Interstellar Plunging Waves: ALMA resolves the physical structure of non-stationary MHD shocks

TL;DR

This study uses ALMA to spatially resolve a non-stationary MHD shock in an interstellar cloud, revealing how stellar feedback can trigger shocks that promote conditions for massive star formation.

Contribution

First direct spatial resolution of a non-stationary MHD shock in an interstellar cloud, linking stellar feedback to shock formation and star formation conditions.

Findings

Resolved the physical structure of an MHD shock with ALMA.

Linked stellar feedback from a supernova remnant to shock triggering.

Identified conditions conducive to massive star formation.

Abstract

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.