April 23, 2009

Active Galaxies Ready For Close-Up


"Now we know for sure that the fastest, most compact, and brightest jets we see with radio telescopes are the ones that are able to kick light up to the highest energies," said Yuri Kovalev, a team member at the Max Planck Institute for Radio Astronomy in Bonn, Germany.



The new findings are an outcome of the MOJAVE program, a long-term study of the jets from active galaxies using the VLBA. "We see the innermost few hundred light-years of these jets for even the most distant active galaxies seen by Fermi," Kovalev noted.

For decades, astronomers have wondered about the nature of these radio-emitting jets. Hints that they also emit radiation at higher energies came from NASA's Compton Gamma-Ray Observatory, which operated throughout the 1990s, and, more recently, from observations by NASA's Chandra X-Ray Observatory.

Fermi's Large Area Telescope (LAT) scans the entire sky every three hours. These quick snapshots of the gamma-ray sky allow astronomers to better monitor sudden flares from active galaxies. The astronomers combined VLBA data of active galaxies with Fermi observations. Active galaxies detected in the LAT's first few months of operations generally possess brighter and more compact radio jets than galaxies the LAT did not see. Moreover, an active galaxy's radio jets tend to be brighter in the months following any gamma-ray flares observed by the LAT.

Kovalev and his colleagues also see a correlation between active galaxies with the brightest gamma-ray emission and those with the fastest jets. Because we see these jets nearly end on, and because the particles within the jets move close to the speed of light, the VLBA can study a phenomenon called "Doppler boosting." This makes radio-emitting blobs look brighter and appear to move much faster that the speed of light.


All this points to the team's conclusion that the portion of an active galaxy's radio jet closest to the galaxy's core is also the source of the gamma-rays Fermi detects. The team's findings appear in two papers to be published in the May 1 issue of The Astrophysical Journal Letters.


Francis Reddy, NASA's Goddard Space Flight Center


Image 1: The radio jets of several active galaxies mapped by the Very Long Baseline Array (VLBA) are inset into the Fermi Gamma-ray Space Telescope's map of the gamma-ray sky. Credit: NASA/DOE/Fermi LAT Collaboration and NRAO/AUI/MOJAVE Team/M. Kadler [ Labeled version ]

Image 2: The blazar 3C 454.3 lies 7 billion light-years away. Nevertheless, VLBA observations detect details less than 100 light-years across in the galaxy's innermost radio jet. Credit: NRAO/AUI/MOJAVE Team/Y. Kovalev

Image 3: The VLBA observes radio-emitting blobs just a few light-years across in jets from the active galaxy NGC 1275, which lies 225 million light-years away. Credit: NRAO/AUI/MOJAVE Team/Y. Kovalev


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