Composite image showing the emission of the HCN(J = 3 → 2) line (green contours), the C¹⁸O(J = 2 → 1) line (white/black contours), and the 870 μm continuum emission (grayscale) in the central region of NGC 3503. The velocity ranges are −26.2 km s^-1 to −25.2 km s^-1 and −26.5 km s^-1 to −25.5 km s^-1 for the C¹⁸O and HCN lines, respectively. Red triangles depict the position of WISE candidate YSOs 71, 76, and 85. The C¹⁸O counterparts of clumps C3, C4, and C7 are indicated. The beam sizes are shown in the lower-left corner.