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

Abstract

We show that the principal differences in appearance of the main and counter-jets within 30 arcsec of the nucleus of the low-luminosity radio galaxy 3C 31 can result entirely from the effects of relativistic aberration in two symmetrical, antiparallel, axisymmetric, time-stationary relativistic flows. We develop empirical parameterized models of the jet geometry and the three-dimensional distributions of the velocity, emissivity and magnetic-field structure and optimize their parameters by fitting the predicted synchrotron intensity and polarization to deep 8.4-GHz VLA observations. We conclude that the jets are at roughly 52 degrees to the line of sight, that they decelerate and that they have transverse velocity gradients. They have three regions with distinct kinematics: a narrow inner region; a flaring region of rapid expansion followed by recollimation and a conical outer region. There is a discontinuity in the flow between inner and flaring regions. The on-axis velocity is close to 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 has primarily toroidal and longitudinal components except in the flaring region, where there is a significant radial component at the jet edge. Simple adiabatic models fail in the inner and flaring regions. 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. [Abridged]Comment: 29 pages, 25 figures, accepted by MNRAS. For additional material, including higher-resolution figures and animations, see http://www.cv.nrao.edu/~abridle/3c31free

Particle acceleration and jet dynamics in Centaurus A

“…The jets decelerate from relativistic to subrelativistic speeds on scales of a few kpc, showing significant transverse velocity structure [1]. The deceleration is likely to be a result of entrainment of external material, and the jet is stabilized by the strong pressure gradient in the centre of the host galaxy [1][2][3]. It is now possible to make detailed quantitative models of these jets, which agree well with observation (e.g.…”

“…There is evidence on larger scales (Fig. 3) that the diffuse emission is edge-brightened in places, which, as the edges of the jets are regions of strong shear [1] might mean that it is associated with genuinely diffuse acceleration processes such as second-order Fermi acceleration. Although the jet will naturally contain a large number of normal stars (a few ×10 6 in the inner kpc), each of which will act, on a smaller scale, like the obstacles we have discussed above, it is easy to show that the cross-sectional area of the jet that they cover, assuming solar mass loss rates (Ṁ ≈ 4 × 10 −14 M ⊙ yr −1 ) and wind speeds (v w ≈ 600 km s −1 ), is too small to produce the observed bolometric X-ray luminosity in any X-ray jet source, including Cen A, even assuming 100% energy conversion efficiency.…”

“…On scales of a few kpc, however, the FRI jets disrupt as they come into pressure balance with the external medium, becoming much less well collimated and less efficient as energy transport mechanisms, and producing the characteristic bright, broad jet emission seen on arcsecond scales with instruments such as the VLA. The jets decelerate from relativistic to subrelativistic speeds on scales of a few kpc, showing significant transverse velocity structure [1]. The deceleration is likely to be a result of entrainment of external material, and the jet is stabilized by the strong pressure gradient in the centre of the host galaxy [1][2][3].…”

Structural Transition in the NGC 6251 Jet: An Interplay With the Supermassive Black Hole and Its Host Galaxy

“…One possibility would be the conversion from the bulk kinetic energy to the jet internal energy. Indeed, Laing & Bridle (2002) suggest a gradual and substantial deceleration of the jet velocity located at 1−3 kpc from the central engine for the FR I type jets based on the VLA polarimetric observations. It is interesting to measure the velocity structure of the conical jet.…”

Clusters of Galaxies and the Cosmic Web with Square Kilometre Array

“…To answer (i), observationally the transverse velocity profile of the radio jets can give clues to understand which of these two processes is more relevant; if former, no velocity profile across the jet is expected, whereas in latter case, there would be a velocity profile with slower layers at the jet boundaries (Laing & Bridle, 2002). Next to answer (ii), comparisons between the radio morphologies of tailed sources in cluster environments and the distribution of the thermal gas as seen in Chandra and XMM-Newton images, indicate that the thermal gas is almost always asymmetric and aligned towards the direction of the bending (Venkatesan et al, 1994), and many a times these clusters are undergoing mergers, resulting in large-scale flows of hot gas owing to the changing gravitational potential (Klamer, Subrahmanyan & Hunstead, 2004).…”