Composite image of NGC 3503 and its environs. The red color shows the IRAC-GLIMPSE emission at 8.0 μm, while green/yellow tonalities show the ATLASGAL 870 μm continuum emission. The ¹³CO(J = 2 → 1) emission line integrated in the velocity ranges from −27.8 km s^-1 to −23.7 km s^-1 and −17.1 km s^-1 to −15.3 km s^-1 are shown in blue contours. Thick light blue circles indicate the position of the MSX CHii  region candidates, while yellow crosses depict the position of the 2MASS sources with IR excess. Green and white diamonds indicate the position of WISE Class I and Class II candidate YSOs, respectively. Numerical references of YSOs are based on Tables 5 and 6. The size of the symbols do not match the angular size of the sources.