Chandra X-ray image of the core of the cooling-flow cluster Abell 2052.
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My general area of research is the study of the X-ray and optical properties of clusters of galaxies. In particular, I am studying a subset of clusters known as "cooling flows." Cooling-flow clusters have quickly cooling gas in their centers which should form large amounts of stars and cold gas; however, little star formation and cold gas have been found.
A theory to account for this discrepancy is that cooling of the intracluster medium (ICM) in cooling-flow clusters is proceeding at much reduced rates due to the input of large amounts of heat (up to 1045 - 1046 ergs s-1) to the ICM by the active galactic nucleus (AGN). In this picture, the reduced cooling should lead eventually to star formation, whose rates should be less than or equal to the cooling rates.
An agreement of these rates has been shown for a number of objects; however, the details of the process are not clear. In particular, questions of the star formation histories, the role of the AGN in triggering star formation, and the effect of star formation on the cooling flow have yet to be fully answered. My thesis research is a study of star formation and cooling rates and AGN heating for a sample of objects with high quality optical, X-ray, and radio data.
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The radio source in Hydra A (from Taylor et al. 1990 shown here as 4.8 GHz contours) has pushed out cavities in the ICM in a Chandra X-ray image.
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The AGN interacts with the ICM though its radio-emitting jets, which are seen in many cluster to push out cavities and shocks in the cluster atmosphere. These cavities and shocks are capable in some objects of balancing cooling losses, but are unable to account alone for the radiation losses in all systems.
The AGN heating scenario probably requires that a feedback mechanism operates between the cooling gas and the source of heat. This feedback could be in the form of a self-regulated loop in which cooling gas accretes onto the central black hole, fueling episodic radio outbursts. The outbursts temporarily quench or significantly reduce cooling through shocks and buoyant bubbles of hot plasma that give up their enthalpy to the surrounding ICM. Eventually, in the absence of cooling gas and accreting material, the AGN weakens or turns off entirely and cooling begins again. The cooling gas would eventually form cold clouds and stars, but at rates much less than previously expected in the classical cooling flow picture.
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