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The magnetic field across the molecular warped disk of Centaurus A


Magnetic fields are amplified as a consequence of galaxy formation and turbulence-driven dynamos. Galaxy mergers can potentially amplify the magnetic fields from their progenitors, making the magnetic fields dynamically important. However, the effect of mergers on magnetic fields is still poorly understood. We use thermal polarized emission observations to trace the magnetic fields in the molecular disk of the nearest radio active galaxy, Centaurus A, which is thought to be the remnant of a merger. Here, we detect that the magnetic field orientations in the plane of the sky tightly follow the ~3.0 kpc-scale molecular warped disk. Our simple regular large-scale axisymmetric spiral magnetic field model can explain, to some extent, the averaged magnetic field orientations across the disk projected on the sky. Our observations also suggest the presence of small-scale turbulent fields, whose relative strengths increase with velocity dispersion and column density. These results have strong implications for understanding the generation and role of magnetic fields in the formation of galaxies across cosmic time.

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Fig. 1: The measured B-field of Centaurus A using far-infrared polarimetric observations with HAWC+/SOFIA.
Fig. 2: Three-dimensional representation of the best B-field morphological model of Centaurus A.
Fig. 3: B-field model versus observations.
Fig. 4: Physical regions of Centaurus A as a function of their polarization properties.
Fig. 5: Polarization measurements as a function of the multi-phase ISM.

Data availability

The data that support the plots within this paper and other findings of this study are available from or from the corresponding author upon reasonable request. Source data are provided with this paper.

Code availability

The code that support the algorithms within this paper and other findings of this study are available from or from the corresponding author upon reasonable request.


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I thank K. Subramanian, K. Tassis, R. Davies, B.-G. Andersson and T. Osterloo for many useful discussions on theoretical approaches, hydromagnetic simulation, gas dynamics and dust grain alignment theories. This work is based on observations made with the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA) under the Program 07_0032. SOFIA is jointly operated by the Universities Space Research Association, Inc. (USRA), under NASA contract NAS2-97001, and the Deutsches SOFIA Institut (DSI) under DLR contract 50 OK 0901 to the University of Stuttgart.

Author information




E.L.-R. led the project, carried out observations, developed the analysis methods and data reductions, interpreted results and wrote the text.

Corresponding author

Correspondence to Enrique Lopez-Rodriguez.

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The author declares no competing interests.

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Peer review information Nature Astronomy thanks Dmitry Sokoloff and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Summary of OTFMAP polarimetric observations.

Columns, from left to right: filter central wavelength, filter bandwidth, angular resolution of the observations, scan rate, scan phase, scan amplitude, scan duration, number of observation sets obtained, and total observation time on-source. Source data

Extended Data Fig. 2 Polarization measurements of the several regions of the galactic disk.

Columns, from left to right: region of the galaxy, median magnetic field orientation, uncertainty of the magnetic field orientation, median polarization degree, uncertainty of the polarization degree. Source data

Extended Data Fig. 3 Physical regions based on B-field orientation and degree of polarization.

Histograms of P (a) and PA (b) of polarization for measurements with P/σP > 3. Three distinct regions are found for the PA of polarization, which are identified with the west (orange), east (red) and low polarized (black) regions. The boundaries of each region are shown with vertical black dashed lines. (c), The spatial correspondence of the three regions identified using the PA distributions are shown with the same colors as the plots at b. The total intensity contours are shown as in Fig. 1. A legend polarization of 10% (black) and beam size of 7.8” (red circle) are shown.

Extended Data Fig. 4 Magnetic field of the central 50” x 50” (0.8 x 0.8 kpc2) of Centaurus A.

a, Total flux (colorscale) with overlaid B-field orientations (white lines). b, Polarized flux (colorscale) with overlaid B-field orientation (white lines). A legend polarization of 5% (black) and beam size of 7.8” (red circle) are shown.

Extended Data Fig. 5 Parameters of the magnetic field morphological model.

Columns, from left to right: Free parameters used in the magnetic field model, symbols associated with the free parameter, boundaries of the flat pior distribution, median value of the posterior distribution with 1σ uncertainty values. Source data

Extended Data Fig. 6 Posterior distributions of the magnetic field morphological model.

A reference of the parameter definitions, used priors, and median values is shown in Extended Data Fig. 5.

Extended Data Fig. 7 Polarization map vs physical parameters.

Temperature (a) and column density (b) maps of Centaurus A with overlaid B-field orientation (while lines) with P/σP > 2.5 and PI/σPI > 2. Temperature contours start at 20 K increasing in steps of 0.5 K, and column density density contours start at log(NH+H2 [cm−2]) = 20.6 increasing in steps of 0.1. 12CO(1-0) integrated line emission (c) and velocity dispersion (d) of the warped disk of Centaurus A with overlaid B-field orientation (white lines) with P/σP > 2.5 and PI/σPI > 2.

Extended Data Fig. 8 Polarized flux vs. total intensity plots.

P-I and PI-I plots at 89 μm vs temperature (a,b) and column density (c,d). The trend of the bulk of the P-I plot, P τ−1, is shown as a black solid line in panels (a) and (c). The uncertainties of the debiased polarized intensity in plots (b) and (d) are shown. The blue dotted vertical lines at I = 1000 and 2700 MJy sr−1 show the limits of the three physical regions found in this analysis. The black dotted lines in panels (b) and (d) show the maximum expected polarization, P  I0 = 15, 6.5, and 1.5% for each of these physical regions, respectively.

Extended Data Fig. 9 Power-law index of plots from Fig. 5.

Columns, from left to right: Parameters of the y-axis used in each fit, regions of the galaxy used for the fit, power-law indexes for the parameters used in the x-axis T, NH+H2, and σν,12CO(1-0). Source data

Extended Data Fig. 10 Velocity dispersion of the outer and molecular disk.

12CO(1-0) velocity dispersion histograms of the outer disk (red) and molecular disk (blue) as identified in Fig. 4. The median (solid line) and 1σ (dashed line) are shown for each physical structure. These values correspond to σv,12CO(1−0) = 18.4 ± 9.2 (km s−1), and σv,12CO(1−0) = 6.4 ± 6.0 (km s−1) for the molecular disk and outer disk, respectively.

Source data

Source Data Table 1

Medians of the physical parameters of each region identified in Fig. 4.

Source Data Extended Data Fig. 1

Summary of OTFMAP polarimetric observations.

Source Data Extended Data Fig. 2

Polarization measurements of the several regions of the galactic disk.

Source Data Extended Data Fig. 5

Parameters of the magnetic field morphological model.

Source Data Extended Data Fig. 9

Power-law index of plots from Fig. 5.

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Lopez-Rodriguez, E. The magnetic field across the molecular warped disk of Centaurus A. Nat Astron 5, 604–614 (2021).

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