ALMA Polarization Study of the Magnetic Fields in Two Massive Clumps in the 20 km s$^{-1}$ Cloud of the Central Molecular Zone

TL;DR

This study uses high-resolution ALMA polarization at 870 μm to map the magnetic field in two massive clumps within the CMZ's 20 km s$^{-1}$ cloud, deriving $B_{ ext{pos}}$ via the Angular Dispersion Function and assessing magnetic support across cloud, core, and condensation scales. The results show cloud-scale magnetic fields dominating while gravity governs core- and condensation-scale structures; multi-scale comparisons with JCMT data reveal systematic changes in field orientation consistent with a transition from magnetically regulated to gravity-dominated dynamics. Magnetic tension can oppose gravity but generally does not prevent gas from infalling toward dense cores, and several cores exhibit sub- or near-virial states with signs of ongoing star formation. Overall, the CMZ environment demonstrates a complex interplay of turbulence, magnetic fields, and gravity, with magnetic fields partially regulating but not prohibiting star formation in these extreme conditions.

Abstract

We present the Atacama Large Millimeter/submillimeter Array (ALMA) observations of linearly polarized 870 $μ$m continuum emission at a resolution of $\sim$0.2$^{\prime\prime}$ (2000 au) toward the two massive clumps, Clump 1 and Clump 4, in the 20 km s$^{-1}$ cloud. The derived magnetic field strengths for both clumps range from $\sim$0.3 to 3.1 mG using the Angular Dispersion Function (ADF) method. The magnetic field orientations across multiple scales suggests that the magnetic field dominates at the cloud scale, whereas gravity likely governs the structures at the core (0.01$-$0.1 pc) and condensation ($\le$ 0.01 pc) scales. Furthermore, the study on the angular difference between the orientations of the local gravity gradient and the magnetic field suggests that the magnetic field predominantly governs the dynamics in the diffuse regions, while gravity and star formation feedback become increasingly significant within the dense regions. The ratio of the magnetic field tension force $F_\textrm{B}$ to the gravitational force $F_\textrm{G}$ suggests that the magnetic field may provide some support against gravity, but it is insufficient to prevent gas from infalling toward the dense cores.

Figures (13)

• Figure 1: Left: false-color image of the 20 km s$^{-1}$ cloud created using the MeerKAT 1.28 GHz continuum in red heywood2022, Spitzer 3.6 $\mu$m in blue, and Spitzer 8 $\mu$m in green stolovy2006. The white line segments are the inferred magnetic field orientation obtained from James Clerk Maxwell Telescope (JCMT) observations of polarized dust emission yang2025. The ALMA observation fields are denoted by the lime circles. Right: The inferred magnetic field orientation (black line segments) overlaid on Stokes $I$ emission (color) toward (b) Clump 1 and (c) Clump 4. The grey contours are the Stokes $I$ emission, which correspond to [5, 25, 50, 100, 150, 200, 350, 500, 650] $\times$$\sigma$ (1$\sigma$ = 65 $\mu$Jy beam$^{-1}$) in panel (b) and [5, 25, 50, 75, 100, 150, 250, 350, 450, 650] $\times$$\sigma$ (1$\sigma$ = 48 $\mu$Jy beam$^{-1}$) in panel (c). The magenta contour in panel (c) denote the area used to derive the magnetic field properties for Clump 4. The cores enclosed by red solid contours in these two clumps are identified by astrodendro. Clump 1-tail is enclosed using the orange solid contour, which is identified as the diffuse emission extended along the northeast-southwest direction that correspond to $\sim$5$\sigma$ in panel (b). Clump 4-C and Clump 4-D, identified as one core using astrodendro, are manually divided by the red solid line using the 75$\sigma$ contour as the reference. The synthesized beam size is denoted by filled black ellipse in the bottom left corner. The vectors in all panels are selected using Super-Nyquist sampling hull2020 and set to have a uniform length. The outer and inner blue dashed contours in panels (b) and (c) are the FHWM of the primary beam and the inner 1/3 of it, respectively. The absolute coordinates of the origins are (a) [266.408$^{\circ}$, $-$29.0858$^{\circ}$], (b) [266.407$^{\circ}$, $-$29.0635$^{\circ}$], and (c) [266.407$^{\circ}$, $-$29.0956$^{\circ}$], respectively.
• Figure 2: The inferred magnetic field orientations at multi-scales for Clump 1. The inferred magnetic field orientations (blue line segments) of Clump 1 at a resolution of $\sim$14 revealed by JCMT data yang2025 in panel (a). The inferred magnetic field orientations (blue line segments) Clump 1 revealed by the ALMA C-2 configuration in panels (b) with a resolution of $\sim$0.71$\times$0.54. The inferred magnetic field orientations (blue line segments) in Clump 1 revealed by the ALMA C-5 configuration in panels (c) and (d) with a resolution of $\sim$0.24$\times$0.17. The synthesized beam is denoted by a filled blue ellipse at the bottom left corner in each panel. The grey contours shows the Stokes $I$ intensity in each panel. The contour levels are [5, 25, 45, 65, 85, 105, 125, 145, 165, 185] $\times$$\sigma$ (1$\sigma$ = 27 mJy beam$^{-1}$) in panel (a), [5, 25, 50, 100, 150, 200, 300, 400] $\times$$\sigma$ (1$\sigma$ = 0.22 mJy beam$^{-1}$) in panel (b), [25, 50, 100, 200, 300, 400, 600, 800, 1000, 1200] $\times$$\sigma$ (1$\sigma$ = 32 $\mu$Jy beam$^{-1}$) in panels (c) and (d), respectively. The vectors in panels (b), (c), and (d) are selected using super-Nyquist sampling hull2020 and set to have a uniform length.
• Figure 3: The inferred magnetic field orientations at multi-scales for Clump 4. The inferred magnetic field orientations (blue line segments) of Clump 4 at a resolution of $\sim$14 revealed by JCMT data yang2025 in panel (a). The inferred magnetic field orientations (blue line segments) in Clump 4 revealed by the ALMA C-2 configuration in panels (b) with a resolution of $\sim$0.71$\times$0.55. The inferred magnetic field orientations (blue line segments) in Clump 4 revealed by the ALMA C-5 configuration in panels (c)$-$(h) with a resolution of $\sim$0.24$\times$0.16. The synthesized beam is denoted by a filled blue ellipse at the bottom left corner in each panel. The grey contours shows the Stokes $I$ intensity in each panel. The contour levels are [5, 25, 45, 65, 85, 105, 125, 145, 165, 185] $\times$$\sigma$ (1$\sigma$ = 27 mJy beam$^{-1}$) in panel (a), [5, 25, 50, 100, 150, 200, 300, 400] $\times$$\sigma$ (1$\sigma$ = 0.21 mJy beam$^{-1}$) in panel (b), [50, 100, 200, 300, 400, 600, 800, 1000, 1200] $\times$$\sigma$ (1$\sigma$ = 22 $\mu$Jy beam$^{-1}$) in panels (c)$-$(h), respectively. The vectors in panels (b)$-$(h) are selected using super-Nyquist sampling hull2020 and set to have a uniform length.
• Figure 4: Absolute angular difference between the JCMT data and the ALMA C-2 configuration (resolution $\sim$0$\hbox{.}\arcsec71\times0\hbox{.}\arcsec54$) data shown in color for Clump 1 and Clump 4 in panels (a) and (b), respectively. The blue contours in each panel are the stokes $I$ intensity from the ALMA C-2 configuration data. The blue contours in panels (a) and (b) are identical to the black contours in \ref{['jcmt-alma-c1']} (b) and \ref{['jcmt-alma-c4']} (b), respectively. The cores enclosed by black solid contours in these two clumps are identified by astrodendro, and the magenta contour in panel (a) denotes Clump 1-tail. The purple line segments in each panel show the inferred magnetic field orientation revealed by JCMT yang2025.
• Figure 5: Top panels: Histogram of the absolute angular difference between the JCMT data and the ALMA C-2 configuration (resolution $\sim$5000 au) for Clump 1 and Clump 4 in panels (a) and (b), respectively. Bottom panels: Histogram of the absolute angular difference between the ALMA C-2 (resolution $\sim$5000 au) and C-5 configurations (resolution $\sim$2000 au) for Clump 1 and Clump 4 in panels (c) and (d), respectively.