Similar complex kinematics within two massive, filamentary infrared dark
  clouds
  Infrared dark clouds (IRDCs) are thought to be potential hosts of the elusive
early phases of high-mass star formation. Here we conduct an in-depth kinematic
analysis of one such IRDC, G034.43+00.24 (Cloud F), using high sensitivity and
high spectral resolution IRAM-30m N$_2$H$^+$ ($1-0$) and C$^{18}$O ($1-0$)
observations. To disentangle the complex velocity structure within this cloud
we use Gaussian decomposition and hierarchical clustering algorithms. We find
that four distinct coherent velocity components are present within Cloud F. The
properties of these components are compared to those found in a similar IRDC,
G035.39-00.33 (Cloud H). We find that the components in both clouds have: high
densities (inferred by their identification in N$_2$H$^+$), trans-to-supersonic
non-thermal velocity dispersions with Mach numbers of $\sim$ $1.5-4$, a
separation in velocity of $\sim$3 km s$^{-1}$, and a mean red-shift of $\sim$
0.3 km s$^{-1}$ between the N$_2$H$^+$ (dense gas) and C$^{18}$O emission
(envelope gas). The latter of these could suggest that these clouds share a
common formation scenario. We investigate the kinematics of the larger-scale
Cloud F structures, using lower-density-tracing $^{13}$CO ($1-0$) observations.
A good correspondence is found between the components identified in the
IRAM-30m observations and the most prominent component in the$^{13}$CO data. We
find that the IRDC Cloud F is only a small part of a much larger structure,
which appears to be an inter-arm filament of the Milky Way.
