Distant sources - methods based on radio galaxies can give...

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Distant sources Radio galaxies and radio-loud quasars have been widely used, particularly in the 80s and 90s, to find distant galaxies: by selecting based on radio spectrum and then observing the host galaxy it was possible to find objects at high redshift at modest cost in telescope time. The problem with this method is that hosts of active galaxies may not be typical of galaxies at their redshift. Similarly, radio galaxies have in the past been used to find distant X-ray emitting clusters, but unbiased selection methods are now preferred. Standard rulers Some work has been done attempting to use radio galaxies as standard rulers to determine cosmological parameters . This method is fraught with difficulty because a radio galaxy's size depends on both its age and its environment. When a model of the radio source is used, though,
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Unformatted text preview: methods based on radio galaxies can give good agreement with other cosmological observations. [15] Effects on environment Whether or not a radio source is expanding supersonically, it must do work against the external medium in expanding, and so it puts energy into heating and lifting the external plasma. The minimum energy stored in the lobes of a powerful radio source might be 10 53 J . The lower limit on the work done on the external medium by such a source is several times this. A good deal of the current interest in radio sources focuses on the effect they must have at the centres of clusters at the present day. [16] Equally interesting is their likely effect on structure formation over cosmological time: it is thought that they may provide a feedback mechanism to slow the formation of the most massive objects...
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