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.

We present a comprehensive multiband spectral and polarimetric study of the jet of 3C 264 (NGC 3862). Included in this study are three HST optical and ultraviolet polarimetry data sets, along with new and archival VLA radio imaging and polarimetry, a re-analysis of numerous HST broadband data sets from the near infrared to the far ultraviolet, and a Chandra ACIS-S observation. We investigate similarities and differences between optical and radio polarimetry, in both degree of polarization and projected magnetic field direction. We also examine the broadband spectral energy distribution of both the nucleus and jet of 3C 264, from the radio through the X-rays. From this we place constraints on the physics of the 3C 264 system, the jet and its dynamics. We find significant curvature of the spectrum from the near-IR to ultraviolet, and synchrotron breaks steeper than 0.5, a situation also encountered in the jet of M87. This likely indicates velocity and/or magnetic field gradients and more efficient particle acceleration localized in the faster/higher magnetic field parts of the flow. The magnetic field structure of the 3C 264 jet is remarkably smooth; however, we do find complex magnetic field structure that is correlated with changes in the optical spectrum. We find that the X-ray emission is due to the synchrotron process; we model the jet spectrum and discuss mechanisms for accelerating particles to the needed energies, together with implications for the orientation of the jet under a possible spine-sheath model.

We present new NICMOS and ACS observations of the quasar jet PKS 0637-752, and we use them, together with existing multiwavelength observations, to produce the most complete spectral coverage of the source to date. We explore the implications of these observations in the context of models for the jet Xray emission. By relaxing the assumption of equipartition, we undertake an exhaustive study of the parameter space for external Compton off the CMB (EC/CMB) model. We find that the multiwavelength observations exclude a magnetic field dominated jet. Using the method proposed by Georganopoulos et al. (2005) for probing the jet matter content we show that protons are needed for practically all jet configurations, extending a previous application of the method by Uchiyama et al. (2005) that was based on exploring three particular jet configurations. We also show that equipartition is the only configuration that can reproduce the observations and have one proton per radiating lepton. We finally present a rather model -independent argument that the jet has a spine-sheath flow pattern, with the spine being faster and emitting most of the IR-optical-Xray emission.

We investigate the structure of the kpc‐scale jet in the nearby (z= 0.073) radio galaxy 3C 15, using new optical Hubble Space Telescope (HST) ACS/F606W polarimetry together with archival multiband HST imaging, Chandra X‐ray data and 8.4‐GHz VLA radio polarimetry. The new data confirm that synchrotron radiation dominates in the optical. With matched beams, the jet is generally narrower in the optical than in the radio, suggesting a stratified flow. We examine a simple two‐component model comprising a highly relativistic spine and lower‐velocity sheath. This configuration is broadly consistent with polarization angle differences seen in the optical and radio data. The base of the jet is relatively brighter in the ultraviolet and X‐ray than at lower energies, and the radio and optical polarization angles vary significantly as the jet brightens downstream. Further out, the X‐ray intensity rises again and the apparent magnetic field becomes simpler, indicating a strong shock. Modelling the synchrotron spectrum of this brightest X‐ray knot provides an estimate of its minimum internal pressure, and a comparison with the thermal pressure from X‐ray emitting gas shows that the knot is overpressured and likely to be a temporary, expanding feature.