Interstellar Plunging Waves: ALMA resolves the physical structure of
  non-stationary MHD shocks
  Magneto-hydrodynamic (MHD) shocks are violent events that inject large
amounts of energy in the interstellar medium (ISM) dramatically modifying its
physical properties and chemical composition. Indirect evidence for the
presence of such shocks has been reported from the especial chemistry detected
toward a variety of astrophysical shocked environments. However, the internal
physical structure of these shocks remains unresolved since their expected
spatial scales are too small to be measured with current instrumentation. Here
we report the first detection of a fully spatially resolved, MHD shock toward
the Infrared Dark Cloud (IRDC) G034.77-00.55. The shock, probed by Silicon
Monoxide (SiO) and observed with the Atacama Large Millimetre/sub-millimetre
Array (ALMA), is associated with the collision between the dense molecular gas
of the cloud and a molecular gas flow pushed toward the IRDC by the nearby
supernova remnant (SNR) W44. The interaction is occurring on sub-parsec spatial
scales thanks to the enhanced magnetic field of the SNR, making the dissipation
region of the MHD shock large enough to be resolved with ALMA. Our observations
suggest that molecular flow-flow collisions can be triggered by stellar
feedback, inducing shocked molecular gas densities compatible with those
required for massive star formation.
