Density map from a two-dimensional simulation of a jet propagating out of the cloudy centre of its host galaxy. The nozzle of the jet (the nucleus) is in the middle of the left border. The clouds (green/amber) are about 1000 times denser than the ambient interstellar medium (blue). Once engulfed by the bow-shock (cyan) driven by the jet, the external overpressure drives strong radiative shocks (red) into the clouds. The clouds' radiative cooling is dynamically significant. Depending on size, density and location, individual clouds may either shrink and persist (with efficient cooling) or ablate and erode in the turbulent flows surrounding the jet. The clouds are obstacles to the jet's propagation, and can cause bending or loss of collimation (either by direct contact or indirectly via reflected disturbances).

Refer to this site for illustrative animations of computational hydrodynamics simulations of AGN jets interacting with external medium and clouds.

• refereed papers
• conference papers

1. Interactions of Jets with Inhomogeneous Cloudy Media
   C. J. Saxton, G. V. Bicknell, R. S. Sutherland, S. Midgley
   2005, MNRAS, 359, 781.
   • colour version
   • greyscale version

   We present two-dimensional slab-jet simulations of jets in inhomogeneous media consisting of a tenuous hot medium populated with a small filling factor by warm, dense clouds. The simulations are relevant to the structure and dynamics of sources such as Gigahertz Peak Spectrum and Compact Steep Spectrum radio galaxies, High Redshift Radio Galaxies and radio galaxies in cooling flows. The jets are disrupted to a degree depending upon the filling factor of the clouds. With a small filling factor, the jet retains some forward momentum but also forms a halo or bubble around the source. At larger filling factors channels are formed in the cloud distribution through which the jet plasma flows and a hierarchical structure consisting of nested lobes and an outer enclosing bubble results. We suggest that the CSS quasar 3C48 is an example of a low filling factor jet — interstellar medium interaction whilst M87 may be an example of the higher filling factor type of interaction. Jet disruption occurs primarily as a result of Kelvin-Helmholtz instabilities driven by turbulence in the radio cocoon not through direct jet-cloud interactions, although there are some examples of these. In all radio galaxies whose morphology may be the result of jet interactions with an inhomogeneous interstellar medium we expect that the dense clouds will be optically observable as a result of radiative shocks driven by the pressure of the radio cocoon. We also expect that the radio galaxies will possess faint haloes of radio emitting material well beyond the observable jet structure.

2. GPS and CSS sources — theory and modelling
   G. V. Bicknell, C. J. Saxton, R. S. Sutherland,
   2003, Publ. Astron. Society of Australia, 20, 102.

   We review theoretical ideas that seem to be currently important for the physics of GPS and CSS radio sources. These include models for their evolution, the production of emission lines, and the origin of the low frequency turnover. We also describe the initial phases of a program of simulations that is aimed at understanding the radiative interactions between jets, lobes, and dense clouds in the nuclei of these objects.

3. Jets: an environmental impact statement
   G. V. Bicknell, C. J. Saxton, R. S. Sutherland, S. Midgley & S. J. Wagner,
   2003, NewAR, 47, 537.

   We review our recent work on the interaction of jets with the interstellar medium of both classical double and young radio galaxies. We propose explanations for the intriguing radio morphology of Hercules A and Pictor A in terms of the complex structure produced near the terminal shock in the jet. This also has ramifications for the interpretation of the high energy emission from Pictor A. Gigahertz Peak Spectrum (GPS) and Compact Steep Spectrum (CSS) sources are now believed to be young radio galaxies in which the jets are interacting with an inhomogeneous interstellar medium. Two dimensional slab-jet simulations of jets interacting with a clumpy interstellar medium show the way in which radiative shocks are driven into the clouds and how the jet is disrupted. There are interesting morphological comparisons between one of our simulations and the GPS source 4C31.05 and the large scale structure of M87. Another simulation with a lower filling factor, resembles the CSS quasar 3C48.

4. Production of ring-like structure in the cocoon of Hercules A
   C. J. Saxton, G. V. Bicknell & R. S. Sutherland,
   2002, ApJ, 579, 176.
   • http://arXiv.org/abs/astro-ph/0204475

   The radio lobes of the radio galaxy Hercules A contain intriguing ring-like structures concentric with the jet axis. To investigate the occurrence of such features, we have used hydrodynamic simulations of jets with a range of Mach numbers (from M=2 to 50) and densities (down to a ratio of 10^-4 relative to the background) to generate ray-traced images simulating synchrotron emission from the time-dependent shock structures. We compare these images with observations of Hercules A, and consider the physical nature and temporal evolution of the most plausible configurations. We find that the observed ring-like structures are well explained as nearly annular shocks propagating in the backflow surrounding the jet. We infer that the jet is oriented at between 30° and 70° to the line of sight, consistent with radio depolarization observations of Gizani & Leahy. The observational lack of hot-spots at the extremities of the radio lobes, and the possible presence of a buried hot-spot near the base of the western lobe, are explained in terms of the intrinsic brightness fluctuations and dynamics of the terminal shock of an ultra-light, low Mach number jet that surges along its axis due to intermittent pinching and obstruction by turbulent backflow in the cocoon. We conclude from the appearance of both sides of the Hercules A, that both jets are on the verge of becoming fully turbulent.

5. Complex shock structure in the western hot-spot of Pictor A
   C. J. Saxton, R. S. Sutherland, G. V. Bicknell, G. F. Blanchet & S. J. Wagner,
   2002, A&A, 393, 765.
   • http://arXiv.org/abs/astro-ph/0203414

   We have carried out simulations of supersonic light jets in order to model the features observed in optical and radio images of the western hot-spot in the radio galaxy Pictor A. We have considered jets with density ratios η=10^-2 - 10^-4, and Mach numbers ranging between 5 and 50. From each simulation, we have generated ray-traced maps of radio surface brightness at a variety of jet inclinations, in order to study the appearance of time-dependent luminous structures in the vicinity of the western hot-spot. We compare these rendered images with observed features of Pictor A. A remarkable feature of Pictor A observations is a bar-shaped "filament" inclined almost at right angles to the inferred jet direction and extending 24" (10.8 h^-1 kpc) along its longest axis. The constraints of reproducing the appearance of this structure in simulations indicate that the jet of Pictor A lies nearly in the plane of the sky. The results of the simulation are also consistent with other features found in the radio image of Pictor A. This filament arises from the surging behaviour of the jet near the hot-spot; the surging is provoked by alternate compression and decompression of the jet by the turbulent backflow in the cocoon. We also examine the arguments for the jet in Pictor A being at a more acute angle to the line of sight and find that our preferred orientation is just consistent with the limits on the brightness ratio of the X-ray jet and counter-jet. We determine from our simulations, the structure function of hot-spot brightness and also the cumulative distribution of the ratio of intrinsic hot-spot brightnesses. The latter may be used to quantify the use of hot-spot ratios for the estimation of relativistic effects.

6. The Centaurus A middle lobe as a Buoyant Bubble
   C. J. Saxton, R. S. Sutherland & G. V. Bicknell,
   2001, ApJ, 563, 103
   • http://arXiv.org/abs/astro-ph/0107558

   We model the northern middle radio lobe of Centaurus A (NGC 5128) as a buoyant bubble of plasma deposited by an intermittently active jet. The extent of the rise of the bubble and its morphology imply that the ratio of its density to that of the surrounding ISM is less than 10^-2, consistent with our knowledge of extragalactic jets and minimal entrainment into the precursor radio lobe. Using the morphology of the lob to date the beginning of its rise through the atmosphere of Centaurus A, we conclude that the bubble has been rising for approximately 140 Myr. This time scale is consistent with that proposed by Quillen and coworkers for the settling of post-merger gas into the presently observed large scale disk in NGC 5128, suggesting a strong connection between the delayed re-establishment of radio emission and the merger of NGC 5128 with a small gas-rich galaxy. This suggests a connection, for radio galaxies in general, between mergers and the delayed onset of radio emission. In our model, the elongated X-ray emission region discovered by Feigelson and coworkers, part of which coincides with the northern middle lobe, is thermal gas that originates from the ISM below the bubble and that has been uplifted and compressed. The “large-scale jet” appearing in the radio images of Morganti and coworkers may be the result of the same pressure gradients that cause the uplift of the thermal gas, acting on much lighter plasma, or may represent a jet that did not turn off completely when the northern middle lobe started to buoyantly rise. We propose that the adjacent emission line knots (the “outer filaments”) and star-forming regions result from the disturbance, in particular the thermal trunk, caused by the bubble moving through the extended atmosphere of NGC 5128.

1. The dynamical evolution of narrow line regions *
   M.A. Dopita, G.V. Bicknell, R.S. Sutherland & C.J. Saxton, 2003, Proceedings of Conference Winds, Bubbles & Explosions, Patzchuaro, Mexico, 2003, Rev Mex A&A, 15, 323, ed. Jane Arthur.
2. Jet Interations with the ISM of Young Radio Galaxies
   G.V. Bicknell, R.S. Sutherland & C.J. Saxton, 2003, IAUS, Recycling Intergalactic and Interstellar Matter, IAU Symposium no. 217, 14-17 July, 2003, Sydney, Australia.
3. Interactions of supersonic jets with inhomogeneous interstellar media
   G.V. Bicknell, C.J. Saxton & R.S. Sutherland, 2003, IAUJD, Quasar Cores and Jets, 25th meeting of the IAU, Joint Discussion 18, 23-24 July 2003, Sydney, Australia.
4. Interactions between jets and the interstellar medium
   C.J. Saxton, G.V. Bicknell, R.S. Sutherland & S. Midgley, 2003, to appear in Proceedings of Conference Radio Galaxies Past, Present and Future, Leiden, the Netherlands, November 2002.