Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Astronomers find key evidence supporting theory of quasars

07.06.2005


The office that astronomer Lei Hao shares with her fellow research associates on the first floor of the Space Sciences Building at Cornell University is tidy and organized. But Hao has been thinking a lot lately about dust.



Actually, she’s recently found a great deal of it. And she’s thrilled.

The dust in question is between 0.88 billion and 2.4 billion light years away from Hao’s office, in galaxies scientists classify as active galactic nuclei (AGNs). By confirming that the dust exists, Hao and her team of researchers from Cornell and several other institutions have given new weight to a popular, but not universally accepted, theory of AGNs. Their new evidence is published in the June 1 issue of Astrophysical Journal Letters (Vol. 625, pp. L75-L78).


Since the early 1980s, the most widely accepted model of AGNs, called the unified theory, involves a basic structure: a black hole at the center, an accretion disc (a round, flat sheet of gas) around it and a doughnut-shaped ring of dusty gas, called a torus, around the accretion disc. Jets of matter are propelled out from the center perpendicular to the plane of the accretion disc.

The model holds that all AGNs share the same fundamental characteristics, but it allows for different radiation patterns with the premise that how an AGN looks depends on the perspective of the observer. An AGN viewed face-on, classified as type 1, will show features from its central region; an AGN viewed from the side (type 2) will have those features obscured by the dusty torus. AGNs include quasars, which look like stars in optical telescopes but emit massive amounts of radiation; Seyfert galaxies, low-energy counterparts of quasars; and blazars, which are AGNs viewed pole-on and which show rapid variations in radiation output over short intervals.

From an observational standpoint, the model has been largely successful. But for years, a key piece of evidence has been missing.

Astronomers can determine the composition and temperature of extragalactic material by analyzing the way radiation passing through it is distributed along an infrared spectrum. When radiation passes through silicate dust (a fine, sandy substance common in interstellar dust), the dust grains absorb it at specific wavelengths and leave dips in the infrared spectrum around 10 and 18 microns.

When scientists observe type 2 AGNs, they recognize the silicate component of the dusty torus by the telltale 10- and 18-micron absorption dips. But in order for the unified theory to be correct, scientists looking down from the top or up from below a type 1 AGN would expect to see excess radiation from the silicate dust at 10 and 18 microns. They didn’t -- and that inconsistency led some to wonder if the theory was flawed.

Hao’s observations of silicate emission bands from type 1 AGNs are likely to quell those doubts.

In their paper, Hao and her colleagues describe five quasars (type 1 AGNs) for which clear bumps in infrared emissions have been discovered at 10 and 18 microns. The measurements were taken by the Spitzer Space Telescope’s infrared spectrograph, which was developed by Cornell professor of astronomy James Houck and is one of three instruments on the orbiting space telescope.

"People have been expecting this feature for a long time," says Hao. And it has always been there, she adds, but nobody had recognized it until now -- partly because the Spitzer’s technology is more sensitive than earlier versions and partly because other instruments didn’t include a wide enough spectral range to catch the 10 and 18 micron features.

Finding evidence of dust may not seem important to non-astronomer types, Hao allows. But she’s not letting that dampen her enthusiasm. "For us it’s quite dramatic," she says. And by comparing the two emission bumps, scientists can begin to learn even more about the AGNs. "The relative ratio between the two features can give some information on the inner temperature of the dusty torus," she says. Those calculations are just preliminary, but finding long-sought evidence of the dust in the first place is enough to make Hao grin. "You can see," she says, "that we verified the unification model."

Co-authors of the paper are Henrik Spoon, Gregory C. Sloan, J.A. Marshall, Daniel Weedman, Vassilis Charmandaris and James Houck of Cornell; L. Armus of the California Institute of Technology; A.G.G.M. Tielens of the Netherlands’ SRON National Institute for Space Research and Kapteyn Institute; Benjamin A. Sargent of the University of Rochester; and Ilse M. van Bemmel of Baltimore’s Space Telescope Institute.

Lauren Gold | EurekAlert!
Further information:
http://www.cornell.edu

More articles from Physics and Astronomy:

nachricht Light-emitting bubbles captured in the wild
28.02.2017 | Georg-August-Universität Göttingen

nachricht Scientists reach back in time to discover some of the most power-packed galaxies
28.02.2017 | Clemson University

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Safe glide at total engine failure with ELA-inside

On January 15, 2009, Chesley B. Sullenberger was celebrated world-wide: after the two engines had failed due to bird strike, he and his flight crew succeeded after a glide flight with an Airbus A320 in ditching on the Hudson River. All 155 people on board were saved.

On January 15, 2009, Chesley B. Sullenberger was celebrated world-wide: after the two engines had failed due to bird strike, he and his flight crew succeeded...

Im Focus: Breakthrough with a chain of gold atoms

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

Im Focus: DNA repair: a new letter in the cell alphabet

Results reveal how discoveries may be hidden in scientific “blind spots”

Cells need to repair damaged DNA in our genes to prevent the development of cancer and other diseases. Our cells therefore activate and send “repair-proteins”...

Im Focus: Dresdner scientists print tomorrow’s world

The Fraunhofer IWS Dresden and Technische Universität Dresden inaugurated their jointly operated Center for Additive Manufacturing Dresden (AMCD) with a festive ceremony on February 7, 2017. Scientists from various disciplines perform research on materials, additive manufacturing processes and innovative technologies, which build up components in a layer by layer process. This technology opens up new horizons for component design and combinations of functions. For example during fabrication, electrical conductors and sensors are already able to be additively manufactured into components. They provide information about stress conditions of a product during operation.

The 3D-printing technology, or additive manufacturing as it is often called, has long made the step out of scientific research laboratories into industrial...

Im Focus: Mimicking nature's cellular architectures via 3-D printing

Research offers new level of control over the structure of 3-D printed materials

Nature does amazing things with limited design materials. Grass, for example, can support its own weight, resist strong wind loads, and recover after being...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Booth and panel discussion – The Lindau Nobel Laureate Meetings at the AAAS 2017 Annual Meeting

13.02.2017 | Event News

Complex Loading versus Hidden Reserves

10.02.2017 | Event News

International Conference on Crystal Growth in Freiburg

09.02.2017 | Event News

 
Latest News

Light-emitting bubbles captured in the wild

28.02.2017 | Physics and Astronomy

Triboelectric nanogenerators boost mass spectrometry performance

28.02.2017 | Materials Sciences

Calculating recharge of groundwater more precisely

28.02.2017 | Ecology, The Environment and Conservation

VideoLinks
B2B-VideoLinks
More VideoLinks >>>