Example of recombination line stacking toward the Hii region G18.9-0.3. The images show one recombination line emission region for various numbers of stacked images in a single channel of 10 km s^-1 width (around 70 km s^-1). The left panel represents a single recombination line (H152α), the middle contains 5 RRLs, and the right panel combines 10 RRLs. The synthesized beam of 40′′ is shown in the bottom left corner of each panel. For reference, the contours in the right panel present the corresponding 1.4 GHz continuum data (see also Fig. 8) with contour levels from 50 to 150 K in steps of 20 K.