SETI public: FW: Cornell News: Study supports theory of quasars

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>From: cunews_at_cornell.edu
>Reply-To: cunews_at_cornell.edu
>To: CUNEWS-PHYSICAL_SCIENCE-L_at_cornell.edu, CUNEWS-SCIENCE-L_at_cornell.edu
>Subject: Cornell News: Study supports theory of quasars
>Date: Mon, 6 Jun 2005 09:07:46 -0400
>
>Cornell astronomers find key evidence supporting theory of quasars
>
>June 6, 2005
>
>Contact: Lauren Gold
>Phone: (607) 255-9736
>E-mail: lg34_at_cornell.edu
>
>Media Contact: Press Relations Office
>Phone: (607) 255-6074
>E-mail: pressoffice_at_cornell.edu
>
>ITHACA, N.Y. -- 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 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.
>
>
>-30-
>
>
>--Spitzer Space Telescope: <http://www.spitzer.caltech.edu/>
>--Cornell's Department of Astronomy: <http://www.astro.cornell.edu/>
>--James Houck:
><http://www.astro.cornell.edu/people/facstaff-detail.php?pers_id=106>
>
>-30-
>
>The web version of this story, with accompanying photos, is available at
>http://www.news.cornell.edu/stories/June05/spitzer.quasars.hao.lg.html
>--
>
>Cornell University News Service
>312 College Ave.
>Ithaca, NY 14850
>607-255-4206
>cunews_at_cornell.edu
>http://www.news.cornell.edu
>

• Previous message: LARRY KLAES: "SETI public: Young, Jupiter-Mass Objects in Ophiuchus"

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