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August 21st, 2000

PG 1700+518: A quasar with a companion starburst galaxy

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PG 1700+518: A quasar with a companion starburst galaxy

Credit: A. Stockton , G. Canalizo & L.M. Close
Institute for Astronomy, University of Hawaii, Honolulu, HI 96822 USA

(This image is published in : A. Stockton, G. Canalizo & L. M. Close, 1998, ApJ, 500, L121)

Quasars or QSO (Quasi-Stellar Objects) are the most luminous known objects. They emit between 10 and 1000 times the energy of our entire Galaxy. In the optical however, they are very disappointing to look at even with the most modern telescopes; they just look like ordinary galactic stars. Hence their name. We now know that these objects are associated with the cores of distant galaxies and that the size of their emitting region is no larger than our own solar system ! The energy source is commonly attributed to the accretion of gas onto a massive central black hole. A powerful source of gas fueling may be strong interactions between the QSO host and companion galaxies. A clear case of such a strong interaction is seen in the QSO PG 1700+518.

PG 1700+518, one of the more luminous low-redshift (z=0.3) quasar, was known from previous ground-based imaging to show a bright, arclike structure extending about 2 arcseconds to the north of its nucleus. However, due of unsufficient angular resolution, the exact nature of this extension was unclear. We show here recent near-infrared images (J and H band) obtained with the University of Hawaii Adaptive Optics System at the CFHT. Unaltered images of the QSO are shown in the top row. They are strongly dominated by the light of the central point-like QSO but some extended emission can be clearly seen at the top. The middle row shows the same images after subtraction of the central unresolved quasar emission and the bottom row shows the result of a deconvolution process. These high angular resolution images clearly demonstrate that the extension is a separate companion galaxy, apparently in the process of merging with the QSO host galaxy. The bright condensation a is probably the nucleus of the companion galaxy while b may be a bright star-forming region. The intense star formation activity of both the quasar and the companion galaxy is likely powered by the interaction itself.

Technical description:

These images were obtained on July 1997 at the CFHT with the University of Hawaii Adaptive Optics system and a 1024x1024 infrared camera in the J (1.2 microns) and H (1.6 microns) photometric bands. Integration times were 2 hrs at J and 1.5 hrs at H. The image is 6" wide.

next week: Abell 383: A Massive Lensing Cluster at z~0.2

editors: François Ménard, Jean-Charles Cuillandre & Catherine Dougados
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