We use the Chandra X-ray Observatory to study the dark matter halos of 34 massive , dynamically relaxed galaxy clusters , spanning the redshift range 0.06 < z < 0.7 .
The observed dark matter and total mass ( dark-plus-luminous matter ) profiles can be approximated by the Navarro Frenk & White ( hereafter NFW ) model for cold dark matter ( CDM ) halos ; for \sim 80 per cent of the clusters , the NFW model provides a statistically acceptable fit .
In contrast , the singular isothermal sphere model can , in almost every case , be completely ruled out .
We observe a well-defined mass-concentration relation for the clusters with an intrinsic scatter in good agreement with the predictions from simulations .
The slope of the mass-concentration relation , c \propto M _ { vir } ^ { a } / ( 1 + z ) ^ { b } with a = -0.45 \pm 0.12 at 95 per cent confidence , is steeper than the value a \sim - 0.1 predicted by CDM simulations for lower mass halos .
With the slope a included as a free fit parameter , the redshift evolution of the concentration parameter , b = 0.71 \pm 0.52 at 95 per cent confidence , is consistent with the same simulations ( b \sim 1 ) .
Fixing a \sim - 0.1 leads to an apparent evolution that is significantly slower , b = 0.30 \pm 0.49 , although the goodness of fit in this case is significantly worse .
Using a generalized NFW model , we find the inner dark matter density slope , \alpha , to be consistent with unity at 95 per cent confidence for the majority of clusters .
Combining the results for all clusters for which the generalized NFW model provides a good description of the data , we measure \alpha = 0.88 \pm 0.29 at 95 per cent confidence , in agreement with CDM model predictions .