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Quasars & Active Galaxies

According to a popular theory, Type 1 and Type 2 AGNs are the same objects viewed from a different angle. The central black hole is assumed to be surrounded by a thick donut-shaped cloud of gas and dust. The source appears different, depending on whether it is observed from the side through the edge of the donut (Type 2), or from the top through the hole (Type 1).
Illustration of Black Hole with Accretion Disk and Torus (CXC/M.Weiss)

This theory explains many of the observations of active galaxies, and has won widespread acceptance among astronomers, though questions remain. Chief among these is whether the theory applies to quasars whose central black hole is at least a thousand times more energetic than the typical AGN. Could a cloud of gas exist for any length of time around a giant black hole that is generating energy at such a prodigious rate? Chandra observations of quasars have shown that they can.

Several strong candidates for Type 2, or obscured quasars have been discovered. These objects are inconspicuous at optical wavelengths, so the X-ray observations have led to the conclusion that the giant black hole population in the universe is much higher than observations with optical telescopes indicate.

More: Quasar 3C273

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