Radio jets
Radio jets, are collimated outflows of fully ionized plasma that originate very close to the supermassive black hole and can reach huge distances, out to Mpc scales. These outflows are threaded by strong magnetic fields that are thought to be instrumental in launching, collimating, and accelerating them. Charged particles gyrating around the magnetic field lines at relativistic speeds emit synchrotron radiation that usually is observed at radio wavelengths, hence the name "radio" jets. The phenomenology and intrinsic properties of these radio jets are intimately connected to the conditions at or close to the event horizons of the supermassive black holes that launch them. Radio jets are usually studied using radio interferometric techniques, which allow us to spatially resolve their structure and kinematics at very high resolutions.
Kinematics of radio jets
One of the first and most surprising findings concerning radio jets was the apparent superluminal motions observed within them. Very long baseline interferometry (VLBI) monitoring of luminous radio-loud AGN (often called blazars) revealed that jets can be described as a conglomeration of, often distinct, blobs of emitting plasma that expand with time, moving away from the core of the radio jet. The combination of relativistic, but obviously sub-luminal, speeds and small viewing angles leads to the apparent (measured) velocities above the speed of light.
We used a large statistically complete sample of luminous radio-loud and flat-spectrum quasars and BL Lac objects to understand the kinematics and evolution of these radio jets. We found that quasars and BL Lacs show distinct kinematic properties, with the latter showing on average smaller outward velocities, but considerably wider jets with strong motions perpendicular to the jet axis. By using simple geometric models we argued that an intrinsic helical jet structure combined with on average smaller viewing angles than flat-spectrum quasars can explain the observed properties.
Related papers: Karouzos et al. 2012a, Karouzos et al. 2012b