ACES: The Magnetic Field in Large Filaments in the Galactic Center

The Galactic Center (GC) is an extreme region of the Milky Way that is host to a complex set of thermal and non-thermal structures. In particular, the GC contains high-density gas and dust that is collectively referred to as the Central Molecular Zone (CMZ). In this work, we study a subset of HNCO filaments identified in band 3 ALMA observations of the GC obtained by the ALMA CMZ Exploration Survey (ACES) that are comparable to high density filaments identified in the Galactic Disk. We compare the orientation of the magnetic field derived from 214 um SOFIA and 850 um JCMT observations with the filament orientation to determine which mechanisms dominate the formation of these filaments. We observe a large range of magnetic orientations in our observed filaments indicating the complex environments the filaments are located in. We also compare the observational results to synthetic data sets created using an MHD model of the GC. Our analysis reveals that the dominant mechanisms local to the HNCO filaments vary throughout the GC with some filaments being dominated by supersonic turbulence and others by subsonic turbulence. The comparison to synthetic observations indicates that the observed filaments are in magnetically dominated environments that could be supporting these filaments against collapse. Our results on the CMZ filaments are also compared to results obtained on similar filaments located in the Galactic Disk, and we find that the filaments studied here are possible CMZ analogs to the dense filamentary “bones” observed previously in the Galactic Disk.

💡 Research Summary

This paper presents a comprehensive analysis of the role of magnetic fields in the formation and support of large-scale, dense filaments within the Galactic Center, specifically the Central Molecular Zone (CMZ). The study utilizes multi-wavelength observational data combined with magnetohydrodynamic (MHD) simulations to unravel the complex interplay between gas dynamics, turbulence, and magnetic fields in this extreme environment.

The research is driven by the long-standing puzzle of the CMZ’s low star formation efficiency despite its high density of molecular gas. The authors investigate whether large filaments, analogous to the “Bones” of the Galactic Disk that trace spiral arms, exist in the CMZ and how magnetic fields influence their stability. The primary data comes from the ALMA CMZ Exploration Survey (ACES), which provides high-resolution maps of HNCO emission tracing dense, shocked gas and revealing filamentary structures. The magnetic field orientation on the plane of the sky is inferred from dust polarization observations at 214 µm (from the SOFIA/FIREPLACE survey) and 850 µm (from the JCMT/BISTRO survey).

The core methodology involves identifying a subset of large-scale HNCO filaments (lengths ~10 pc), masking out prominent massive clouds, and then statistically comparing the orientation of the magnetic field vectors from FIREPLACE and BISTRO with the geometrical orientation of each filament. This comparison aims to discern whether the field is predominantly parallel, perpendicular, or randomly aligned with the filament, which in turn indicates the dominant physical mechanism (e.g., flow-driven compression, gravitational collapse, or turbulence) responsible for its formation and morphology.

The key findings are multifaceted. First, the observed filaments exhibit a wide range of magnetic field alignments relative to their axes, from ordered to random. This diversity contrasts with the more consistently perpendicular fields found in many Galactic Disk “Bones,” suggesting that the CMZ filaments reside in more complex and dynamically active environments. Second, comparison with synthetic observations generated from a state-of-the-art MHD simulation of the GC indicates that the observed filaments are located in regions where the magnetic field is strong and dynamically important—specifically, in magnetically dominated environments. This implies that magnetic pressure likely provides significant support against gravitational collapse, potentially contributing to the regulation of star formation in the CMZ. Third, the analysis suggests that the dominant local physical mechanisms governing individual filaments vary across the CMZ, with some being influenced more by supersonic turbulence and others by subsonic turbulence.

In conclusion, the study posits that the large HNCO filaments studied are plausible CMZ analogs to the Galactic Disk “Bones.” However, their magnetic field morphology is more complex, likely due to the unique and violent conditions of the Galactic Center. The work underscores the critical role of magnetic fields in shaping interstellar structures and regulating star formation, even in the most extreme galactic environments. It successfully demonstrates the power of combining high-resolution molecular line mapping, multi-wavelength polarimetry, and numerical simulations to advance our understanding of astrophysical magnetohydrodynamics.