Magnetic threads and gravity: ALMA observations of the infrared dark cloud G14.225-0.506

¹ Institut de radioastronomie millimétrique (IRAM), 300 rue de la piscine, 38406 Saint Martin d’Hères, France
² Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
³ Institut de Ciències de l’Espai (ICE, CSIC), Can Magrans s/n, 08193 Cerdanyola del Vallès, Catalonia
⁴ Institut d’Estudis Espacials de Catalunya (IEEC), Campus del Baix Llobregat–UPC, Esteve Terradas 1, 08860 Castelldefels, Catalonia, Spain
⁵ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), Martí i Franquès 1, 08028 Barcelona, Catalonia, Spain
⁶ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Catalonia, Spain
⁷ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁸ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁹ Academia Sinica, Institute of Astronomy and Astrophysics, Taipei 10617, Taiwan
¹⁰ Department of Physics, National Sun Yat-sen University, No. 70, Lien-Hai Road, Kaohsiung City 80424, Taiwan, ROC
¹¹ School of Astronomy and Space Science, Nanjing University, Nanjing 210023, PR China
¹² Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210023, PR China
¹³ Astronomy Department, University of Virginia, Charlottesville, VA 22904, USA
¹⁴ Institute of Astronomy & Department of Physics Institute of Astronomy and Department of Physics, National Tsing Hua University, Hsinchu 300044, Taiwan

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 18 August 2025
Accepted: 22 January 2026

Abstract

Context. In the star formation process, the interplay between gravity, turbulence, and magnetic fields is significant, with magnetic fields apparently serving a regulatory function by opposing gravitational collapse. Nonetheless, the extent to which magnetic fields are decisive relative to turbulence and gravity, as well as the specific environments and conditions involved, remains uncertain.

Aims. This study aims to ascertain the role of magnetic fields in the fragmentation of molecular clouds into clumps down to core scales.

Methods. We examined the magnetic field as observed with ALMA at core scales (approximately 10 000 AU/0.05 pc) toward the infrared dark cloud (IRDC) G14.225-0.506, focusing on three regions with shared physical conditions. We juxtaposed these data with prior observations at the hub-filament system scale (approximately 0.1 pc).

Results. Our findings indicate a similar magnetic field strength and fragmentation level between the two hubs. However, distinct magnetic field morphologies have been identified across the three regions where the polarized emission is detected. In region N (i.e., the northern Hub: Hub-N), the large-scale magnetic field, perpendicular to the filamentary structure, persists at smaller scales in the southern half; however, it 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 (i.e., the southern Hub: Hub-S), the magnetic field is considerably inhomogeneous among the detected condensations and we did 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 Hub-N, the magnetic field aligns parallel to the elongated emission, suggesting a transition in the field geometry.

Conclusions. The magnetic field shows a clear evolution with spatial scales. We propose that the most massive condensations detected in Hub-N are undergoing gravitational collapse, as revealed by the relative significance of the magnetic field and gravitational potential (Σ_B) and mass-to-flux ratio. The distortion of the magnetic field could be a response to the flow of material as a result of such a collapse.