Filamentary Structure in the Lobes of Radio Galaxies


Observations of the radio galaxies Pictor A and Hercules A have revealed intriguing structures that appear to represent the effects of complex shock structures in jets and the surrounding cocoon. From the research we have carried out in this project, it is clear that this structure is both density and Mach number dependent, giving us an opportunity to estimate these parameters in a completely different way from methods previously used. Our extension of jet simulations down to a ratio of jet density to background density of 10-4 is new and charts a previously unexplored region of parameter space. We use a scalar tracer to distinguish the distributions and evolution of matter originating from the jet and from the ambient medium of thermal gas. We use hydrodynamic data output to generate ray-traced images simulating synchrotron emission from the three-dimensional structures. We compare these images with astronomical observations, and study the temporal evolution of filamentary features in detail.


Principal Investigator

Curtis J. Saxton
Mt Stromlo Observatory
RSAA
and
Dept. of Physics & Theoretical Physics
Faculty of Science
ANU

Project

x34

Facilities Used

SC

Co-Investigators

Geoffrey V. Bicknell
Mt Stromlo Observatory
RSAA
and
Dept. of Physics & Theoretical Physics
Faculty of Science
ANU

Ralph S. Sutherland
Mt Stromlo Observatory
RSAA
ANU

RFCD Codes

240101, 240502


Significant Achievements, Anticipated Outcomes and Future Work

We have investigated the structure and behaviour of the termination shock and backflow cocoon in extragalactic jets ranging over two orders of magnitude in density. We find that the distribution of radio emitting plasma in the cocoon depends substantially on the jet density. Moreover, the density dependent turbulence in the jet backflow also alters the surging action of the jet through compression and decompression. This affects the structure of luminous shocks. We have also found that boundary conditions of the system have a significant effect on the width and mixing of the jet cocoon.

We have reproduced images of radio surface brightness similar to the hot-spot and bar-like filament revealed in radio and optical observations of the western radio lobe of Pictor A. These structures are well explained in terms of the surging termination shock of the jet, surrounded by bright transitory ring-like shocks in the jet’s backflow. The transient simulated structures match observation in episodes amounting to approximately 4% of the time, implying that such morphologies would be uncommon in radio galaxies, yet could be observed in a small fraction. Our studies suggest that the jet of Pictor A is almost in the plane of the sky; the observed morphology is not reproduced by a jet that is rendered at inclinations of 45 or less. Furthermore, we predict that these bright structures have dynamical lifetimes that are shorter than the light travel time across the source. The shock luminosity can vary in time by over an order of magnitude even though the power and thrust of the jet source is constant. This provides an important qualification to comparisons between the brightnesses of the independent eastern and western hot-spots in the radio source.

The ring-like structures observed in the radio lobes of Hercules A are well explained as shocks propagating in the backflow surrounding the jet. In this case, we infer that the jet is oriented ~45 to the line of sight. The lack of bright hot-spots in these radio lobes can be understood if the head of the Hercules A jet is presently in one of the low-luminosity phases seen in our simulation, during the reformation or surging of the termination shock.

Figure 1. Cross-sectional map of the concentration of radio plasma, derived from the jet, in a snapshot from a simulation with (,M)=(10-4,10). The jet is the white horizontal feature. It propagates to the right through the denser background medium.

 

Figure 2. Log density map with velocity vectors superimposed, corresponding to the previous image. Within the bow shock, a turbulent cocoon surrounds the jet. The compression and decompression of the jet by the turbulence in the cocoon is evident through the velocity vectors.

Figure 3. Raytraced image of a snapshot from a jet with (,M)=(10-4,5) viewed at an inclination of 80.

Figure 4. Raytraced image of a snapshot from a jet with (,M)=(10-4,5) viewed at an inclination of 45.

 

Computational Techniques Used

Our hydrodynamic simulations were conducted using the VH-1 code (Blondin and Lukfin 1993) which is an implementation of the Piecewise Parabolic Method (PPM) (Colella and Woodward 1984). An advantage of PPM for this type of simulation is its excellent resolution of shocks. We have enhanced the code to achieve greater efficiency, and have added a scalar tracer to distinguish and follow the evolution of various constituents of the physical system.

Our images of simulated radio surface brightness were rendered by a special-purpose ray-tracing program that projects three-dimensional structures obtained from the PPM output. At a given orientation, the volumetric emissivity (a function of the PPM output variables) is integrated along the line of sight for each pixel of the sky/screen image.

 

Publications, Awards and External Funding

This project is connected with the ARC large grant A69905341: "Jet-cloud interactions in active galactic nuclei."

C.J. Saxton, R.S. Sutherland, G.V. Bicknell, Evidence of time-dependent ultra-light jet behaviour in Pictor A, Astronomy & Astrophysics, 2002, submitted.

C.J. Saxton, R.S. Sutherland, G.V. Bicknell, Production of ring-like structure in the cocoon of Hercules A, Astrophysical Journal, 2002, submitted.