3D Structures on Relativistic Jets

The properties of wave-like helically twisted normal mode structures on steady relativistic jets are summarized. Wave speeds are a function of the wavelength and less than the jet speed. However, normal mode interference can lead to both stationary and superluminal phase effects. A maximum pressure fluctuation criterion suggested by numerical simulations of axisymmetric relativistic jets is used to find the maximum asymmetric jet distortions and velocity fluctuations. Cyclic transverse velocity fluctuation can lead to variation in the flow direction on the order of the relativistic beaming angle. Resulting variation in the Doppler boost factor can lead to significant brightness asymmetries as helical structures twist around the jet beam. Growth of these structures is reduced as the jet density, Lorentz factor or Mach number are increased. Maximum jet distortion is reduced as the Lorentz factor increases and this suggests a reduction in mass entrainment or other non-linear disruptive processes that influence the morphological development of radio sources.Comment: to appear in Life Cycles of Radio Galaxies, ed. J. Biretta et al., New Astronomy Reviews; 8 pages, including 3 figure

Anatomy of helical relativistic jets: The case of S5 0836+710

Helical structures are common in extragalactic jets. They are usually attributed in the literature to periodical phenomena in the source (e.g., precession). In this work, we use VLBI data of the radio-jet in the quasar S5 0836+710 and hypothesize that the ridge-line of helical jets like this corresponds to a pressure maximum in the jet and assume that the helically twisted pressure maximum is the result of a helical wave pattern. For our study, we use observations of the jet in S5 0836+710 at different frequencies and epochs. The results show that the structures observed are physical and not generated artificially by the observing arrays. Our hypothesis that the observed intensity ridge-line can correspond to a helically twisted pressure maximum is confirmed by our observational tests. This interpretation allows us to explain jet misalignment between parsec and kiloparsec scales when the viewing angle is small, and also brings us to the conclusion that high-frequency observations may show only a small region of the jet flow concentrated around the maximum pressure ridge-line observed at low frequencies. Our work provides a potential explanation for the apparent transversal superluminal speeds observed in several extragalactic jets by means of transversal shift of an apparent core position with time.Comment: Accepted for publication in the Astrophysical Journa

Dynamics and Structure of Three-Dimensional Poloidally Magnetized Supermagnetosonic Jets

A set of 3D MHD simulations of magnetized jets has been performed. The jets contain an equipartition primarily poloidal magnetic field and the effect of jet density on jet dynamics and structure is evaluated. The jet is precessed at the origin to excite Kelvin-Helmholtz unstable helical modes. We extensively compare the structure in these simulations with linear stability theory. The jet that is dense with respect to the external medium develops a high speed core surrounded by a less dense sheath consisting of slower moving jet fluid. These simulations suggest that extended extragalactic jets propagate to such large distances because they are surrounded by a lobe or cocoon whose density is less than the jet density. (Abridged abstract.)Comment: 30 pages, AASTeX, to appear in ApJ, much better versions of Figures 2-5 are available at http://crux.astr.ua.edu/~rosen/hcr/hcr.htm

The Effect of Expansion on Mass Entrainment and Stability of Super-Alfv\'enic Jets

We extend investigations of mass entrainment by jets, which previously have focused on cylindrical supermagnetosonic jets and expanding trans-Alfv\'enic jets, to a set of expanding supermagnetosonic jets. We precess these jets at the origin to excite the helical mode of the Kelvin-Helmholtz (or KH) instability, in order to compare the results with predictions from linear stability analysis. We analyze this simulation set for the spatial development of magnetized mass, which we interpret as jet plus entrained, initially unmagnetized external mass. As with the previous simulation sets, we find that the growth of magnetized mass is associated with the growth of the KH instability through linear, nonlinear, and saturated stages and with the expansion of magnetized material in simulated observations of the jet. From comparison of measured wavelengths and wave speeds with the predictions from linear stability analysis, we see evidence that the KH instability is the primary cause for mass entrainment in these simulations, and that the expansion reduces the rate of mass entrainment. This reduced rate can be observed as a somewhat greater distance between the two transition points separating the three stages of expansion.Comment: 18 pages, 6 figures, AASTeX, to appear in Nov 1 issue of ApJ (vol 543), postscript versions of Figures 3 and 5 are available at http://crux.astr.ua.edu/~rosen/supcon/rh.htm

The role of Kelvin-Helmholtz instability in the internal structure of relativistic outflows. The case of the jet in 3C 273

Relativistic outflows represent one of the best-suited tools to probe the physics of AGN. Numerical modelling of internal structure of the relativistic outflows on parsec scales provides important clues about the conditions and dynamics of the material in the immediate vicinity of the central black holes in AGN. We investigate possible causes of the structural patterns and regularities observed in the parsec-scale jet of the well-known quasar 3C 273. We present here the results from a 3D relativistic hydrodynamics numerical simulation based on the parameters given for the jet by Lobanov & Zensus (2001), and one in which the effects of jet precession and the injection of discrete components have been taken into account. We compare the model with the structures observed in 3C 273 using very long baseline interferometry and constrain the basic properties of the flow. We find growing perturbation modes in the simulation with similar wavelengths to those observed, but with a different set of wave speeds and mode identification. If the observed longest helical structure is produced by the precession of the flow, longer precession periods should be expected. Our results show that some of the observed structures could be explained by growing Kelvin-Helmholtz instabilities in a slow moving region of the jet. However, we point towards possible errors in the mode identification that show the need of more complete linear analysis in order to interpret the observations. We conclude that, with the given viewing angle, superluminal components and jet precession cannot explain the observed structures.Comment: Accepted for publication in Astronomy & Astrophysics. 14 pages. Higher resolution plots available on request to [email protected] and at http://www.mpifr-bonn.mpg.de/staff/mperuch

The fearful face and beyond: fMRI studies of the human amygdala

The amygdala has been labeled as a detector of threat , evidenced by its heightened response to fearful faces in human neuroimaging studies. A critical element of the fearful face is an increase in eye white area, hypothesized to be crucial for the rapid detection of fear in another\u27s face. Yet other facial expressions can also increase eye white area in a manner that is similar (a lateral shift in gaze) or identical (surprise) to fear. It is unknown if the amygdala can differentiate between these types of increases in eye white area and those that are specifically associated with fear when using only the eye region of the face. Furthermore, whether the fearful mouth can elicit an amygdala response when shown in isolation is unknown. Using functional magnetic resonance imaging, we found that the amygdala responded robustly to fearful eyes as well as eye stimuli that were ambiguous in nature. The fearful mouth, on the other hand, was unable to generate a significant response from the amygdala, however the happy condition elicited a slight response from the right amygdala, most likely due to the visual salience of the smile. We also observed a functional laterality between the two amygdalae in that the left amygdala responded only to fearful eyes while the right amygdala activated to any change in the eyes; the same laterality was also evident when eye stimuli were ambiguous in nature indicating that the left amygdala is more tuned to detect fear in the eyes while the right amygdala acts as a general detector of eye changes. This lends more evidence to the existence of parallel mechanisms for processing visual threat. Together, our results indicate that while the amygdala is primarily a detector of fearful faces, it has evolved to respond to other facial expressions that are also behaviorally relevant or potentially threatening to the viewer