supercluster. The Catalogue of Principal Galaxies lists 18 within 10 arc minutes - 21 within 12 arc minutes and the next closest at 16.7 arc minutes. Conclusion:- galaxy cluster radius 10 to 12 arc minutes. Hyperleda lists 28 galaxies within 10 arc minutes - 35 within 12 arc minutes and the next closest is at 14.55 arc minutes. Conclusion:- galaxy cluster radius 10 to 12 arc minutes NED lists 72 and 91 galaxies respectively but there is a less clear cut boundary if you consider the whole group. The good news is that there is red shift data available and this is very revealing. If you just consider galaxies with a red shift in the narrow range between 0.060 and 0.072 then 41 lies within 10 arc minutes, 45 within 12 arc minutes with only six more if you go out to 15 arc minutes. Increasing the red shift range to 0.057 to 0.075 only increases these numbers to 42, 46 and 52 so the chosen group is well defined. Conclusion:- galaxy cluster radius 10 to 12 arc minutes with all members with a red shift of between 0.060 and 0.072. Of course red shifts do not provide an entirely reliable indication of distance because of distortions to the local velocity field due to in- fall onto concentrations of mass but it is a lot more to go on than Abell et al had! I think that this shows that there is significant scope for new galaxy clusters to be "extracted" from already listed examples but that the criteria for any discovery claim need to be examined and discussed. Martin Nicholson, Daventry, England. http://www.martin-nicholson.info/1/1a.htm Visit the Astronomical Hall of Shame at http://www.geocities.com/queen5658/ |
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