Magnetic threads and gravity: ALMA Observations of IRDC G14.225-0.506

TL;DR

This study uses ALMA 1.39 mm polarization to probe magnetic fields at core scales (~$0.05$ pc) in IRDC G14.225-0.506, focusing on three regions (N,S,F1) and comparing with older hub-filament-scale observations. It shows region-specific B-field morphologies, with a predominantly perpendicular orientation at larger scales becoming distorted near massive condensations, where gravity dominates as indicated by Sigma$_{B}<1$ and supercritical mass-to-flux ratios, especially in N-7. The work combines the IG-method with DCF variants to quantify magnetic support and mass loading, revealing multiscale collapse consistent with a Global Hierarchical Collapse framework. Collectively, the results demonstrate gravity-driven field distortions emerging at core scales and provide observational links between filamentary accretion, core collapse, and magnetic-field evolution, emphasizing the progressive dominance of gravity in shaping magnetic structure during early star formation.

Abstract

During the star formation process, the interplay between gravity, turbulence, and B-fields is significant, with B-fields apparently serving a regulatory function. However, the extent to which B-fields are decisive relative to turbulence and gravity remains uncertain. This study aims to ascertain the role of B-fields in the fragmentation of molecular clouds. We examine the B-field observed with ALMA at core scales towards the infrared dark cloud G14.225-0.506, focusing on 3 regions with shared physical conditions, and juxtapose it with prior observations at the Hub-filament system scale. Our findings indicate a similar B-field strength and fragmentation level between the 2 hubs. However, distinct B-field morphologies are identified across the 3 regions where polarized emission is detected. In the region N, the large-scale B-field, which is perpendicular to the filamentary structure, persists at smaller scales in the southern half but becomes distorted near the more massive condensations in the northern half. Notably, these condensations exhibit signs of impending collapse, as evidenced by supercritical mass-to-flux values. In the region S, the B-field is considerably inhomogeneous among the detected condensations, and we do not observe a direct correlation between the field morphology and the condensation density. Lastly, in an isolated dust clump located within a southern filament of the northern hub, the B-field aligns parallel to the elongated emission, suggesting a transition in the field geometry. The B-field shows a clear evolution with spatial scales. We propose that the most massive condensations detected in the northern Hub are undergoing gravitational collapse, as revealed by the relative significance of the magnetic field and gravitational potential and mass-to-flux ratio. The distortion of the B-field could be a response to the flow of material due to the collapse.

Figures (16)

• Figure 1: CSO continuum emission at 350 $\mu$m (gray map) and red contours which depict [3, 6, 9, 15, 25, 35, 45, 55, 65, 75, 85, 95] times rms noise (80 mJy beam$^{-1}$). The red and black circles in the lower left corner show the CSO and ALMA beam size, respectively. Blue contours represent the 80%, 40%, and 33% levels of the ALMA primary beam for each target region, labeled N, F1, F2, F3, and S.
• Figure 2: Continuum emission at 1.39 mm toward region N (upper panel) and region S (bottom panel). Contours depict [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 300, 500, 700] times rms noise (0.12 mJy beam$^{-1}$), overlapped with magnetic field segments in blue, where thin and thick segments depict segments within the inner 50$\%$ and 40$\%$ of the primary beam model respectively. Red segments show the CSO magnetic field at 350 $\mu$m where thin and thick segments indicate polarization intensity between 2 and 3 sigmas, and larger than 3 times sigma respectively AnezLopez2020. Color contours show leaves from dendrogram analysis, and the black labels present the corresponding ID label. The blue solid circle in the bottom left corner of each panel depict the ALMA beam size. The Pink triangle shows the H$_2$O maser detected in Wang2006. Green tripods show cores detected with the SMA telescope at 1.3 mm Busquet2016. Yellow and purple tripods depict radio sources detected at 6 cm (C-band) and at 3.6 cm (X-band) by DiazMarquez2024 respectively. White tripods show NH$_3$ cores detected by Ohashi2016.
• Figure 3: similar to Fig. \ref{['fig:leavesN']}-left, but for region F1.
• Figure 4: Magnetic field position angle histogram toward region N (top) and S (bottom). Black solid line depict the angle median value i.e., 95$^{\circ}$ for region N and (44$^{\circ}$, 153$^{\circ}$) for region S. Red dotted line shows the main direction of the CSO magnetic field orientation, 105$^{\circ}$ and (33º, 134º) for region N and S respectively, as showed in AnezLopez2020. Magenta dashed line shows perpendicular to main filament orientation (i.e., F10-E, 100$^{\circ}$, Busquet2013. Magenta solid and dotted lines indicate the overall orientation of the polarization in H-band and R-band, respectively Santos2016.
• Figure 5: Magnetic field position angle histogram, toward region F1. Black solid line depict the median value over the distribution ($\sim$26º). Magenta dashed lined shows the orientation of main filament orientation ($\sim$10º) Busquet2013. Magenta solid and dotted lines indicate the overall orientation of the polarization in H-band and R-band, respectively Santos2016.