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Evidence for buried quasars unites galaxies.

Hidden quasars may lie at the heart of at least five radio-emitting galaxies, according to observational evidence reported this week. The new data strongly suggest that certain galaxies once categorized as different types actually represent only one type, with their seeming differences resulting from their orientations relative to Earth.

Radio galaxies belong to a group of luminous objects known for their "active galactic nuclei" (AGN). The AGN class also includes quasars and certain bright objects that don't emit much radiation at radio wavelengths -- Seyfert Type I and Type II galaxies.

Since the mid-1980s, studies have suggested that members of the AGN menagerie have more in common than meets the eye. Specifically, several researchers have proposed that the active nuclei in all of these galaxies contian a quasar surrounded by a doughnut-shaped cloud of dust and gas. If viewed edge-on visible light-and through the encircling dust -- the brilliant quasars would remain shrouded from view. The parent galaxies would therefore appear to emit only the narrow bands of light characteristic of certain radio or Type II Seyfert galaxies. But if observers looked straight through the hole of the dusty doughnut, the same galaxies would instead broadcast the brilliant, wideband spectral emissions typical of a naked, radio-emitting quasar or Type I Seyfert.

Recent experiments using polarized light from Seyferts supported this unified galactic theory (SN: 6/25/88, p.404; SN: 4/27/91, p.261), but no one had ever imaged the crucial hidden quasar. To search for that missing link, Stanislav Djorgovski and his colleagues at the California Institute of Technology in Pasadena have now examined eight radio galaxies in the infrared -- a wavelength band which dust cannot attenuate as completely as visible light.

The group focused special attention on Cygnus A, the brightest radio galaxy in the northern sky. Using a camera attached to the 5-meter telescope at the Palomar Observatory near Escondido, Calif., the researchers photographed Cygnus A at four successively longer near-infrared wavelengths -- each penetrating the dust more deeply than the last.

The core of the galaxy can't be discerned in visible light. But at the shortest infrared wavelength, the nucleus begins to emerge as round, somewhat focused object, Djorgovski says. And when viewed at the longest wavelength, the core resembles a point source, as if a compact object lurked there. Moreover, the location of this impact source matches the position of the galaxy's known radio-emitting core, the researchers report.

According to Djorgovski, this all but proves that the core of Cygnus A harbors a quasar. "It's really too much to be a coincidence," he explains. "The logical interpretation is that we've found a buried quasar that we couldn't see optically."

He and his colleagues describe the Cygnus A data in the May 10 ASTROPHYSICAL JOURNAL LETTERS. And at a meeting of the American Astronomical Society in Seattle this week, Djorgovski announced that other radio galaxies studied by his team show similar evidence of a hidden quasar -- M87, Perseus A, 3C236 and 3C264.

The Caltech team also estimated the apparent absorption of visible light by the dust believed to encircle Cygnus A's core. Comparing the observed intensity of the galaxy's central radio source -- which emerges from the core unimpeded by the dust -- with the muted emission in infrared, the team calculated that dust along the line of sight must be absorbing all but one out of every [10.sup.21] visible-light photons radiated by the core.

At the Seattle meeting, Andrew S. Wilson of the University of Maryland at College Park reported that he and his colleagues had independently calculated the same absorption. They made their estimate by comparing X-ray emissions from the galaxy's core with their own observations of dust-obscured, near-infrared radiation.

Wilson's team analyzed infrared spectra from Cygnus. A using an infrared telescope on Mauna Kea in Hawaii. They detected warm molecular hydrogen, often associated with dense clouds of gas and dust, he says. Wilson notes that the hydrogen finding supports the notion that dust surrounds a hot, quasar-like object at the core of radio galaxies and perhaps other active galactic nuclei. In other words, all such nuclei may be the same at heart.
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Title Annotation:hidden quasars may lie at the heart of at least five radio-emitting galaxies
Author:Cowen, Ron
Publication:Science News
Date:Jun 1, 1991
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