The astronomers found that, rather than flowing smoothly, the jets are full of dense clouds of gas that appear to be colliding and merging as they are blasted outward at nearly the speed of light. The collisions are causing the hot spots to temporarily flare in brightness as shock waves move through them, then recede as the shock waves dissipate.
"We've known for some time that the jets aren't smooth and uniform, but these new observations finally let us see what is actually going on inside these jets," said Nathan Smith of the University of California, Berkeley, lead author of a paper accepted for publication in The Astrophysical Journal.
The discovery suggests that the flickering of the hot spots may offer a new way to analyze these jets, called relativistic jets, which are common in the universe and are thought to contain some of the most energetic particles known.
Jets are one of the most powerful phenomena in the universe. They consist of narrow beams of particles, mostly electrons and protons, that are ejected from black holes and neutron stars at speeds approaching the speed of light. It is still unclear how jets are able to accelerate particles so efficiently.
The observations focused on the central region of the radio galaxy M87, located about 54 million light-years away in the constellation Virgo. At its heart is a supermassive black hole with a mass about 3 billion times that of our Sun.
Smith's team used Hubble's Space Telescope Imaging Spectrograph (STIS) instrument to obtain long-exposure images of the inner jet over many months. They then created movies by combining a series of 22 short-exposure images taken over 20 minutes. The movies reveal that the brightest knots of emission are constantly changing, while some remain fixed.
"For the first time, we can directly observe material colliding inside a jet," Smith said. "Before, the best we could do was study the aftermaths of these events. Now we can see the interactions in real time."
One possibility raised by the results is that the knots in the jet may be part of a natural "focusing" effect. As the jets flow outward from the black hole, they entrain ambient gas from their surroundings, which slows them down. The slower material creates a kind of "focusing collar" around the jet that causes it to pinch, allowing the knots to collide and merge more easily.
The observations show that the clumps of material appear to be "surfing" along shock waves moving down the jet at more than 99.5 percent of the speed of light. The astronomers have estimated the sizes of the clumps to be about 1,000 times the size of our solar system.
"These clumps might be analogous to the traffic jams we experience on the freeway, where cars slow down and bunch up behind a bottleneck," Smith said. "Instead of cars, these clumps of material are being slowed down by the collision with the ambient gas."
The scientists found that the brightness of some hot spots remained constant for the duration of the observations. They believe these spots may be caused by standing shock waves, akin to the bow shock wave in front of a ship moving through the water, rather than the moving shock waves created by the clumps.
The movies also reveal that the speed at which the clumps are moving is decreasing as they travel down the jet. This is the first time that astronomers have been able to directly observe this deceleration inside a jet.
The observations will allow scientists to refine models of jet physics and develop a better understanding of how galaxies are able to convert the gravitational energy of black holes into the enormous energy carried by jets.
