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Black Hole Hide and Seek

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see captionMarch 21, 2000 -- Could giant black holes be hiding in the centers of galaxies that lead double lives, appearing as mild-mannered "Clark Kent-like" normal galaxies in visible light, but revealing a powerful "Superman-like" inner core in X-rays? Many astronomers think so, but no one has ever been able to prove it.

Now, using the Chandra X-ray Observatory, a group of astronomers has published observations of an object that appears to be an optical "Clark Kent" galaxy with a camouflaged "Superman" in the middle -- in other words, they may have found an elusive Type 2 quasar.

Right: The left-hand panel shows the Chandra X-ray Observatory image of a powerful point-like source of X rays. The Hubble Space Telescope image (right panel) shows the spiral galaxy with which the X-ray source is associated. The X-ray source is located at the center of the galaxy, and has a deficit of low energy X rays, consistent with absorption by a thick cloud of gas. The combination of powerful X-ray emission, absorption of low energy X-rays, and the relatively normal optical appearance of the galaxy suggests that the source is a rare type of black hole called a Type 2 quasar.

Type 2 quasars are predicted by a popular theory of these distant objects called the unified model. The unified model is an extension of an earlier concept that explains galaxies with unusual energetic activity in their central regions. These regions are called Active Galactic Nuclei, or AGNs, and have been categorized into two types:
  • Type 1 AGNs show evidence of energetic activity in the form of hot, rapidly moving gas clouds, and bright radio emission, and low energy X-ray emission.
  • Type 2 AGNs show evidence of less energetic activity in the form of scattered light and gas moving at moderate speeds, some radio activity in the form of jets, and no low energy X rays.

The AGN unified model, developed by astronomers Robert Antonucci, of the University of California, Santa Barbara, and Joseph

Miller of the University of California, Santa Cruz, proposed that Type 1 and Type 2 AGNs could have a common explanation -- a donut-shaped veil of gas and dust around a central black hole.

Below: This Hubble Space Telescope image shows a spiral galaxy with an active galactic nucleus. The "S" is actually a lane of stars, gas and dust circling the core. Designated NGC 3393, the bright core makes this galaxy a Seyfert and the infrared glow of central dust help distinguish it as "type 2." Seyfert galaxies have extremely energetic nuclei similar to more powerful quasars. Seyferts are thought to have black holes in their centers.

see captionAccording to the model, a thick doughnut of gas and dust surrounds a central black hole. The source looks different, depending on whether it is observed through the doughnut, through the hole, or at an intermediate angle. In extreme cases representing a Type 2 quasar, the optical radiation from the quasar is absorbed while the high-energy X rays penetrate the veil. The energy absorbed by the gas and dust is re-radiated at lower energy infrared and submillimeter wavelengths.

A team of scientists led by British astronomers Andrew Fabian of the Institute of Astronomy in Cambridge and Ian Smail of the University of Durham recently reported strong evidence for the existence of a Type 2 quasar. They used NASA's Chandra X-ray Observatory, the Hubble Space Telescope, and the James Clerk Maxwell submillimeter telescope on Mauna Kea in Hawaii to examine numerous candidate galaxies. By concentrating their search near two galaxy clusters, the astronomers exploited a gravitational lensing effect that can lead to a significant brightening of distant sources.

They discovered four new X-ray sources and seven new submillimeter sources. The brightest X-ray source is concentrated in the center of a distant galaxy. The point-like object has a deficit of low energy X rays, consistent with absorption by a thick cloud of gas. The combination of powerful X-ray emission, absorption of low energy X rays, and the relatively normal optical appearance of the galaxy led the scientists to conclude that the source is a strong contender to be a genuine Type 2 quasar.

Six of the submillimeter sources that were discovered were not detected in X rays. This could mean that a central black hole is shrouded by an unusually thick cloud of dust and gas, or an additional source of submillimeter radiation is present, perhaps due to a burst of star formation.

A paper describing these results will be published in the Monthly Notices of the Royal Astronomical Society. The Chandra observations were made on November 5, 1999 using the Advanced CCD Imaging Spectrometer (ACIS). The team involved scientists from the Institute of Astronomy, the University of Durham, University College London, and the Observatoire Midi-Pyrenees in France.

The ACIS instrument was built for NASA by the Massachusetts Institute of Technology, Cambridge, and Pennsylvania State University, University Park. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. TRW, Inc., Redondo Beach, Calif., is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, Mass.
Web Links

Chandra home page -from Harvard

Chandra News -from NASA

Galaxy Clusters -a tutorial from Harvard's Chandra home page

X-Rays - Another Form of Light - the basics of X-rays from the Chandra home page at Harvard