The converging gas flow around the infrared dark cloud G28.3

TL;DR

This work uses NOEMA interferometry merged with IRAM 30 m data to map G28.37+0.07 across ~81 pc^2, resolving a west-east converging gas flow from cloud scales to cores. It finds a dense-gas mass-flow rate of about $10^{-3}$ $M_\odot$ yr$^{-1}$ along the flow, with line-of-sight infall toward central sources ~25× smaller, indicating dominance of longitudinal motions. CH$_3$CN-derived gas temperatures are higher than Herschel dust temperatures in dense regions, suggesting mechanical heating from kinetic-energy dissipation contributes to the thermal budget. The results imply that interfaces of converging flows channel gas into multidirectional infall, feeding high-mass star formation, and highlight a decoupling between gas and dust temperatures in this environment.

Abstract

Aims: The G28.37+0.07 star-forming region is a prototypical infrared dark cloud (IRDC) located at the interface of a converging gas flow. This study characterizes the properties of this dynamic gas flow. Methods: Combining data from the Northern Extended Millimeter Array (NOEMA) with single-dish data from the IRAM30m observatory, we mapped large spatial scales (~81pc^2) at high angular resolution (7.0''x2.6'' corresponding ~2.3x10^4au or ~0.1pc) down to core scales. The spectral setup in the 3mm band covers many spectral lines as well as the continuum emission. Results: The data reveal the proposed west-east converging gas flow in all observed dense gas tracers. We estimate a mass-flow rate along that flow around 10^-3M_sun/yr. Comparing these west-east flow rates to infall rates toward sources along the line of sight, the gas flow rates are roughly a factor of 25 greater than than those along the line of sight. This confirms the dominance of longitudinal motions along the converging gas flow in G28.37. For comparison, in the main north-south IRDC formed by the west-east converging gas flow, infall rates along the line of sight are about an order of magnitude greater than those along the west-east flow. In addition to the kinematic analysis, a comparison of CH_3CN-derived gas temperatures with Herschel-derived dust temperatures typically show higher gas temperatures toward high-density sources. We discuss whether mechanical heating from the conversion of the flow's kinetic energy into thermal energy may explain some of the observed temperature differences. Conclusions: The differences between flow rates along the converging flow, perpendicular to it, and toward the sources at the IRDC center indicate that at the interfaces of converging gas flows - where most of the active star formation takes place - originally more directed gas flows can convert into multidirectional infall motions.

Figures (11)

• Figure 1: Overview of the G28 region. Left panel: Color scale showing GLIMPSE 8 $\mu$m emission churchwell2009 for the G28 IRDC. The contours show the corresponding ATLASGAL 870 $\mu$m dust continuum emission, starting at 4$\sigma$ (200 mJy beam$^{-1}$) schuller2009 and continuing in 8$\sigma$ steps. The box outlines the area shown to the right. Right panel: Zoom into the G28 target region. The color scale presents the velocity field (first moment) observed in $^{13}$CO(3–2) with APEX beuther2020. The contours show the 870 $\mu$m continuum emission from the ATLASGAL survey schuller2009. The red circles outline the observed NOEMA mosaic. A scale bar is shown to the right in both panels.
• Figure 2: Continuum images of the G28 IRDC. Color scale and solid contours showing the NOEMA 3 mm continuum emission. Contour levels range from 0.4 to 1.6 mJy beam$^{-1}$ ($1\sigma \sim 0.1$ mJy beam$^{-1}$). The dotted contours show 870 $\mu$m single-dish continuum data (ATLASGAL, schuller2009) starting at 0.15 Jy beam$^{-1}$ and continuing in steps of 0.3 Jy beam$^{-1}$ up to 1.5 Jy beam$^{-1}$ ($1\sigma \sim 0.05$ Jy beam$^{-1}$). Middle: Color scale showing the Spitzer 8 $\mu$m emission ($2"$ resolution, churchwell2009) with 3 mm emission overlaid as contours. Right: Color scale showing the 1.95 GHz continuum emission from the THOR survey ($10.6"\times 9.0"$ resolution, beuther2016wang2020a) with 3 mm emission overlaid as contours. All panels show a scale bar and the NOEMA synthesized beam. Source labels follow carey2000, wang2008; G28N and G28fil are newly labeled here.
• Figure 3: 3.6 mm continuum data with source identifications. The contour levels start at the 5$\sigma$ level (0.5 mJy beam$^{-1}$) and continue in 5$\sigma$ steps. The sources from the Clumpfind identification are labeled; the star marks the phase-center position for the source offsets in Table \ref{['cont']}. A scale bar and the synthesized beam are shown as well.
• Figure 4: Temperature maps. Left: Temperature map from combined fitting of CH$_3$CN$(5_k-4_k)$ and $(4_k-3_k)$$k$-ladders ($k$ from 0 to 4). Middle: Dust temperature map from Herschel far-infrared data marsh2017. Right: CH$_3$CN temperature map, smoothed to the same $12"$ spatial resolution of the Herschel map. The contours outline the NOEMA 3.6 mm continuum emission; contour levels are from 0.4 to 1.6 mJy beam$^{-1}$ ($1\sigma \sim 0.1$ mJy beam$^{-1}$). The source numbers and a scale bar are shown in the left panel.
• Figure 5: Integrated intensity data for G28. Top: 30 m observations. Bottom: Merged NOEMA+30 m data (except NH$_2$D, which shows NOEMA-only data as that line was not covered by the 30 m observations). The integration range is 76 to 84 km s$^{-1}$. All maps were created by clipping the data below the $3\sigma$ level; intensity levels were chosen for each panel separately to better highlight the emission features. The contours on the 30 m data show 870 $\mu$m continuum schuller2009 in $3\sigma$ steps of 0.15 Jy beam$^{-1}$. The contours on the NOEMA+30 m data show the NOEMA-only 3.6 mm continuum from 0.4 to to 1.6 mJy beam$^{-1}$ ($1\sigma \sim 0.1$ mJy beam$^{-1}$). Molecules are labeled in all panels, and the beam and scale bar are shown in the top-left panels.