Analysis of the non-radial Mach number, i.e. the Mach number perpendicular to the direction of the shell’s expansion. Only the region with positive X-coordinate, which corresponds to the undisturbed part of the 60 M_⊙ bubble, is taken into account. Left: 4.05 Myr, right: 7.47 Myr. The upper parts show the non-radial Mach number versus the logarithm of the density. Colour encodes the mass per bin, where each bin spans 0.05 in Mach number and 0.06 dex in logarithmic density. No appreciable non-radial motions are found for the hot bubble interior, whereas the dense shell material shows Mach numbers of the order of unity. The lower parts show mass weighted non-radial Mach number histograms (vertically collapsed versions of the plots above). The plots are dominated by the quiescent ambient medium. The mass with given non-radial Mach number declines strongly around a Mach number of unity towards higher Mach numbers as expected for shells dominated by the Vishniac instability.