Composite image of NGC 3503 and its environs. Red and green show emission at 8.0 and 4.5 μm (IRAC-GLIMPSE), respectively. White and blue contours show the radio continuum 4800 MHz and integrated CO line emissions. Circles in light blue indicate the position of the three candidates to compact Hii  region identified using the MSX Catalog (see Table 3 in Paper I). The CO temperature contours are 0.5 K km s^-1 (~10 rms), 1.5, 6.6, 10.5, 15.5, 20.5, 25.5, and 30.5 K km s^-1. The radio continuum 4800 MHz contours levels go from 2.4 mJy beam^-1 (~3 rms) to 10.4 mJy beam^-1 in steps of 2 mJy beam^-1, and from 10.4 mJy beam^-1in steps of 4 mJy beam^-1.