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Modeling the Invisible Interstellar Medium

Submitter: Jo-Anne Brown, Cameron Van Eck and Marijke Haverkorn
Description: Of all the constituents making up the interstellar medium (gas, dust, cosmic rays and a magnetic field), only the magnetic field is truly invisible, since it does not radiate. Therefore, it is with great excitement that a new model of the magnetic field of the Milky Way Galaxy is being proposed by Cameron Van Eck and Jo-Anne Brown from the University of Calgary and collaborators, including Marijke Haverkorn. The model, which will be published in the Astrophysical Journal next month, is sketched in the upper left in a bird's eye view of the Milky Way, where the blue/white shows the gaseous spiral arms and the lines show the direction of the large-scale magnetic field relative to the spiral arms. The magnetic field is directed mostly clockwise (as viewed from the North Galactic Pole), with a counter-clockwise region spiraling out from the centre. Parts of this "reversed" region have been observed before, and additional, separate reversed regions have been claimed previously, but this study provides firm evidence for a model with only one coherent reversed region.

The global model is actually a combination of three separate models in different sections, as shown in the lower left. The fact that the correspondence between the three models is not perfect may indicate that the spiral arms are not perfectly log-normal, but have slight wiggles in them (as frequently observed in external galaxies).

The two images on the right give a visual impression of how well the data (the little circles) match with the model (the background). (For afficionado's: these are rotation measure data for pulsars and extragalactic background sources, the latter averaged over a few degrees). If it weren't for the little black circles around the data points, many of the data points disappear into the background, owing to a good fit by the model. Deviating points likely indicate small-scale structure.

Want more information? Be sure to read the complete text that can be found on astro-ph at: http://lanl.arxiv.org/abs/1012.2938
Copyright: Astrophysical Journal
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