The high sensitivity and resolution of the Chandra satellite has allowed the detection in the clusters A2142 and A3667 of sharp discontinuities in the X-ray surface brightness, the so called cold fronts [151, 152, 153]. Ettori and Fabian [154] pointed out that the observed temperature jumps in A2142 require that thermal conduction across cold fronts must be suppressed by a factor of 100 or more, compared to the classical Spitzer value [155]. Actually, a tangled magnetic field has been found [156] to reduce the thermal conductivity from the Spitzer value by a factor of order 102 - 103.
Similar features have now been detected in several other clusters [65, 157, 158]. These structures are apparently contact discontinuities between the gas which was in the cool core of one of the merging sub-clusters and the surrounding intracluster gas. They are not shocks because the density increase across the front is accompanied by a temperature decrease such that there is no dramatic change in the pressure and entropy.
In the cluster A3667
[152,
153]
(see Fig. 7),
the temperature discontinuity in the cold front
occurs over a scale of 3.5"
(
4 kpc). Vikhlinin et al.
[153]
suggested that in order to reproduce
such a sharp feature, magnetic fields are required to suppress
both thermal conduction and Kelvin-Helmholtz instabilities
along the contact discontinuity.
They found that the front sharpness and its gradual smearing at large
angles are most likely explained by the existence of a layer
with a 10
µG magnetic field parallel to the front.
The magnetic filed in the layer is probably
amplified by the stretching of the field lines.
 |
Figure 7. Chandra X-ray image (left) and temperature map (right) of A3667 [152, 153]. |