Hubble Space Telescope Resolves Braided Galactic Jet
15 January 1991NASA's Hubble Space Telescope has provided a detailed view of a ten thousand light-year long jet of plasma which has been ejected from the core of a galaxy 270 million light-years away. Observations made with the European Space Agency's Faint Object Camera (FOC) reveal that the jet has an unusual braided structure, like a twisted pair of wires. "This is the first time that such a structure has been seen in an optical jet," says F. Duccio Macchetto, ESA's Principal Investigator on the FOC and Head of the Science Programs Division at the Space Telescope Science Institute.
The results of this observation were presented at the meeting of the American Astronomical Society in Philadelphia, Pennsylvania on 15 January 1991.
The FOC image provides intriguing new details for understanding how the core of an active galaxy generates such a narrow beam of energy and then propagates the jet across millions of light years, at velocities approaching the speed of light.
The jet appears as a bright "finger" extending in the northeast direction from radio galaxy 3C 66B. The FOC image has an angular resolution of 0.1 arc seconds, which is twelve times better than previous ground based optical images, and even three times better than high resolution radio maps obtained with the Very Large Array radio telescope at Socorro, New Mexico. In addition to the unique double stranded structure, the FOC image reveals filaments, bright knots, kinks and other complex features never before seen in an optical jet. Many of these features overlay the radio structure of the jet.
The jet was observed on 28 August 1990 as part of an early program to assess the optical performance of the HST. "The HST is a uniquely important instrument for studying synchrotron jets," says Macchetto. Image reconstruction techniques applied to the data to bring out additional detail.
In addition to HST's extraordinary resolution its ultraviolet sensitivity is ideal for studying optical jets because they are relatively bright in UV compared to their host galaxies. Once the image of the host galaxy is subtracted through computer processing, details in the jet can be traced all the way to the galactic core.
The bluish, highly polarized light of the jet in galaxy 3C 66B is produced by electrons which are spiraling along magnetic fields at velocities approaching the speed of light. The light and radio emissions produced by the electrons are synchrotron radiation - so named because similar radiation is observed in particle accelerator machines.
But what is the "machine" which is the powerhouse behind the jet in 3C 66B? The favored mechanism is a super-massive black hole which may lie at the core of the galaxy. Stars, dust and gas swirl deep into the hole's intense gravitational field along a broad, flattened accretion disk. The hot plasma in the spinning disk creates powerful electric currents which in turn generate twisted magnetic fields which align to the black hole's spin axis.
The black hole's spin axis is also an escape route for the high speed electrons. As the electrons spiral outward along magnetic field lines, they lose energy in proportion to their frequency and the strengths of the magnetic field. In a timespan of only a few hundred years the electrons responsible for the optical emission lose much of their energy. However, the electrons which produce radio emission can survive in the same magnetic field for tens of thousands of years. Hence most galactic jets which extend for thousands of light-years are detected in radio wavelengths. In only a few cases have optical counterparts been observed.
Nevertheless, the long optical jet in 3C 66B presents a mystery for astronomers. How do the electrons remain energetic enough to radiate visible light throughout their 10 000 year journey along the jet?
One possibility is that the electrons are boosted back to higher energy levels as they move along the jet, perhaps by instabilities at the edge of the plasma flow. Another possibility, which the jet's braided appearance suggests, is that the electrons speed along a channel which has a much lower magnetic field strength and hence lower energy loss.
Two sharp bends and kinks in the strand (3000 and 8000 light-years out from the nucleus) are also hard to explain. They may indicate that the galaxy's "central engine" doesn't release energy at a steady rate but rather "hiccoughs" or fluctuates in output. The kinks may also be produced by a complex magnetic field structure along the jet, or collisions with dense regions in interstellar gas.
This is only the second galaxy with an optical jet which has been observed with HST (the previous optical jet studied is in the galaxy PKS 0521-36). Both observations show a close match between the optical and radio features in the jets in each respective galaxy. However there are significant differences between the structure of the jets of these two galaxies. This suggests that different mechanisms are at work for transporting material from the galactic core.
Further observation with HST will provide fundamental new information about the nature of galactic jets, in particular how energy is transported along the jet and the role of magnetic fields in channeling material from the core of a galaxy to intergalactic space.