We present a set of high-resolution 3D MHD simulations of steady light, supersonic jets, exploring the influence of jet Mach number and the ambient medium on jet propagation and energy deposition ...over long distances. The results are compared to simple self-similar scaling relations for the morphological evolution of jet-driven structures and to previously published 2D simulations. For this study we simulated the propagation of light jets with internal Mach numbers 3 and 12 to lengths exceeding 100 initial jet radii in both uniform and stratified atmospheres. The propagating jets asymptotically deposit approximately half of their energy flux as thermal energy in the ambient atmosphere, almost independent of jet Mach number or the external density gradient. Nearly one-quarter of the jet total energy flux goes directly into dissipative heating of the ICM, supporting arguments for effective feedback from AGNs to cluster media. The remaining energy resides primarily in the jet and cocoon structures. Despite having different shock distributions and magnetic field features, global trends in energy flow are similar among the different models. As expected the jets advance more rapidly through stratified atmospheres than uniform environments. The asymptotic head velocity in King-type atmospheres shows little or no deceleration. This contrasts with jets in uniform media with heads that are slowed as they propagate. This suggests that the energy deposited by jets of a given length and power depends strongly on the structure of the ambient medium. While our low-Mach jets are more easily disrupted, their cocoons obey evolutionary scaling relations similar to the high-Mach jets.
We present the first three-dimensional MHD radio galaxy simulations that explicitly model transport of relativistic electrons, including diffusive acceleration at shocks as well as radiative and ...adiabatic cooling in smooth flows. We discuss three simulations of light Mach 8 jets, designed to explore the effects of shock acceleration and radiative aging on the nonthermal particle populations that give rise to synchrotron and inverse-Compton radiations. We also conduct detailed synthetic radio observations of our simulated objects. We have gained several key insights from this approach: 1. The jet head in these multidimensional simulations is extremely complex. The classical jet termination shock is often absent, but motions of the jet terminus spin a ``shock-web complex'' within the backflowing jet material of the head. 2. Understanding the spectral distribution of energetic electrons in these simulations relies partly upon understanding the shock-web complex, for it can give rise to distributions that confound interpretation in terms of the standard model for radiative aging of radio galaxies. 3. The magnetic field outside of the jet itself becomes very intermittent and filamentary in these simulations, yet adiabatic expansion causes most of the cocoon volume to be occupied by field strengths considerably diminished below the nominal jet value. Thus population aging rates vary considerably from point to point.
We report on an effort to study the connections between dynamics in simulated radio galaxy plasma flows and the properties of nonthermal electron populations carried in those flows. To do this we ...have introduced a new numerical scheme for electron transport that allows a much more detailed look at this problem than has been possible before. Especially when the dynamics is fully three dimensional the flows are generally chaotic in the cocoon, and the jet itself can flail about violently. The bending jet can pinch itself off and redirect itself to enhance its penetration of the ambient medium. These behaviors often eliminate the presence of a strong jet termination shock, which is assumed present in all modern cartoon models of the RG phenomenon. Instead a much more complex ``shock web'' forms near the end of the jet that leads to a far less predictable pattern of particle acceleration. Similarly, the magnetic fields in these flows are highly filamented, as well as spatially and temporally intermittent. This leads to a very localized and complex pattern of synchrotron aging for relativistic electron populations, which makes it difficult to use properties of the electron spectrum to infer the local rate of aging.
We perform a series of so-called ``synthetic observations'' on a set of 3D MHD jet simulations which explicitly include energy-dependent transport of relativistic electrons, as described in the ...companion paper by Jones, Tregillis, & Ryu. Analyzing them in light of the complex source dynamics and energetic particle distributions described in that paper, we find that the standard model for radiative aging in radio galaxies does not always adequately reflect the detailed source structure.
We report an extensive set of two-dimensional MHD simulations exploring the
role and evolution of magnetic fields in the dynamics of supersonic plasma
clumps. We examine the influence of both ambient ...field strength and orientation
on the problem. Of those two characteristics, field orientation is far more
important in the cases we have considered with $\beta_0 = p_g/p_b \ge 1$. That
is due to the geometry-sensitivity of field stretching/amplification from
large-scale shearing motions around the bullet. When the ambient magnetic field
is transverse to the bullet motion, even a very modest field, well below
equipartition strength, can be amplified by field line stretching around the
bullet within a couple of bullet crushing times so that Maxwell stresses become
comparable to the ram pressure associated with the bullet motion. The
possibility is discussed that those situations might lead to large, induced
electric potentials capable of accelerating charged particles. When the ambient
field is aligned to the bullet motion, on the other hand, reconnection- prone
topologies develop that shorten the stretched field and release much of the
excess energy it contains. In this geometry, the Maxwell stresses on the bullet
never approach the ram pressure level. In both cases, however, the presence of
a field with even moderate initial strength acts to help the flow realign
itself around the bullet into a smoother, more laminar form. That reduces
bullet fragmentation tendencies caused by destructive instabilities. Eddies
seem less effective at field amplification than flows around the bullet,
because fields within eddies tend to be expelled to the eddy perimeters.
Similar effects cause the magnetic field within the bullet itself to be reduced
below its initial value over time.
We have applied an effective numerical scheme for cosmic-ray transport to 3D MHD simulations of jet flow in radio galaxies (see the companion paper by Jones et al. 1999). The marriage of relativistic ...particle and 3D magnetic field information allows us to construct a rich set of ``synthetic observations'' of our simulated objects. The information is sufficient to calculate the ``true'' synchrotron emissivity at a given frequency using explicit information about the relativistic electrons. This enables us to produce synchrotron surface-brightness maps, including polarization. Inverse-Compton X-ray surface-brightness maps may also be produced. First results intended to explore the connection between jet dynamics and electron transport in radio lobes are discussed. We infer lobe magnetic field values by comparison of synthetically observed X-ray and synchrotron fluxes, and find these ``inverse-Compton'' fields to be quite consistent with the actual RMS field averaged over the lobe. The simplest minimum energy calculation from the synthetic observations also seems to agree with the actual simulated source properties.
We have developed an economical, effective numerical scheme for cosmic-ray transport suitable for treatment of electrons up to a few hundreds of GeV in multidimensional simulations of radio galaxies. ...The method follows the electron population in sufficient detail to allow computation of synthetic radio and X-ray observations of the simulated sources, including spectral properties (see the companion paper by Tregillis et al. 1999). The cosmic-ray particle simulations can follow the effects of shock acceleration, second-order Fermi acceleration as well as radiative and adiabatic energy losses. We have applied this scheme to 2-D and 3-D MHD simulations of jet-driven flows and have begun to explore links between dynamics and the properties of high energy electron populations in radio lobes. The key initial discovery is the great importance to the high energy particle population of the very unsteady and inhomogeneous flows, especially near the end of the jet. Because of this, in particular, our simulations show that a large fraction of the particle population flowing from the jet into the cocoon never passes through strong shocks. The shock strengths encountered are not simply predicted by 1-D models, and are quite varied. Consequently, the emergent electron spectra are highly heterogeneous. Rates of synchrotron aging in "hot-spots" seem similarly to be very uneven, enhancing complexity in the spectral properties of electrons as they emerge into the lobes and making more difficult the task of comparing dynamical and radiative ages.
We report an extensive set of two-dimensional MHD simulations exploring the role and evolution of magnetic fields in the dynamics of supersonic plasma clumps. We examine the influence of both ambient ...field strength and orientation on the problem. Of those two characteristics, field orientation is far more important in the cases we have considered with \(\beta_0 = p_g/p_b \ge 1\). That is due to the geometry-sensitivity of field stretching/amplification from large-scale shearing motions around the bullet. When the ambient magnetic field is transverse to the bullet motion, even a very modest field, well below equipartition strength, can be amplified by field line stretching around the bullet within a couple of bullet crushing times so that Maxwell stresses become comparable to the ram pressure associated with the bullet motion. The possibility is discussed that those situations might lead to large, induced electric potentials capable of accelerating charged particles. When the ambient field is aligned to the bullet motion, on the other hand, reconnection- prone topologies develop that shorten the stretched field and release much of the excess energy it contains. In this geometry, the Maxwell stresses on the bullet never approach the ram pressure level. In both cases, however, the presence of a field with even moderate initial strength acts to help the flow realign itself around the bullet into a smoother, more laminar form. That reduces bullet fragmentation tendencies caused by destructive instabilities. Eddies seem less effective at field amplification than flows around the bullet, because fields within eddies tend to be expelled to the eddy perimeters. Similar effects cause the magnetic field within the bullet itself to be reduced below its initial value over time.
We apply magnetohydrodynamic (MHD) modeling to the radio galaxy Hercules A for investigating the jet-driven shock, jet/lobe transition, wiggling, and magnetic field distribution associated with this ...source. The model consists of magnetic tower jets in a galaxy cluster environment, which has been discussed in a series of our papers. The profile of underlying ambient gas plays an important role in jet-lobe morphology. The balance between the magnetic pressure generated by axial current and the ambient gas pressure can determine the lobe radius. The jet body is confined jointly by the external pressure and gravity inside the cluster core radius R_c, while outside R_c it expands radially to form fat lobes in a steeply decreasing ambient thermal pressure gradient. The current-carrying jets are responsible for generating a strong, tightly wound helical magnetic field. This magnetic configuration will be unstable against the current-driven kink mode and it visibly grows beyond R_c where a separation between the jet forward and return currents occurs. The reversed pinch profile of global magnetic field associated with the jet and lobes produces projected B-vector distributions aligned with the jet flow and the lobe edge. AGN-driven shock powered by the expanding magnetic tower jet surrounds the jet/lobe structure and heats the ambient ICM. The lobes expand subsonically; no obvious hot spots are produced at the heads of lobes. Several key features in our MHD modeling may be qualitatively supported by the observations of Hercules A.
