Optical Polarimetry of the Jets of Nearby Radio Galaxies. I. The Data

In this paper, the first in a series, we present an overview of new Hubble Space Telescope (HST) imaging polarimetry of six nearby radio galaxies (3C 15, 3C 66B, 3C 78, 3C 264, 3C 346, and 3C 371) with optical jets. These observations triple the number of extragalactic jets with subarcsecond-resolution optical polarimetry. We discuss the polarization characteristics of each jet and, as our Stokes I images also represent by far the deepest optical images yet obtained of each of these jets, we also discuss the morphology in total flux of each jet in detail. We find evidence of high optical polarization, averaging 20%, but reaching upwards of 50% in some objects, confirming that the optical emission is synchrotron, and that the components of the magnetic fields perpendicular to the line of sight are well ordered. We find a wide range of polarization morphologies, with each jet having a somewhat different relationship between total intensity and polarized flux and the polarization position angle. We find two trends in all of these jets. First, jet “edges” are very often associated with high fractional optical polarizations, as also found in earlier radio observations of these and other radio jets. In these regions, the magnetic field vectors appear to track the jet direction, even at bends, where we see particularly high fractional polarizations. This indicates a strong link between the local magnetic field and jet dynamics. Second, optical flux maximum regions are usually well separated from maxima in fractional polarization and often are associated with polarization minima. This trend is not found in radio data and was found in our optical polarimetry of M87 with HST. However, unlike in M87, we do not find a general trend for near-90◦ rotations in the optical polarization vectors near flux maxima. We discuss possibilities for interpreting these trends, as well as implications for jet dynamics, magnetic field structure and particle acceleration. (Refer to PDF file for exact formulas)