Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...451..645v&link_type=abstract
Astrophysical Journal v.451, p.645
Astronomy and Astrophysics
Astronomy
17
Ism: Magnetic Fields, Methods: Data Analysis, Radio Lines: Galaxies
Scientific paper
Observations of the Zeeman effect in interstellar H I emission profiles have been carried out with the 140 foot (43 m) radio telescope in 39 high-latitude directions where magnetic field detections have been claimed in the literature. In half of these directions no agreement was found. For nine directions in this set, no detectable Zeeman effect was measured, while to others gave apparent detections at odds with published results. These 10, as well as t8 other directions where apparent agreement was found, exhibited signals that are shown to have been produced by interaction between polarized beam structure and H I brightness gradients around the sources. A general correction was applied to the data by removing the signature of the brightness gradient that most closely accounted for the observations. The majority of sources exhibited the signature of the H I brightness gradients in the north-south direction in the V-channel output of the receiver at a level consistent with the amplitude of the dominant polarized sidelobes, of order 0.3%. Interpretation of the data shows that none of the previously published magnetic field detections could therefore be confirmed. Instead, low limits of the order of a few microgauss are set for the directions studied. In addition, a previously published claim of a magnetic detection toward a high-velocity cloud is not confirmed either.
A review of available data suggests that the apparent magnetic field strength in H I emission features is a function of the maximum amplitude of polarized sidelobes during the observations. Until such sidelobes are reduced by a factor of 10 below their present levels (that is, to ˜0.03%) it is unlikely that the Zeeman effect in H I emission profiles will be detected without a great deal more work than has heretofore been applied to correct the data for artifacts.
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