Spectral Polarization of the Redshifted 21 cm Absorption Line Toward 3C
  286
  A re-analysis of the Stokes-parameter spectra obtained of the z=0.692 21 cm
absorption line toward 3C 286 shows that our original claimed detection of
Zeeman splitting by a line-of-sight magnetic field, B_los = 87 microgauss is
incorrect. Because of an insidious software error, what we reported as Stokes V
is actually Stokes U: the revised Stokes V spectrum indicates a 3-sigma upper
limit of B_los < 17 microgauss. The correct analysis reveals an absorption
feature in fractional polarization that is offset in velocity from the Stokes I
spectrum by -1.9 km/s. The polarization position-angle spectrum shows a dip
that is also significantly offset from the Stokes I feature, but at a velocity
that differs slightly from the absorption feature in fractional polarization.
We model the absorption feature with 3 velocity components against the core-jet
structure of 3C 286. Our chisquare minimization fitting results in components
with differing (1) ratios of H I column density to spin temperature, (2)
velocity centroids, and (3) velocity dispersions. The change in polarization
position angle with frequency implies incomplete coverage of the background jet
source by the absorber. It also implies a spatial variation of the polarization
position angle across the jet source, which is observed at frequencies higher
than the 839.4 MHz absorption frequency. The multi-component structure of the
gas is best understood in terms of components with spatial scales of ~100 pc
comprised of hundreds of low-temperature (T < 200 K) clouds with linear
dimensions of about 1 pc.
