New clues have been unveiled to understand the functioning of the most violent and energetic objects known: the jets produced by super massive black holes in active galaxies. An international team of astronomers, including Dan Homan, professor of physics and astronomy at Denison University, 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 super massive 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 May 1, 2009, issue of the Astrophysical Journal Letters. Dr. 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 super massive 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 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 these two powerful facilities is generating exciting insights into the acceleration and emission mechanisms of these mysterious black hole phenomena.

The gamma-ray observations have been performed with NASA’s Fermi Gamma-Ray Telescope, which has been operational since the summer of 2008. Fermi 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 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 at 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!”

Denison’s Professor Homan notes: “Our goal now is to use these gamma-ray and radio observations together to understand how these amazing jets are created and accelerated at the hearts of these distant active galaxies.”

The work for astronomers does not stop here: it has been seen that the galaxies that flicker the most in the gamma-ray and radio sky also host the fastest jets. Compared to those jets not yet detected by Fermi, they are much brighter and more active in the radio. In another words, the region of the jet closest to the black hole is undoubtedly the region where the gamma-ray and the radio bursts of light originate. 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.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

NASA’s Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership mission, developed in collaboration with the U.S. Department of Energy and important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

Original publications:

The Relation Between AGN Gamma-Ray Emission and Parsec-Scale Radio Jets, Y. Y. Kovalev et al., The Astrophysical Journal Letters, 696, L17-L21 (2009)

A Connection Between Apparent VLBA Jet Speeds and Initial AGN Detections Made by the Fermi Gamma-ray Observatory, M. Lister et al., The Astrophysical Journal Letters, 696, L22-L26 (2009)

Related Press Releases:

Fermi's Best-Ever Look at the Gamma-Ray Sky, NASA, March 3^rd, 2009

VLBA Movies Reveal New Details of Cosmic Jets, NRAO, January 8^th, 2008

NRAO Teams with NASA Gamma-Ray Satellite, NRAO, June 5^th, 2007

VLBA Monitoring of AGN Jets: The MOJAVE Project

VLBA

Fermi Gamma-ray Space Telescope LAT Group

About Denison:

Denison University, founded in 1831, is an independent, residential liberal arts institution located in Granville, Ohio. A highly selective college enrolling 2,100 full-time undergraduate students from all 50 states and dozens of foreign countries, Denison is a place where innovative faculty and motivated students collaborate in rigorous scholarship, civic engagement and the cultivation of independent thinking.

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