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A quasar outshines its host galaxy in this artist's interpretation.
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.
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.
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.
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.
This work was published in The Astrophysical Journal 606 (2004) 126-138
And is also available at: Astro-ph: http://xxx.lanl.gov/abs/astro-ph/0401442
|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 falls above the smooth line which correspond to the stellar profile of the AO guide star. This excess flux betrays the faint host galaxy.|
|The distribution of all quasars found in the 2dF QSO Redshift Survey at the Anglo-Australian Telescope. We are located at the centre of the plot, with quasars at increasingly large distances moving away from the centre. Even amongst all these quasars, only a handful had nearby bright stars so that adaptive optics could be used. The quasars imaged by Gemini are circled in blue and are typically about 12 billion light years away.|
Quasars are a class of objects that are located at great distances from us and thus represent the universe at a relatively young age. They are intrinsically extremely bright (considering their distances from us) and this extreme luminosity has been a challenge to explain. Astronomers think that quasars shine due to intense activity in cores of young galaxies where supermassive black holes fuel these intensely luminous beacons. Today, we see what may be the remnant black holes of this youthful excess at the cores of normal stable galaxies like our Milky Way.
The work at Gemini shows that the galaxies responsible for a quasar’s luminosity were not exceptional or even undergoing extraordinary events (like collisions) to produce the excessive radiation. To explain this, astronomers speculate that in the distant history of our universe, black holes grew by swallowing large quantities of cold, dense gas from which stars form. This gas was much more common then than it is now, having mostly been turned into the stars we see today.
The Gemini Observatory is an international collaboration that has built two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai`i (Gemini North) and the Gemini South telescope is located on Cerro Pachón in central Chile (Gemini South), and hence provide full coverage of both hemispheres of the sky. Both telescopes incorporate new technologies that allow large, relatively thin mirrors under active control to collect and focus both optical and infrared radiation from space.
The Gemini Observatory provides the astronomical communities in each partner country with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the UK Particle Physics and Astronomy Research Council (PPARC), the Canadian National Research Council (NRC), the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT), the Australian Research Council (ARC), the Argentinean Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). The Observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.
Australian involvement in the Gemini partnership is supported by the Australian Research Council and by the Department of Education, Science and Training through the Major National Research Facilities component of the "Backing Australia's Ability" program.
UK participation in the Gemini Observatory is funded by the Particle Physics and Astronomy Research Council (PPARC), the UK's strategic science investment agency which funds research, education and public understanding in four broad areas of science - particle physics, astronomy, cosmology and space science.
Peter Michaud (US Gemini)
Gemini Observatory, Hilo, HI
Helen Sim (Australia)
Julia Maddock (UK)
Dr. Scott Croom (Australia)
Professor Brian Boyle (Australia)
Dr. David Schade (Canada)
|Professor Tom Shanks (UK)
University of Durham, Durham, UK
|Dr. Lance Miller (UK)
University of Oxford, Oxford, UK
|Dr. Robert J. Smith (UK)
Liverpool John Moores
University, Liverpool, UK