Wednesday, March 02, 2005

Biman Nath: Entropy in the ICM and AGN

Biman Nath gave the UMCP Astronomy Colloquium. He reviewed the entropy of the ICM and in particular that in groups. S = T/n^2/3. If clusters are created at the same time we expect S to go as T but it goes as T^0.65. Further we expect L to go as T^2 however it goes as almost T^3. These imply something other than gravity determines the structure of the ICM. The effect is particularly marked in groups. One possibility is that galaxies form more efficiently in groups leading to the low S gas to cool out and be replaced by high S gas falling into the center. This may have some effect but cannot explain the observations on its own. The alternative is heating either by AGN or cosmic rays. There is good evidence now for AGN energy input into the ICM. The unknown is whether this can happen at higher redshifts and to the extent required. A fascinating new result (Croston et al 2005) seems to show that those groups containing radio-loud AGN have higher entropy than those having radio-quiet AGN. I pointed out in a question that since entropy is a time integral over the lifetime of the cluster this would imply that AGN which are radio-loud now were radio-loud in the past (and similarly for radio-quiet). Nath wondered whether the observed entropy would change while a radio-loud AGN was active.

Keywords: clusters, AGN, entropy

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