Left panels: integrated ¹³CO emission in the velocity range from ~−17.1 km s^-1 to −15.3 km s^-1 (upper panel), and from 18.7 km s^-1 to 21.7 km s^-1 (lower panel). The velocity interval of each image is indicated in the upper-left corner. In the upper panel, the lowest ¹³CO temperature contour is 0.35 K km s^-1 (~5 rms) and the contour spacing temperature is 0.7 K km s^-1. In the lower panels, the lowest ¹³CO temperature contour is 0.75 K km s^-1 (~15 rms) and contour spacing temperature is 1 K km s^-1. The beam size is shown by a circle in the lower-left corner of each image. Right panels: overlay of the IRAC GLIMPSE 4 (8 μm) emission of NGC 3503 (grayscale) and the mean ¹³CO emission in the velocity intervals shown in the left panels (contour lines).