This heating rate due to radio-loud AGN activity can be compared with the radiative cooling rates of the haloes of hot gas surrounding elliptical galaxies, as determined using X-ray observations. Figure 3 shows the bolometric X-ray luminosities (LX) of the 401 early-type galaxies from the sample of O'Sullivan et al. , compared with their optical luminosities (Lb). Also shown are the predicted radio-loud AGN heating rates for three different assumed values of /. It is remarkable how well the predicted radio-loud AGN
170 P. N. Best heating rates for f =1 match the radiative cooling rates at all optical luminosities. Provided that all or most of the energetic output of the radio-loud AGN can be coupled effectively to their host galaxy then recurrent radio-loud AGN activity provides a sufficient feedback effect to suppress the cooling of gas and thus control the rate of growth of massive elliptical galaxies.
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