Radio image (black contours) of the field containing the unidentified HESS source HESS J1858+020 obtained with GMRT at 610 MHz. The contours correspond to 2, 3, and 4 times 0.22 mJy beam^-1, the rms, and the beam size is 12.2″ × 4.8″. The colour image shows the 12 μm WISE image of this region (Wright et al. 2010). The white contours show the ¹³CO (J = 1–0) emission integrated between 51 and 59 km s^-1 from the Galactic Ring Survey (Jackson et al. 2006). The red circles correspond to X-ray sources. Some relevant radio sources are labelled as R1,.., R5, respectively (see Table 2). The shell-like structure clearly visible in the upper right region of the figure corresponds to the SNR G35.6−0.4. The HESS significance contours (4, 5, and 6σ, in magenta) for the location of HESS J1858+020 are overlaid. The blue ellipse indicates the 95% confidence location of the Fermi-LAT gamma-ray source 2FGL J1857.6+0211. The positions of the three pulsars are indicated with yellow crosses, and the positions of two young stellar objects, close to X5, are indicated by triangles.