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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The code that support the algorithms within this paper and other findings of this study are available from https://github.com/galmagfields 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.
The author declares no competing interests.
Peer review information Nature Astronomy thanks Dmitry Sokoloff and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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.
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.
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.
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.
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.
A reference of the parameter definitions, used priors, and median values is shown in Extended Data Fig. 5.
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.
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.
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).
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.
Medians of the physical parameters of each region identified in Fig. 4.
Summary of OTFMAP polarimetric observations.
Polarization measurements of the several regions of the galactic disk.
Parameters of the magnetic field morphological model.
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). https://doi.org/10.1038/s41550-021-01329-9