A New Catalog of Faraday Rotation Measures and Redshifts for
  Extragalactic Radio Sources
  We present a catalog of Faraday rotation measures (RMs) and redshifts for
4003 extragalactic radio sources detected at 1.4 GHz, derived by identifying
optical counterparts and spectroscopic redshifts for linearly polarized radio
sources from the NRAO VLA Sky Survey. This catalog is more than an order of
magnitude larger than any previous sample of RM vs. redshift, and covers the
redshift range 0 < z < 5.3 ; the median redshift of the catalog is z = 0.70,
and there are more than 1500 sources at redshifts z > 1. For 3650 of these
sources at Galactic latitudes |b| >= 20 degrees, we present a second catalog in
which we have corrected for the foreground Faraday rotation of the Milky Way,
resulting in an estimate of the residual rotation measure (RRM) that aims to
isolate the contribution from extragalactic magnetic fields. We find no
significant evolution of RRM with redshift, but observe a strong
anti-correlation between RRM and fractional polarization, p, that we argue is
the result of beam depolarization from small-scale fluctuations in the
foreground magnetic field or electron density. We suggest that the observed
variance in RRM and the anti-correlation of RRM with p both require a
population of magnetized intervening objects that lie outside the Milky Way but
in the foreground to the emitting sources.
