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A magnetic torsional wave near the Galactic Centre traced by a ‘double helix’ nebula


The magnetic field in the central few hundred parsecs of the Milky Way has a dipolar geometry and is substantially stronger than elsewhere in the Galaxy, with estimates ranging up to a milligauss (refs 1–6). Characterization of the magnetic field at the Galactic Centre is important because it can affect the orbits of molecular clouds by exerting a drag on them, inhibit star formation, and could guide a wind of hot gas or cosmic rays away from the central region. Here we report observations of an infrared nebula having the morphology of an intertwined double helix about 100 parsecs from the Galaxy's dynamical centre, with its axis oriented perpendicular to the Galactic plane. The observed segment is about 25 parsecs in length, and contains about 1.25 full turns of each of the two continuous, helically wound strands. We interpret this feature as a torsional Alfvén wave propagating vertically away from the Galactic disk, driven by rotation of the magnetized circumnuclear gas disk. The direct connection between the circumnuclear disk and the double helix is ambiguous, but the images show a possible meandering channel that warrants further investigation.

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Figure 1: The double helix nebula (DHN), observed at the infrared wavelength of 24 µm with the MIPS camera on the Spitzer Space Telescope.
Figure 2: Images of a 1° field (from -0.1° to +0.9° in Galactic latitude) that include the Galactic plane, the Galactic Centre, and the DHN.


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M.M. is grateful to S. Cowley and S. Stolovy for discussions. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. Partial support for this work was provided by NASA through an award issued by JPL/Caltech. This research also made use of data products from the Midcourse Space Experiment, available via the NASA/IPAC Infrared Science Archive.

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Correspondence to Mark Morris.

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Supplementary information

Supplementary Discussion

Alternatives to thermal dust emission to account for the spectral energy distribution of the Double Helix Nebula. While these alternative mechanisms are unlikely, they warrant consideration. (DOC 26 kb)

Supplementary Table

MSX surface brightnesses and parameter fits for seven representative nebular positions. The reported MSX fluxes and their excesses relative to the local background are given for all four MSX bands. The best-fit power-law indices and blackbody temperatures are also shown for two assumptions about the grain emissivity. (DOC 70 kb)

Supplementary Figure Legend

Text describing the details of the supplementary figure, stating the purpose of each panel and clarifying the nomenclature used in the figure. (DOC 28 kb)

Supplementary Figure

Four views of the cylindrical channel segments that lie along the hypothesized channel connecting the circumnuclear disk (CND) with the Double Helix Nebula (DHN). These are shown as four panels with different color scales. (PDF 268 kb)

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Morris, M., Uchida, K. & Do, T. A magnetic torsional wave near the Galactic Centre traced by a ‘double helix’ nebula. Nature 440, 308–310 (2006).

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