Subsonic islands within a high-mass star-forming IRDC

TL;DR

This study reveals that a significant portion of dense gas in a high-mass star-forming IRDC exhibits subsonic motions, challenging previous assumptions about turbulence levels in such regions and impacting theories of star formation.

Contribution

It provides the first detailed evidence of widespread subsonic turbulence in a high-mass star-forming IRDC using high-resolution ammonia observations.

Findings

Over a third of ammonia spectra show subsonic motions.

Line widths are narrower than previously observed, with a mean of 0.71 km/s.

Subsonic turbulence suggests stronger magnetic support is needed against collapse.

Abstract

High-mass star forming regions are typically thought to be dominated by supersonic motions. We present combined Very Large Array and Green Bank Telescope (VLA+GBT) observations of NH$_3$ (1,1) and (2,2) in the infrared dark cloud (IRDC) G035.39-00.33, tracing cold and dense gas down to scales of 0.07 pc. We find that, in contrast to previous similar studies of IRDCs, more than a third of the fitted ammonia spectra show subsonic non-thermal motions (mean line width of 0.71 $\mathrm{km~s^{-1}}$), and the sonic Mach number distribution peaks around $\mathcal{M} = 1$. As possible observational and instrumental biases would only broaden the line profiles, our results provide strong upper limits to the actual value of $\mathcal{M}$, further strengthening our findings of narrow line widths. This finding calls for a reevaluation of the role of turbulent dissipation and subsonic regions in massive-star and cluster formation. Based on our findings in G035.39, we further speculate that the coarser spectral resolution used in the previous VLA NH$_3$ studies may have inhibited the detection of subsonic turbulence in IRDCs. The reduced turbulent support suggests that dynamically important magnetic fields of the 1 mG order would be required to support against possible gravitational collapse. Our results offer valuable input into the theories and simulations that aim to recreate the initial conditions of high-mass star and cluster formation.