69 research outputs found
Simulating Electron Transport and Synchrotron Emission in Radio Galaxies: Shock Acceleration and Synchrotron Aging in Three-Dimensional Flows
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.Comment: 44 pages, 6 figures; to be published in the Astrophysical Journal
(August 2001); higher-quality figures can be found at
http://www.msi.umn.edu/Projects/twj/radjet/radjet.htm
The magnetohydrodynamics of supersonic gas clouds: MHD cosmic bullets and wind-swept clumps
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) greater than or equal to 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 held 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. For oblique fields, we expect that the transverse field cases modeled here are more generally relevant. What counts is whether field lines threading the face of the bullet are swept around it in a fashion that folds them (leading to reconnection) or that keeps them unidirectional one each side of the bullet. In the second instance, behaviors should resemble those of the transverse field cases. We estimate that this quasi-transverse behavior is appropriate whenever the angle, theta, between the motion and the field satisfies tan theta greater than or similar to 1/M, where M is the bullet Mach number. From these simulations, we find support in either held geometry for the conclusions reached in previous studies that nonthermal radio emission associated with supersonic clumps is likely to be controlled largely by the generation of strong magnetic fields around the perimeters of the clumps, rather than local particle acceleration and field compression within the bow shock. In addition, since the magnetic pressure on the nose of the bullet likely becomes comparable to the ram pressure and hence the total pressure behind the bow shock, the gas pressure there could be substantially lower than that in a gasdynamical bullet. That means, as well, that the temperature in the region on the nose of the bullet would be lower than that predicted in the gasdynamical case. That detail could alter expectations of the thermal emission, including X-rays and UV-IR linesopen554
Three-Dimensional Simulations of Bi-Directed Magnetohydrodynamic Jets Interacting with Cluster Environments
We report on a series of three-dimensional magnetohydrodynamic simulations of
active galactic nucleus (AGN) jet propagation in realistic models of magnetized
galaxy clusters. We are primarily interested in the details of energy transfer
between jets and the intracluster medium (ICM) to help clarify what role such
flows could have in the reheating of cluster cores. Our simulated jets feature
a range of intermittency behaviors, including intermittent jets that
periodically switch on and off and one model jet that shuts down completely,
naturally creating a relic plume. The ICM into which these jets propagate
incorporates tangled magnetic field geometries and density substructure
designed to mimic some likely features of real galaxy clusters. We find that
our jets are characteristically at least 60% efficient at transferring thermal
energy to the ICM. Irreversible heat energy is not uniformly distributed,
however, instead residing preferentially in regions very near the jet/cocoon
boundaries. While intermittency affects the details of how, when, and where
this energy is deposited, all of our models generically fail to heat the
cluster cores uniformly. Both the detailed density structure and nominally weak
magnetic fields in the ICM play interesting roles in perturbing the flows,
particularly when the jets are non-steady. Still, this perturbation is never
sufficient to isotropize the jet energy deposition, suggesting that some other
ingredient is required for AGN jets to successfully reheat cluster cores.Comment: 19 pages, 18 figures, Accepted for publication in the Astrophysical
Journa
Synthetic Observations of Simulated AGN Jets: X-ray Cavities
Observations of X-ray cavities formed by powerful jets from AGN in galaxy
cluster cores are widely used to estimate the energy output of the AGN. Using
methods commonly applied to observations of clusters, we conduct synthetic
X-ray observations of 3D MHD simulated jet-ICM interactions to test the
reliability of measuring X-ray cavity power. These measurements are derived
from empirical estimates of the enthalpy content of the cavities and their
implicit ages. We explore how such physical factors as jet intermittency and
observational conditions such as orientation of the jets with respect to the
line of sight impact the reliability of observational measurements of cavity
enthalpy and age. An estimate of the errors in these quantities can be made by
directly comparing "observationally" derived values with "actual" values from
the simulations. In our tests, cavity enthalpy derived from observations was
typically within a factor of two of the simulation values. Cavity age and,
therefore, cavity power are sensitive to the accuracy of the estimated
inclination angle of the jets. Cavity age and power estimates within a factor
of two of the actual values are possible given an accurate inclination angle.Comment: 34 pages, 14 figures, accepted for publication in Ap
MHD Simulations of AGN Jets in a Dynamic Galaxy Cluster Medium
We present a pair of 3-d magnetohydrodynamical simulations of intermittent
jets from a central active galactic nucleus (AGN) in a galaxy cluster extracted
from a high resolution cosmological simulation. The selected cluster was chosen
as an apparently relatively relaxed system, not having undergone a major merger
in almost 7 Gyr. Despite this characterization and history, the intra-cluster
medium (ICM) contains quite active "weather". We explore the effects of this
ICM weather on the morphological evolution of the AGN jets and lobes. The
orientation of the jets is different in the two simulations so that they probe
different aspects of the ICM structure and dynamics. We find that even for this
cluster that can be characterized as relaxed by an observational standard, the
large-scale, bulk ICM motions can significantly distort the jets and lobes.
Synthetic X-ray observations of the simulations show that the jets produce
complex cavity systems, while synthetic radio observations reveal bending of
the jets and lobes similar to wide-angle tail (WAT) radio sources. The jets are
cycled on and off with a 26 Myr period using a 50% duty cycle. This leads to
morphological features similar to those in "double-double" radio galaxies.
While the jet and ICM magnetic fields are generally too weak in the simulations
to play a major role in the dynamics, Maxwell stresses can still become locally
significant.Comment: 20 pages, 14 figures, accepted for publication in the Astrophysical
Journa
Simulations of Nonthermal Electron Transport in Multidimensional Flows: Synthetic Observations of Radio Galaxies
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.Comment: 7 pages, 1 figure; to appear in Life Cycles of Radio Galaxies, ed. J.
Biretta et al., New Astronomy Review
Simulations of Nonthermal Electron Transport in Multidimensional Flows: Application to Radio Galaxies
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.Comment: 7 pages, 1 figure; to appear in Life Cycles of Radio Galaxies, ed. J.
Biretta et al., New Astronomy Review
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