Gas dynamics in the star forming region G18.148$-$0.283: Is it a
  manifestation of two colliding molecular clouds?
  We report the results obtained from a multi-wavelength study of the HII
region, G18.148$-$0.283, using the upgraded Giant Metre-wave Radio Telescope
(uGMRT) at 1350 MHz along with other archival data. In addition to the radio
continuum emission, we have detected the H169$\alpha$ and H170$\alpha$ radio
recombination lines towards G18.148$-$0.283 using a correlator bandwidth of 100
MHz. The moment-1 map of the ionized gas reveals a velocity gradient of
approximately 10 km s$^{-1}$ across the radio continuum peaks. The $^{12}$CO
($J$=3$-$2) molecular line data from the COHRS survey also shows the presence
of two velocity components that are very close to the velocities detected in
the ionized gas. The spectrum and position-velocity diagram from CO emission
reveal molecular gas at an intermediate velocity range bridging the velocity
components. We see mid-infrared absorption and far-infrared emission
establishing the presence of a filamentary infrared dark cloud, the extent of
which includes the targeted HII region. The magnetic field inferred from dust
polarization is perpendicular to the filament within the HII region. We have
also identified two O9 stars and 30 young stellar objects towards the target
using data from the 2MASS, UKIDSS, and GLIMPSE surveys. Cumulatively, this
suggests that the region is the site of a cloud-cloud collision that has
triggered massive star formation and subsequent formation of an HII region.
