Daniel C. Homan (NRAO and Denison University)
John F. C. Wardle (Brandeis University)
VLBA Project Code: BH046
Observation Epoch: December 1997
This is an image of the radio jet, 3C 84, in the galaxy NGC 1275. The right-hand image shows the total radio emission of the jet as resolved by the VLBA at a frequency of 15 GHz. The left-hand image shows a enlarged version of the base of the jet where find emission that is strongly circularly polarized. The levels of circular polarization are extremely high, up to 3% polarized at point "A" in the 15 GHz image, and we see a change in sign of the circular polarization from positive to negative (dashed contours) as we look toward the base of the jet. Previous high resolution studies with the VLBA have found that circular polarization of radio jets is very weak on average, with the majority of sources having less than 0.3% polarization, one tenth of the level we measure here, and no previous observations have ever detected both signs of circular polarization in the same jet simulateously.
Radio jets in galaxies are thought to originate near a supermassive black hole (millions or billions of times the mass of our Sun) at the center of the galaxy. The black hole is rapidly spinning with a hot accretion disk of material that is in the process of falling inward. Radio jets originate somewhere above (and below) the accretion disk, and carry magentic fields and charged particles away from the black hole and accretion disk. The jets can be highly relativistic with outflow speeds very near the speed of light. The radio emission we detect with the VLBA is generated by the charged particles spiraling around in the magnetic fields.
The motion of the particles in the magnetic fields imprints a signature upon the radio emission we observe. This signature is in the form of polarization, both linear and circular polarization. Circular polarization is generated in only very small amounts, either directly by the spiraling motion of the radiating particles or by a propagation effect, called Faraday conversion, which turns linear polarization into circular.
The very high levels of circular polarization we've detected here in 3C84 imply an extremely efficient production mechanism. For direct synchrotron emission to be the source of the polarization we see, we calculate that the magnetic field in the jet must be at least 1 Gauss in strength, and that field must be highly ordered and essentially uni-directional. Unfortunately, polarization produced by direct synchrotron emission cannot adequately explain the spectrum of the negative polarization we observe at points "B" and "C".
We conclude that the polarization is most likely generated by the Faraday conversion propagation effect. With circular polarization detected at only two frequencies, this effect is difficult to model uniquely; however, in general, strong conversion implies the jet may contain a significant quantity of low energy, relativistic particles which are responsible for the conversion from linear to circular polarization. Future studies at several frequencies will help us better constrain the polarization spectrum and may allow us to fully model the Faraday conversion effect and place strong constraints on the low-energy particles in the jet.
Accepted in ApJ Letters: astro-ph/0401319, full title is "High Levels of Circularly Polarized Emission from the Radio Jet in NGC 1275 (3C 84)"