  HESS J1858+020 is a weak gamma-ray source that does not have any clear
cataloged counterpart at any wavelengths. Recently, the source G35.6-0.4 was
re-identified as a SNR. The HESS source lies towards the southern border of
this remnant. The purpose of this work is to investigate the interstellar
medium around the mentioned sources in order to look for possible counterparts
of the very-high energy emission. Using the 13CO J=1-0 line from the Galactic
Ring Survey and mid-IR data from GLIMPSE we analyze the environs of HESS
J1858+020 and SNR G35.6-0.4. The 13CO data show the presence of a molecular
cloud towards the southern border of SNR G35.6-0.4 and at the same distance as
the remnant. This cloud is composed by two molecular clumps, one, over the SNR
shell and the other located at the center of HESS J1858+020. We estimate a
molecular mass and a density of ~ 5 X 10^{3} Msun and ~ 500 cm^{-3},
respectively for each clump. Considering the gamma-ray flux observed towards
HESS J1858+020, we estimate that a molecular cloud with a density of at least
150 cm^{-3} could explain the very-high energy emission hadronically. Thus, we
suggest that the gamma-ray emission detected in HESS J1858+020 is due to
hadronic mechanism. Additionally, analyzing mid-IR emission, we find that the
region is active in star formation, which could be considered as an alternative
or complementary possibility to explain the very-high energy emission.
