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Magnetic Fields Near and Far
Galactic and Extragalactic Single-Dish Radio Observations of the Zeeman Effect
Institution: | University of California, Berkeley (USA) |
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Advisor(s): | Carl Heiles |
Degree: | Ph.D. in Astrophysics |
Year: | 2008 |
Volume: | 171 pages |
ISBN-10: | 1599426846 |
ISBN-13: | 9781599426846 |
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According to astrophysical theory, magnetic fields should play an important
role in the structure and dynamics of the interstellar medium. While
astronomical observations confirm this directly, the observational record
is sparse. This is because magnetic fields can only be measured via
polarimetric methods, and most of these methods can only provide an
indirect inference of the magnetic field strength. The Zeeman effect,
however, is the only method by which in situ measurements of astrophysical
magnetic fields can be made.
The spectral signature of Zeeman splitting is imprinted in the circular
polarization spectrum of radiation received from an astronomical source.
In order to make a reliable detection at radio frequencies, one must employ
careful calibrations and account for instrumental effects. We begin this
dissertation by covering the fundamentals of radio spectropolarimetry. We
then offer historical details regarding the Zeeman effect and its use in
single-dish radio observations. We present an outline of how one accurately
measures the Zeeman effect using large single-dish radio telescopes. We
follow this with results from an assessment of the polarization properties
of the 100 m Green Bank Telescope (GBT).
We then present magnetic field detections made via the Zeeman effect from
the Galactic scale to cosmological distances. We begin with GBT
observations of 21 cm emission toward the Taurus Molecular Cloud (TMC)
complex. Recent observations have suggested that fields stronger than 20
microgauss are located at the distance of the TMC. Our Zeeman
observations rule out fields of this strength, but do show a clear +5
microgauss detection from HI emission at the velocity of the TMC. More
surprisingly, we have discovered multiple detections of a line-of-sight
magnetic field of strength roughly +40 microgauss in a filament near -50
km/s. We then present a windfall of detections of milligauss-strength
magnetic fields in starburst galaxies. Detected by means of Zeeman
splitting of 1667 MHz hydroxyl megamaser emission, these Arecibo and GBT
results represent the first extragalactic Zeeman measurements to probe the
field inside an external galaxy. Finally, we climb the cosmological
distance ladder, and present a dramatic GBT detection of a magnetic field
in a damped Lyman-alpha absorber at a redshift of 0.692. We discuss
possible scenarios for the creation of an 84 microgauss field at a
look-back time of 6.4 Gyr.
Tim Robishaw was born and raised in Boston, Massachusetts. He spent an enjoyable 13 years at the University of California, Berkeley, where he accumulated a B.A., an M.A., and a Ph.D. in astrophysics. He has been appointed a Postdoctoral Fellow in Radio Polarimetry in the School of Physics at the University of Sydney.
171 pages
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