* Astronomy

Astronomer Lei Hao has recently found a great deal of dust.
The dust is between 0.88 and 2.4 billion light years away, in active galactic nuclei (AGNs).
By confirming that the dust exists, Hao and her team of researchers from Cornell and several other institutions have got rid of a nagging problem to the a popularly accepted theory of AGNs.
Their new evidence is published in the June 1 issue of Astrophysical Journal Letters (Vol. 625, pp. L75-L78).
The most widely accepted model of AGNs, called the unified theory, involves a basic structure: a black hole at the centre, an accretion disc 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 centre 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 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. Throwing the whole theory into question.
When radiation passes through silicate dust contained in the torus, 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.

"The relative ratio between the two features can give some information on the inner temperature of the dusty torus". 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, that we verified the unification model."

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