Mach number M determined from the LOS velocity and the thermodynamic parameters retrieved from the inversion for the LBG (top row, see Fig. 2) and a QSG (bottom row, see Fig. 4). The maps only show the very deep photospheric layers from log τ = −0.4 (left) to log τ = + 0.2 (right). Supersonic velocities are indicated by red/yellow colors, subsonic velocities by blue colors. Negative values denote the Mach number for upflows.