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ABSTRACT We show that asymmetries in total intensity and linear polarization between the radio jets and counter-jets in two lobed Fanaroff-Riley Class I (FR I) radio galaxies, B2 0206+35 (UGC 1651) and B2 0755+37 (NGC 2484), can be accounted for if these jets are intrinsically symmetrical, with decelerating relativistic outflows surrounded by mildly relativistic backflows. Our interpretation is motivated by sensitive, well-resolved Very Large Array imaging, which shows that both jets in both sources have a two-component structure transverse to their axes. Close to the jet axis, a centrally darkened counter-jet lies opposite to a centrally brightened jet, but both are surrounded by broader collimated emission that is brighter on the counter-jet side. We have adapted our previous models of FR I jets as relativistic outflows to include an added component of symmetric backflow. We find that the observed radio emission, after subtracting contributions from the extended lobes, is well described by models in which decelerating outflows with parameters similar to those derived for jets in plumed FR I sources are surrounded by backflows containing predominantly toroidal magnetic fields. These return to within a few kpc of the galaxies with velocities ≈0.25 c and radiate with a synchrotron spectral index α≈ 0.55. We discuss whether such backflow is to be expected in lobed FR I sources and suggest ways in which our hypothesis can be tested by further observations. [ABSTRACT FROM AUTHOR]

We use high-quality Very Large Array (VLA) images of the Fanaroff–Riley Class I radio galaxy 3C 31 at six frequencies in the range 1365–8440 MHz to explore the spatial scale and origin of the rotation measure (RM) fluctuations on the line of sight to the radio source. We analyse the distribution of the degree of polarization to show that the large depolarization asymmetry between the north and south sides of the source seen in earlier work largely disappears as the resolution is increased. We show that the depolarization seen at low resolution results primarily from unresolved gradients in a Faraday screen in front of the synchrotron-emitting plasma. We establish that the residual degree of polarization in the short-wavelength limit should follow a Burn law and we fit such a law to our data to estimate the residual depolarization at high resolution. We discuss how to interpret the structure function of RM fluctuations in the presence of a finite observing beam and how to address the effects of incomplete sampling of RM distribution using a Monte Carlo approach. We infer that the observed RM variations over selected areas of 3C 31, and the small residual depolarization found at high resolution, are consistent with a power spectrum of magnetic fluctuations in front of 3C 31 whose power-law slope changes significantly on the scales sampled by our data. The power spectrum can only have the form expected for Kolmogorov turbulence [ ] on scales ≲5 kpc. On larger scales, we find . We briefly discuss the physical interpretation of these results. We also compare the global variations of RM across 3C 31 with the results of three-dimensional simulations of the magnetic-field fluctuations in the surrounding magnetoionic medium. We infer that the RM variation across 3C 31 is qualitatively as expected from relativistic-jet models of the brightness asymmetry wherein the apparently brighter jet is on the near side of the nucleus and is seen through less magnetoionic material than the fainter jet. We show that our data are inconsistent with observing 3C 31 through a spherically symmetric magnetoionic medium, but that they are consistent with a field distribution that favours the plane perpendicular to the jet axis – probably because the radio source has evacuated a large cavity in the surrounding medium. We also apply our analysis techniques to the case of Hydra A, where the shape and the size of the cavities produced by the source in the surrounding medium are known from X-ray data. We emphasize that it is essential to account for the potential exclusion of magnetoionic material from a large volume containing the radio source when using the RM variations to derive statistical properties of the fluctuations in the foreground magnetic field. [ABSTRACT FROM AUTHOR]

We present high-quality Very Large Array (VLA) images of the Fanaroff–Riley Class I (FR I) radio galaxy 3C 31 in the frequency range 1365–8440 MHz with angular resolutions from 0.25 to 40 arcsec. Our new images reveal complex, well resolved filamentary substructure in the radio jets and tails. We also use these images to explore the spectral structure of 3C 31 on large and small scales. We infer the apparent magnetic field structure by correcting for Faraday rotation. Some of the intensity substructure in the jets is clearly related to structure in their apparent magnetic field: there are arcs of emission where the degree of linear polarization increases, with the apparent magnetic field parallel to the ridges of the arcs. The spectra of the jets between 1365 and 8440 MHz are consistent with power laws within 60 arcsec of the nucleus. The spectral indices, α (flux density ) are significantly steeper within ≈7 arcsec of the nucleus than between 7 and 50 arcsec ( 0.52–0.57). The spectra of the arcs and of the jet edges are also slightly flatter than the average for their surroundings. At larger distances, the jets are clearly delimited from surrounding larger scale emission both by their flatter radio spectra and by sharp brightness gradients. The spectral index of 0.62 in the first 7 arcsec of 3C 31's jets is very close to that found in other FR I galaxies where their jets first brighten in the radio and where X-ray synchrotron emission is most prominent. Farther from the nucleus, where the spectra flatten, X-ray emission is fainter relative to the radio. The brightest X-ray emission from FR I jets is therefore not associated with the flattest radio spectra, but with a particle acceleration process whose characteristic energy index is . The spectral flattening with distance from the nucleus occurs where our relativistic jet models require deceleration, and the flatter spectra at the jet edges may be associated with transverse velocity shear. [ABSTRACT FROM AUTHOR]

We present new, deep 8.5-GHz VLA observations of the nearby, low-luminosity radio galaxy 3C 296 at resolutions from 0.25 to 5.5 arcsec. These show the intensity and polarization structures of the twin radio jets in detail. We derive the spectral-index distribution using lower-frequency VLA observations and show that the flatter-spectrum jets are surrounded by a sheath of steeper-spectrum diffuse emission. We also show images of Faraday rotation measure and depolarization and derive the apparent magnetic field structure. We apply our intrinsically symmetrical, decelerating relativistic jet model to the new observations. An optimized model accurately fits the data in both total intensity and linear polarization. We infer that the jets are inclined by 58° to the line of sight. Their outer isophotes flare to a half-opening angle of 26° and then recollimate to form a conical flow beyond 16 kpc from the nucleus. On-axis, they decelerate from a (poorly constrained) initial velocity to ≈ 0.4 around 5 kpc from the nucleus, the velocity thereafter remaining constant. The speed at the edge of the jet is low everywhere. The longitudinal profile of proper emissivity has three principal power-law sections: an inner region (0–1.8 kpc), where the jets are faint, a bright region (1.8–8.9 kpc) and an outer region with a flatter slope. The emission is centre brightened. Our observations rule out a globally ordered, helical magnetic field configuration. Instead, we model the field as random on small scales but anisotropic, with toroidal and longitudinal components only. The ratio of longitudinal to toroidal field falls with distance along the jet, qualitatively but not quantitatively as expected from flux freezing, so that the field is predominantly toroidal far from the nucleus. The toroidal component is relatively stronger at the edges of the jet. A simple adiabatic model fits the emissivity evolution only in the outer region after the jets have decelerated and recollimated; closer to the nucleus, it predicts far too steep an emissivity decline with distance. We also interpret the morphological differences between brightness enhancements (‘arcs’) in the main and counter-jets as an effect of relativistic aberration. [ABSTRACT FROM AUTHOR]

We present images of the jets in the nearby radio galaxy NGC 315 made with the Very Large Array at five frequencies between 1.365 and 5 GHz with resolutions between 1.5 and 45 arcsec. Within 15 arcsec of the nucleus, the spectral index of the jets is . Further from the nucleus, the spectrum is flatter, with significant transverse structure. Between 15 and 70 arcsec from the nucleus, the spectral index varies from ≈0.55 on-axis to ≈0.44 at the edge. This spectral structure suggests a change of dominant particle acceleration mechanism with distance from the nucleus and the transverse gradient may be associated with shear in the jet velocity field. Further from the nucleus, the spectral index has a constant value of 0.47. We derive the distribution of Faraday rotation over the inner ±400 arcsec of the radio source and show that it has three components: a constant term, a linear gradient (both probably due to our Galaxy) and residual fluctuations at the level of 1–2 rad m−2. These residual fluctuations are smaller in the brighter (approaching) jet, consistent with the idea that they are produced by magnetic fields in a halo of hot plasma that surrounds the radio source. We model this halo, deriving a core radius of ≈225 arcsec and constraining its central density and magnetic field strength. We also image the apparent magnetic field structure over the first ±200 arcsec from the nucleus. [ABSTRACT FROM AUTHOR]

We apply our intrinsically symmetrical, decelerating relativistic jet model to deep Very Large Array imaging of the inner ±70 arcsec of the giant low-luminosity radio galaxy NGC 315. An optimized model accurately fits the data in both total intensity and linear polarization. We infer that the velocity, emissivity and field structure in NGC 315 are very similar to those of the other low-luminosity sources we have modelled, but that all of the physical scales are larger by a factor of about 5. We derive an inclination to the line of sight of 38°± 2° for the jets. Where they first brighten, their on-axis velocity is . They decelerate to β≈ 0.4 between 8 and 18 kpc from the nucleus and the velocity thereafter remains constant. The speed at the edge of the jet is ≈0.6 of the on-axis value where it is best constrained, but the transverse velocity profile may deviate systematically from the Gaussian form we assume. The proper emissivity profile is split into three power-law regions separated by shorter transition zones. In the first of these, at ≈3 kpc (the flaring point) the jets expand rapidly at constant emissivity, leading to a large increase in the observed brightness on the approaching side. At ≈10 kpc, the emissivity drops abruptly by a factor of 2. Where the jets are well resolved, their rest-frame emission is centre brightened. The magnetic field is modelled as random on small scales but anisotropic and we rule out a globally ordered helical configuration. To a first approximation, the field evolves from a mixture of longitudinal and toroidal components to predominantly toroidal, but it also shows variations in structure along and across the jets, with a significant radial component in places. Simple adiabatic models fail to fit the emissivity variations. [ABSTRACT FROM AUTHOR]

We present a general approach to the modelling of the brightness and polarization structures of adiabatic, decelerating relativistic jets, based on the formalism of Matthews and Scheuer. We compare the predictions of adiabatic jet models with deep, high-resolution observations of the radio jets in the FR I radio galaxy 3C 31. Adiabatic models require coupling between the variations of velocity, magnetic field and particle density. They are therefore more tightly constrained than the models we have previously presented for 3C 31. We show that adiabatic models provide a poorer description of the data in two crucial respects: they cannot reproduce the observed magnetic-field structures in detail, and they also predict too steep a brightness decline along the jets for plausible variations of the jet velocity. We find that the innermost regions of the jets show the strongest evidence for non-adiabatic behaviour, and that the adiabatic models provide progressively better descriptions of the jet emission at larger distances from the galactic nucleus. We briefly discuss physical processes which might contribute to this non-adiabatic behaviour. In particular, we develop a parametrized description of distributed particle injection, which we fit to the observed total intensities. We show that particles are preferentially injected where bright X-ray emission is observed, and where we infer that the jets are overpressured. [ABSTRACT FROM AUTHOR]

We compare deep Very Large Array (VLA) imaging of the total intensity and linear polarization of the inner jets in the nearby, low-luminosity radio galaxy 3C 31 with models of the jets as intrinsically symmetrical, decelerating relativistic flows. We show that the principal differences in appearance of the main and counter-jets within 30 arcsec of the nucleus can result entirely from the effects of relativistic aberration in two symmetrical, antiparallel, axisymmetric, time-stationary relativistic flows. We develop empirical parametrized models of the jet geometry and the three-dimensional distributions of the velocity, emissivity and magnetic-field structure. We calculate the synchrotron emission by integration through the models, accounting rigorously for relativistic effects and the anisotropy of emission in the rest frame. The model parameters are optimized by fitting to our 8.4-GHz VLA observations at resolutions of 0.25 and 0.75 arcsec full width at half maximum (FWHM), and the final quality of the fit is extremely good. The novel features of our analysis are that we model the two-dimensional brightness distributions at large number of independent data points rather than using one-dimensional profiles, we allow transverse as well as longitudinal variations of velocity, field and emissivity and we simultaneously fit total intensity and linear polarization. We conclude that the jets are at ≈ 52° to the line of sight, that they decelerate and that they have transverse velocity gradients. Their magnetic field configuration has primarily toroidal and longitudinal components. The jets may be divided into three distinct parts, based not only on the geometry of their outer isophotes, but also on their kinematics and emissivity distributions: a well-collimated inner region; a flaring region of rapid expansion followed by recollimation and a conical outer region. The inner region is poorly resolved, but is best modelled as the sum of fast (0.8-0.9c) and... [ABSTRACT FROM AUTHOR]