Via Láctea / Milky Way

ScienceWeek
ASTRONOMY: ON THE CENTER OF THE MILKY WAY

The following points are made by T.J. Lazio and T.N. LaRosa (Science 2005 307:686):

1) At a distance of just 25,000 light years (2.5 x 10^(20) m), the center of our Galaxy, the Milky Way, provides the foundation for understanding phenomena in other galaxies. The central black hole (1) and regions of intense star formation in its vicinity can be probed at 100 times the resolution of even the nearest galaxies. Nonetheless, even the basic properties of a key component of the galactic center, its magnetic field, remain poorly understood.

2) Magnetic fields have the potential to transform, store, and explosively release energy, to transport angular momentum, and to confine high-energy plasmas into powerful jet flows. They are therefore central to astrophysical activity from stellar to galactic scales.

3) Magnetic fields are found throughout the Milky Way. Measurements suggest that the magnetic field in the spiral disk of our Galaxy has two components, one globally ordered and the other random, with approximately equal strengths of ~0.3 nT (2); the globally ordered component generally follows the spiral arms of the galaxy. Key questions about the magnetic field in the galactic center are whether it is comparable in strength or much stronger than the field in the disk, and whether it is globally ordered or largely random.

4) approximately 20 years ago, the first high-resolution radio images of the galactic center (3) revealed numerous magnetic structures that are unique to the galactic center. The most striking of these is the galactic center radio arc, a series of parallel linear filaments, each of which is merely a few light years wide yet more than 100 light years long. Also observed were a number of isolated linear features that were variously referred to as streaks, threads, and filaments. The relation between these isolated filaments and the bundled filaments of the radio arc remains unknown.

5) These filamentary structures are distinguished by extreme length-to-width ratios (~10 to 100), nonthermal spectra, and a high intrinsic polarization (~30%, and in some cases approaching the theoretical maximum of 70% for synchrotron radiation). The polarization and nonthermal spectra are consistent with the filaments being produced by synchrotron radiation from relativistic electrons spiraling around a magnetic field. Detailed measurements of individual filaments have shown that the magnetic fields are aligned longitudinally with the filament.(4,5)

References (abridged):
1. G. C. Bower et al., Science 304, [704] (2004)
2. R. Beck, Space Sci. Rev. 99, 243 (2001)
3. F. Yusef-Zadeh et al., Nature 310, 557 (1984)
4. M. Morris, E. Serabyn, Annu. Rev. Astron. Astrophys. 34, 645 (1996)
5. C. C. Lang, K. R. Anantharamaiah, N. E. Kassim, T. J. W. Lazio, Astrophys. J. 521, L41 (1999)

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