We study the distribution of projected offsets between the cluster X-ray centroid and the brightest cluster galaxy ( BCG ) for 65 X-ray selected clusters from the Local Cluster Substructure Survey ( LoCuSS ) , with a median redshift of z = 0.23 .
We find a clear correlation between X-ray/BCG projected offset and the logarithmic slope of the cluster gas density profile at 0.04 r _ { \mathrm { 500 } } ( \alpha ) , implying that more dynamically disturbed clusters have weaker cool cores .
Furthermore , there is a close correspondence between the activity of the BCG , in terms of detected H \alpha and radio emission , and the X-ray/BCG offset , with the line emitting galaxies all residing in clusters with X-ray/BCG offsets of \leq 15 kpc .
Of the BCGs with \alpha < - 0.85 and an offset < 0.02 r _ { \mathrm { 500 } } , 96 per cent ( 23/24 ) have optical emission and 88 per cent ( 21/24 ) are radio active , while none has optical emission outside these criteria .
We also study the cluster gas fraction ( f _ { \mathrm { gas } } ) within r _ { \mathrm { 500 } } and find a significant correlation with X-ray/BCG projected offset .
The mean f _ { \mathrm { gas } } of the ‘ small offset ’ clusters ( < 0.02 r _ { \mathrm { 500 } } ) is 0.106 \pm 0.005 ( \sigma = 0.03 ) compared to 0.145 \pm 0.009 ( \sigma = 0.04 ) for those with an offset > 0.02 r _ { \mathrm { 500 } } , indicating that the total mass may be systematically underestimated in clusters with larger X-ray/BCG offsets .
Our results imply a link between cool core strength and cluster dynamical state consistent with the view that cluster mergers can significantly perturb cool cores , and set new constraints on models of the evolution of the intracluster medium .