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Quasars: Cosmic Powerhouses Dwelling In Humble Galactic Homes

Mauna Kea, Hawaii – May 26, 2004:   Quasars are the most brilliant of cosmic fireworks, shining out across billions of light-years of space. However, a recent study done at Gemini Observatory shows that they appear to blaze forth from humdrum galaxies in the early universe, and surprisingly, not from the giant or disrupted ones astronomers expected.
 
 
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  According to an international team of astronomers that studied an assortment of these luminous objects near the edge of the observable universe, these pedestrian galactic surroundings came as a shock. “It’s like finding a Formula One racing car in a suburban garage,” said Dr Scott Croom of the Anglo-Australian Observatory in Australia who led the study. “These observations should really have been like using a magnifying glass to find an elephant. Instead, the host galaxies turned out to be more like little mice, despite their brilliant roar!” said team member Dr. Tom Shanks, of the University of Durham (UK).

The quasars were imaged using adaptive optics technology on the Frederick C. Gillett Gemini North Telescope at Mauna Kea, Hawai’i. Dr. David Schade of the National Research Council Canada presented the observations at the first Gemini Science Conference in Vancouver, Canada on May 25th.
 
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Astronomers think that quasars are located in the central cores of galaxies where matter falling onto a supermassive black hole releases a blinding torrent of radiation. These powerhouses flourished when the universe was between a tenth and a third of its present age.

“This finding is particularly exciting because it means that we may need to re-think our models of how quasars work. This isn’t the first time quasars have done this to us, it seems that quasars like to keep us guessing!” said Dr. Schade.
 
 
Quasar Host Galaxy Hokupa'a near-infrared K' band imaging of adaptive optics (AO) guide star (left panel), and of quasar and host galaxy (central panel) at redshift z = 1.93. This is the only quasar out of nine that were imaged with Gemini that showed a faint host galaxy detected.
The panel on the right shows the radial light profile. The points fall above the smooth line which corresponds to the stellar profile of the AO guide star. This excess flux betrays the faint host galaxy.
  Image:  © Gemini Observatory

 
Many astronomers expected that a quasar’s host galaxy would be large and massive, and might show signs of having collided with another galaxy – violence that could spark a quasar into brilliance. The team's finding will undoubtedly add fuel to the debate regarding how galaxies and black holes form and grow.

The team’s aim was to obtain some of the first-ever detailed infrared views of the host galaxies – nine in all – each about 10 billion light-years away. “We’d hoped their sizes and shapes might give clues as to what triggered quasar activity,” said Dr Croom. Instead, the team found that all but one of the galaxies were too faint or small to detect, even though Gemini’s sensitivity and resolution were exceptionally high. The one convincing detection was remarkably unremarkable, similar in brightness and size to the Milky Way galaxy.

Astronomers have used other telescopes on the ground and in space to look for very distant quasar host galaxies, but the results have been inconclusive. “For this study, the Gemini telescope was able to produce an image sharpness that is usually only possible by using the Hubble Space Telescope,” said Dr. Shanks. “But Gemini’s larger mirror can collect 10 times more light to study faint objects.” The image detail was achieved with adaptive optics to remove distortions to starlight caused by atmospheric turbulence. This combination gives astronomers a powerful capability to produce some of the sharpest infrared images ever obtained of very faint objects in the early universe.

The adaptive optics system used on Gemini was called Hokupa‘a-36 combined with a near-infrared imager called QUIRC both developed at the University of Hawaii’s Institute for Astronomy.

One of the difficulties inherent in this study was to find quasars close to the relatively bright guide stars necessary to use adaptive optics technology. To find the necessary sample size, the team drew on a database of more than 20,000 quasars gathered with the Anglo-Australian Telescope between 1997 and 2002. This work represents the largest quasar survey to date and, “It’s the only one in which we could hope to find a decent sample of quasars to meet our requirements,” said Dr. Croom.


Reference: 
S. Croom, D. Schade, B.J. Boyle, T. Shanks, L. Miller and R.J. Smith,
"Gemini imaging of QSO host galaxies at z~2," Ap. J. 606, 126–138 (2004)     (ArXiv e-print)
 
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