Letter to the Editor

Subsonic islands within a high-mass star-forming infrared dark cloud

¹ Max Planck Institute for Extraterrestrial Physics, Gießenbachstraßse 1, 85748 Garching bei München, Germany
e-mail: vsokolov@mpe.mpg.de
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
³ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁵ Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
⁶ Department of Physics, University of Florida, Gainesville, FL 32611, USA
⁷ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy UK

⁸ School of Physics and Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS, UK
⁹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received: 1 February 2018
Accepted: 19 February 2018

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 NH₃ (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 km s⁻¹), and sonic Mach number distribution peaks around 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 ℳ, further strengthening our findings of narrow line widths. This finding calls for a re-evaluation 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 NH₃ 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.