Magnetic field morphological diagnostics with ALMA in the G327.29 protocluster: VGT versus dust polarization

TL;DR

This study addresses how magnetic fields, turbulence, and gravity govern the evolution of massive protoclusters by applying the Velocity Gradient Technique (VGT) to four molecular tracers (DCN, C$^{18}$O, HN$^{13}$C, H$^{13}$CO$^{+}$) and comparing with 1.2 mm dust polarization in the G327.29 protocluster using high-resolution ALMA data. It constructs model data cubes via pixel-wise Gaussian decomposition and conducts VGT on matched components to infer $B_{\text{POS}}$ morphologies, revealing sub-Alfvénic turbulence with velocity caustics and a bimodal distribution of angular measurements that indicate concurrent turbulence and gravity. The analysis uncovers large-scale gravitational infall from the surroundings onto filaments and the central hub, while dense cores show a mix of turbulent and gravitational influence that varies with tracer, reflecting different density regimes. Overall, the work demonstrates the viability of VGT as a tool for probing magnetic-field structure and dynamical state in massive protoclusters and provides a path to disentangle inflow and core formation processes using multi-tracer, high-resolution data.

Abstract

Magnetic fields and turbulence may play a key role in the evolution of protoclusters, influencing the formation of dense cores and stars. Here, we examine the morphology of the magnetic fields in the G327.29 protocluster using both the velocity gradient technique (VGT) extracted from molecular line emissions and linear polarization in the dust continuum emission. The VGT analysis is performed using four molecular tracers: DCN (3-2), C18O (2-1), HN13C (3-2), and H13CO+ (3-2) - which probe gas across different density regimes, observed with the ALMA 12 m array. Owing to its sensitivity to gas dynamics, a comparison between VGT and dust polarization provides a powerful probe of the evolutionary processes in massive star-forming regions. From our analysis we reveal a complex magnetic-field structure, shaped by the combined influence of turbulence and gravity. In addition, it also appears that there is a large-scale (beyond the core scale) gravitational infall from the surrounding medium on to the filament and the central densest region. Furthermore, we observe that cores are dominated by a mix of turbulence and gravity. Overall, this work presents, likely for the first time, the application of VGT to a massive protocluster, G327.29, using high-resolution ALMA observations.

Figures (33)

• Figure 1: Left: Color image shows the Spitzer (GLIMPSE) 8 $\mu$m emission of G327.29 star-forming region 2003PASP..115..953B. Contours represent the ATLASGAL 870 $\mu$m emission obtained from the work of 2009AA...504..415S. Contour levels are 0.45, 1, 2, 4, 8, 10, 14, 16, 18, 23, 28, and 34 Jy beam$^{-1}$ respectively with a 19.2$"$ beam. Middle: 1.2 mm continuum emission towards the central clump (protocluster) obtained from the ALMA-IMF large program 2022AA...662A...8M. Here the two maroon dashed circles represent the area where the polarization measurements are carried out using ALMA 12m configurations in this study. Right: 1.2 mm continuum emission obtained from this work after mosaicking the two pointings denoted in blue dashed circles in the bottom left figure.
• Figure 2: Spatial distribution of 1.2 mm cores in this region obtained from Koley et al. (2025, in preparation). Here, blue circles/ellipses indicate 1.2 mm cores, and the numerical number associated with each core indicates its ID mentioned in Table \ref{['tab:table1']}. The red dashed polygon regions indicate the possible outflow regions identified from the SiO (5$-$4) emission 2024ApJ...960...48T and the background color image represents the integrated intensity of the 1.2 mm continuum.
• Figure 3: left column: Overplots of the $B_{\text{pos}}$ obtained from 1.2 mm dust continuum emission (red pseudo-vectors) and VGT of (i) DCN (3$-$2), (ii) C$^{18}$O (2$-$1), and (iii) HN$^{13}$C (3$-$2) line (total model) emissions (blue pseudo-vectors). Here, the color map in each figure indicates the integrated intensity map (moment 0) of the respective line emissions. Right column: Spatial distributions of angular measurements (AM) between dust and corresponding line emissions. Four regions: Region 1, Region 2, Region 3 and Region 4 showed in four different colors. A gravitational infall signature is observed towards these regions which are the arc of a filament or arc of a dense central structure. Here, the 1.2 mm continuum cores are overplotted in pink color.
• Figure 4: Continuation of Fig. \ref{['fig:fig4']} but for H$^{13}$CO$^{+}$ (3$-$2) line emission.
• Figure 5: Histogram plots of angular measurement (AM) between dust and corresponding line emissions. Observed peaks in each figure at $\sim$ -1 and $\sim$ +1, indicating gravity and turbulent dominance respectively.