R. A. Laing

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. n n n nWe 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.

Adiabatic relativistic models for the jets in the radio galaxy 3C 31

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.

Asymmetry of jets, lobe length and spectral index in quasars

ABSTRACT The less depolarized lobe of a radio source is generally the lobe containing the jet(Laing-Garrington correlation) but the less depolarized lobe is also generally thatwith the flatter radio spectrum (Liu-Pooley correlation). Both effects are strong; takentogether they would imply a correlation between jet side and lobe spectral index,i.e. between an orientation-dependent feature and one which is intrinsic. We test thisprediction using detailed spectral imaging of a sample of quasars with well-defined jetsand investigate whether the result can be reconciled with the standard interpretationof one-sided jets in terms of relativistic aberration. Our central finding is that thespectrum of high surface brightness regions is indeed flatter on the jet side, but thatthe spectrum of low surface brightness regions is flatter on the side with the longerlobe. We discuss possible causes for these correlations and favour explanations interms of relativistic bulk motion in the high surface brightness regions and differentialsynchrotron ageing in the extended lobe material.Key words: galaxies: jets – quasars: general – radio continuum: general

Relativistic models of two low-luminosity radio jets: B2 0326+39and B2 1553+24

We apply the intrinsically symmetrical, decelerating relativistic jet model developed by Laing & Bridle for 3C 31 to deep, full-synthesis 8.4-GHz VLA imaging of the two low-luminosity radio galaxies B2 0326+39 and B2 1553+24. After some modifications to the functional forms used to describe the geometry, velocity, emissivity and magnetic-field structure, these models can accurately fit our data in both total intensity and linear polarization. We conclude that the jets in B2 0326+39 and B2 1553+24 are at angles of 64° ± 5° and 7.°7 ± 1.°3 to the line of sight, respectively. In both objects, we find that the jets decelerate from 0.7-0.8c to < 0.2c over a distance of approximately 10 kpc, although in B2 1553+24 this transition occurs much further from the nucleus than in B2 0326+39 or 3C 31. The longitudinal emissivity profiles can be divided into sections, each fitted accurately by a power law; the indices of these power laws decrease with distance from the nucleus. B2 0326+39 also requires a discontinuity in emissivity to in order to fit a region with several bright knots of emission. In B2 1553+24, the sudden brightening of the jet can be explained by a combination of rapid expansion of the jet and a continuous variation of emissivity. The magnetic fields in both objects are dominated by the longitudinal component in the high-velocity regions close to the nucleus and by the toroidal component further out, but B2 0326+39 also has a significant radial component at large distances, whereas B2 1553+24 does not. Simple adiabatic models fail to fit the emissivity variations in the regions of high velocity but provide good descriptions of the emissivity after the jets have decelerated. Given the small angle to the line of sight inferred for B2 1553+24, there should be a significant population of similar sources at less extreme orientations. Such objects should have long (?200 kpc), straight, faint jets and we show that their true sizes are likely to have been underestimated in existing images.

Asymmetry of jets, lobe size and spectral index in radio galaxies and quasars

In this paper we investigate the correlations between spectral index, jet side and extent of the radio lobes for a sample of nearby FRII radio galaxies. In DennettThorpe et al. (1997) we studied a sample of quasars and found that the high surface brightness regions had flatter spectra on the jet side (explicable as a result of Doppler beaming) whilst the extended regions had spectral asymmetries dependent on lobe length. Unified schemes predict that asymmetries due to beaming will be much smaller in narrow-line radio galaxies than in quasars, and we therefore chose to investigate in a similar fashion, a sample of radio galaxies with detected jets. We find that spectral asymmetries in these objects are uncorrelated with jet sidedness at all brightness levels, but depend on relative lobe volume. Our results are not in conflict with unified schemes, but suggest that the differences between the two samples are due primarily to power or redshift, rather than to orientation. We also show directly that hotspot spectra steepen as a function of radio power or redshift. Whilst a shift in observed frequency due to the redshift may account for some of the steepening, it cannot account for all of it, and a dependence on radio power is required.

Synchrotron emission from anisotropic disordered magnetic fields

We derive expressions for the total and linearly polarized synchrotron emissivity of an element of plasma containing relativistic particles and disordered magnetic field that has been sheared or compressed along three independent directions. Our treatment follows that given by Matthews & Scheuer in the special case of a power-law electron energy spectrum. We show that the emissivity integrals depend on a single parameter, making it straightforward to generate one-dimensional look-up tables. We also demonstrate that our formulae give identical results to those in the literature in special cases.

The HST survey of the B2 sample of radio galaxies: Detection of two optical jets

We present HST observations of previously undetected optical jets in the low-luminosity radio galaxies B2u20090755+37 and B2u20091553+24. We show that there is accurate spatial coincidence between optical and radio emission, implying that the former is likely to be synchrotron radiation. The physical properties of the jets are similar to those known previously: their radio-optical spectral indices are ≈ 0.7 and in B2u20090755+37, the spectrum steepens between optical and X-ray wavelengths. Our results support the hypothesis that optical emission is detectable from jets orientated within ≈ 20° of the line of sight for the B2 sample.