The Mightiest Monsters of the Universe
Brightest ultrafast jets in active galaxies found to be powerful gamma-ray beacons
April 22, 2009
New observations reveal further details on the functioning of the most violent and energetic objects known: the jets produced by supermassive black holes in active galaxies. An international team of astronomers, including scientists at the Max Planck Institute for Radio Astronomy in Bonn, has combined data from some of the world's most advanced astronomical instruments, including the U.S. Very Long Baseline Array (VLBA) and the Fermi Gamma-ray Space Telescope, to study how supermassive black holes manage to produce copious quantities of the most energetic form of light: gamma-rays.
The international team has combined observations of the bright gamma-ray sky by NASA's orbiting Fermi Gamma-ray Space Telescope with those from the ground-based Very Long Baseline Array radio telescope to observe the material expelled with enormous speeds away from the black holes in the heart of very remote galaxies. These ejections take the form of narrow jets in radio telescope images, and appear to be producing the gamma-rays detected by Fermi. Their findings are being reported in two publications in the 2009 May 1 issue of the Astrophysical Journal Letters. Yuri Kovalev, Humboldt Fellow and scientist at the Max Planck Institute for Radio Astronomy, is enthusiastic: "These objects are amazing: finally we know for sure that the fastest, most compact, and brightest jets that we see with radio telescopes are the ones which are able to kick the light up to the highest energies".
Distant galaxies host deep in their nucleus spinning supermassive black holes, which are billions of times heavier than our Sun but are confined to a region no larger than our solar system. These rapidly rotating black holes attract stars, gas and dust, creating huge magnetic fields. The magnetic forces can trap some of the infalling gas and focus it into narrow jets that flow away from the core of the galaxy at velocities approaching the speed of light. The jets, when they happen to be pointed directly at Earth, display one of the most spectacular phenomena in the Universe, since a quirk of Einstein's theory of special relativity can make their speeds appear faster than the speed of light. Theoreticians and observers alike have been asking themselves for decades about the nature and composition of these energetic radio-emitting jets, and if they also radiate in other parts of the electromagnetic spectrum. And this is indeed the case! Some hints were provided by the EGRET instrument on the Compton Gamma Ray Observatory telescope in the late 1990s and more recent discoveries of X-ray emission made by the Chandra Observatory. But now, the prediction of the link between radio and gamma-ray mission has been confirmed by the several of world's most advanced astronomical instruments, such as the VLBA and Fermi. "The marriage of a prime instrument such as the VLBA with a new gamma-ray telescope is generating exciting insights into the acceleration and emission mechanisms of these mysterious black hole phenomena", says Anton Zensus, director at the Max Planck Institute for Radio Astronomy and Fermi Affiliated Scientist. "The gamma-ray bright sources are now proven to be brighter, more compact and faster at light year scales than the gamma-ray quiet sources."
The gamma-ray observations have been performed with NASA's Fermi Gamma-ray Space Telescope, which has been operational since the summer of 2008. The Fermi Large Area Telescope records an image of the whole sky every few hours to explore the most extreme environments in the universe, including pulsars and gamma-ray bursts, as well as black holes in galactic nuclei. Gamma-ray observations alone are not enough to discern the exact location of the radiation, however. The VLBA serves as a magnifying glass under which the details of the most energetic processes in the distant universe appear as if they were happening in our own Galactic backyard. Interestingly, many objects found by Fermi to be extreme in gamma-rays are emitting strong bursts of radio emission at the same time.
The Very Long Baseline Array of the U.S. National Science Foundation is a continent-wide system of ten radio telescope antennas, ranging from Hawaii in the west to the U.S. Virgin Islands in the east. Dedicated in 1993, the VLBA is operated by the NRAO and is designed to monitor the brightest objects in the Universe at the highest available resolution in astronomy. Matthew Lister, Physics Professor in Purdue University and guest investigator on the Fermi project, says: "For more than a decade, we have collected images of the brightest galaxies in the radio sky to study the changing structures of their jets. We have waited a long time to compare our measurements with the findings in the gamma-ray sky, and now we finally have it!"
The work for astronomers does not stop here: the team has concluded that the region of the jet closest to the black hole is undoubtedly the place where the gamma-ray and the radio bursts of light originate in about the same time. However, some parts of the puzzle have yet to be resolved: some bright gamma-ray sources in the sky appear to have no radio or optical counterpart - their nature is still completely unknown. With this new joint eye on the universe provided by Fermi and the VLBA, astronomers look forward to investigating these intriguing mysteries, and anticipate many new exciting discoveries to come.