IRAM-30 m maps of the Horsehead edge. Maps were rotated by counterclockwise around the projection center, located at (δx,δy) = (20′′,0′′), to bring the exciting star direction in the horizontal direction, and the horizontal zero was set at the PDR edge, delineated by the red vertical line. The crosses show the positions of the PDR (green) and the dense core (blue), where deep integrations were performed at IRAM-30 m. The white lines delineate the arc-like structure of the DCO⁺ emission. The spatial resolution is plotted in the bottom-left corner. Values of contour levels are shown in the respective image lookup table. The emission of all lines is integrated between 10.1 and 11.1   km   s^-1.