Average cross-correlation function map derived from the subsequent Doppler images plotted in Figs. 6 and 8. Black represents strong correlation, while light pink indicates no correlation. The quadratic sine fit (solid line) applied to the ridge of the correlation pattern (dots with error bars) suggests a solar-type rotational law with a very weak shear of α_DR = 0.0026 ± 0.0006. See Sect. 5.3 for details.