Alan H. Bridle

Dynamical models for jet deceleration in the radio galaxy 3C 31

We present a dynamical analysis of the flow in the jets of the low-luminosity radio galaxy 3C31 based on our earlier geometrical and kinematic model (Laing & Bridle 2002) and on estimates of the external pressure and density distributions from Chandra observations (Hardcastle et al. 2002). We apply conservation of particles, energy and momentum to derive the variations of pressure and density along the jets and show that there are self-consistent solutions for deceleration by injection of thermal matter. We initially take the jets to be in pressure equilibrium with the external medium at large distances from the nucleus and the momentum flux to be � = �/c, where � is the energy flux; we then progressively relax these constraints. With our initial assumptions, the energy flux is well determined: � � 9 – 14 ×10 36 W. We infer that the jets are over-pressured compared with the external medium at the flaring point (1.1kpc from the nucleus) where they start to expand rapidly. Local minima in the density and pressure and maxima in the mass injection rate and Mach number occur at � 3kpc. Further out, the jets decelerate smoothly with a Mach number � 1. The mass injection rate we infer is comparable with that expected from stellar mass loss throughout the cross-section of the jet close to the flaring point, but significantly exceeds it at large distances. We conclude that entrainment from the galactic atmosphere across the turbulent boundary layer of the jet is the dominant mass input process far from the nucleus, but that stellar mass loss may also contribute near the flaring point. The occurrence of a significant over-pressure at the flaring point leads us to suggest that it is the site of a stationary shock system, perhaps caused by reconfinement of an initially free jet. Our results are compatible with a jet consisting of e e + plasma on parsec scales which picks up thermal matter from stellar mass loss to reach the inferred density and mass flux at the flaring point, but we cannot rule out an e p + composition with a low-energy cut-off.

A Chandra observation of the X-ray environment and jet of 3C 31

We have observed the twin-jet radio galaxy 3C 296 with Chandra. X-ray emission is detected from the nucleus, from the inner parts of the radio jet and from a small-scale thermal environment around the jet deceleration region. As we have found in previous observations of other twin-jet radio galaxies, the X-ray jet and a steep pressure gradient in the external thermal environment are associated with the region where strong bulk deceleration of the jet material is suggested by radio observations. Our observations provide additional evidence that the inner jets of twin-jet objects are always associated with a relatively cool, dense central X-ray emitting component with a short cooling time.

Relativistic models and the jet velocity field in the radio galaxy 3C 31

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 ≈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 much slower components. The transition between inner and flaring regions marks a discontinuity in the flow where the emissivity increases suddenly. The on-axis velocity stays fairly constant at ≈0.8c until the end of the flaring region, where it drops abruptly to ≈0.55c, thereafter falling more slowly to ≈0.25c at the end of the modelled region. Throughout the flaring and outer regions, the velocity at the edge of the jet is ≈0.7 of its on-axis value. The magnetic field in the flaring region is complex, with an essentially isotropic structure at the edge of the jet, but a more ordered toroidal + longitudinal configuration on-axis. In the outer region, the radial field vanishes and the toroidal component becomes dominant. We show that the emissivity and field structures are inconsistent with simple adiabatic models in the inner and flaring regions. We suggest that the discontinuity between the inner and flaring regions could be associated with a stationary shock structure and that the inferred transverse velocity profiles and field structure in the flaring region support the idea that the jets decelerate by entraining the external medium. We demonstrate the appearance of our model at other angles to the line of sight and argue that other low-luminosity radio galaxies resemble 3C 31 seen at different orientations.

Jet reorientation in active galactic nuclei : two winged radio galaxies

Winged, or X-shaped, radio sources form a small class of morphologically peculiar extragalactic sources. We present multifrequency radio observations of two such sources. We derive maximum ages since any re-injection of fresh particles of 34 and 17 Myr for the wings of 3C 223.1 and 3C 403 respectively, based on the lack of synchrotron and inverse Compton losses. On morphological grounds we favour an explanation in terms of a fast realignment of the jet axis which occurred within a few Myr. There is no evidence for merger activity, and the host galaxies are found to reside in no more than poor cluster environments. A number of puzzling questions remain about those sources: in particular, although the black hole could realign on sufficiently short time-scales, the origin of the realignment is unknown.

Structures of the magnetoionic media around the Fanaroff–Riley Class I radio galaxies 3C 31 and Hydra A

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 P(f) can only have the form expected for Kolmogorov turbulence [P(f) α f -11/3 ] on scales ≤5 kpc. On larger scales, we find P(f) ∝∼ f -2.3 . 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 3C31 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 3C31 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.

A relativistic model of the radio jets in 3C 296

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 spectralindex distribution using lower-frequency VLA observations and show that the flatterspectrum 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 β = v/c ≈ 0.8 to β ≈ 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.

Origin of the structures and polarization in the classical double 3C 219

Scaled-array VLA observations of the classical double radio galaxy 3C 219 at 1.4, 1.5, 1.7, and 4.9 GHz with 1.4″ resolution have been used to derive the spectral and polarimetric properties of the source. The extended emission of the lobes is filamentary, as in other well-resolved radio galaxies. A second type of filamentation, spatially independent of the total intensity filaments, is found in the depolarization distribution of the source. The depolarization filaments are associated with strong local gradients in the rotation measure and may indicate that a clumpy magnetoionic medium surrounds the radio galaxy

Systematic properties of decelerating relativistic jets in low-luminosity radio galaxies

We model the kiloparsec-scale synchrotron emission from jets in 10 Fanaroff-Riley Class I radio galaxies for which we have sensitive, high-resolution imaging and polarimetry from the Very Large Array. We assume that the jets are intrinsically symmetrical, axisymmetric, decelerating, relativistic outflows and we infer their inclination angles and the spatial variations of their flow velocities, magnetic field structures and emissivities using a common set of fitting functions. The inferred inclinations agree well with independent indicators. The spreading rates increase rapidly, then decrease, in a flaring region. The jets then recollimate to form conical outer regions at distance r0 from the active galactic nucleus (AGN). The flaring regions are homologous when scaled by r0. At ~0.1r0, the jets brighten abruptly at the onset of a high-emissivity region and we find an outflow speed of ~0.8c, with a uniform transverse profile. Jet deceleration first becomes detectable at ~0.2r0 and the outflow often becomes slower at its edges than it is on-axis. Deceleration continues until ~0.6r0, after which the outflow speed is usually constant. The dominant magnetic-field component is longitudinal close to the AGN and toroidal after recollimation, but the field evolution is initially much slower than predicted by flux-freezing. In the flaring region, acceleration of ultrarelativistic particles is required to counterbalance the effects of adiabatic losses and account for observed X-ray synchrotron emission, but the brightness evolution of the outer jets is consistent with adiabatic losses alone. We interpret our results as effects of the interaction between the jets and their surroundings. (Slightly abridged).

A relativistic model of the radio jets in NGC 315

We apply our intrinsically symmetrical, decelerating relativistic jet model to deep VLA imaging of the inner ±70arcsec of the giant low-luminosity radio galaxy NGC315. An optimized model accurately fits the data in both total intensity and linear polarization. We infer that the velocity, emissivity and field structure in NGC315 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 β = v/c � 0.9. They decelerate to β � 0.4 between 8 and 18kpc 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 �3kpc (the flaring point) the jets expand rapidly at constant emissivity, leading to a large increase in the observed brightness on the approaching side. At �10kpc, 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.

Multifrequency observations of the jets in the radio galaxy NGC 315

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 a = 0.61. 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.