Zeeman effect in 21 centimeter line radiation: Methods and initial results

Journal Article · Fri Jan 01 04:00:00 EST 1982 · Astrophys. J.; (United States)

DOI:https://doi.org/10.1086/159544· OSTI ID:5426199

Troland, T H; Heiles, C

We report upon a search for the Zeeman effect in the 21 cm line of galactic neutral hydrogen (H i) using the 26 m radio telescope at Hat Creek Observatory. We have carefully investigated the causes and remedies for instrumental circular polarization. These effects can, in principle, impose the most severe limitation upon H i emission line Zeeman effect measurements. They do not appear to have a serious effect at our current levels of sensitivity. In eight H i emission regions, including several associated with shell-like structures, we derive upper limits to the line-of-sight field of between 4 and 10 microgauss. We have also improved existing field limits for two H i absorption regions: the vroughly-equal0 km s/sup -1/ component toward Cas A(B<0.8 ..mu..G) and the vroughly-equal-85 km s/sup -1/ component toward Cyg A (B<4.6 ..mu..G). The field strengths we derive for H i regions are low, and magnetic energy densities are smaller than energy densities associated with turbulent or other macroscopic motions (but larger than those of thermal motions). Therefore, magnetic forces should not appreciably influence motions of the gas. In this case, the field should scale as Bproportionaln/sup 2/3/, and the absence of detectable fields in most of the observed regions is very difficult to understand. Reversals of magnetic field lines on scales smaller than the telescope beamwidth can reduce the magnitude of the measured field; however, we argue that this effect is unimportant for the current measurements. Owing to the weakness of the magnetic field, alignment of interstellar dust grains by the conventional mechanism is not possible in the H i regions we have studied. Since grain alignment does indeed occur in these regions, a different and more efficient mechanism must be invoked.

• Research Organization: Physics and Astronomy Department, University of Kentucky
• OSTI ID: 5426199
• Journal Information: Astrophys. J.; (United States), Journal Name: Astrophys. J.; (United States) Vol. 252:1; ISSN ASJOA
• Country of Publication: United States
• Language: English

Related Subjects

640105* -- Astrophysics &amp; Cosmology-- Galaxies
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
COSMIC RADIO SOURCES
DATA ANALYSIS
H1 REGIONS
INTERSTELLAR GRAINS
INTERSTELLAR MAGNETIC FIELDS
INTERSTELLAR SPACE
MAGNETIC FIELDS
PARTICLES
POLARIMETERS
POLARIZATION
SPACE
ZEEMAN EFFECT