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Active Galactic Nuclei

The types of galaxies in Figure 05-01b and c seems to be a good classification scheme for the nearby galaxies. However, there are other kinds of galaxies, which do not fit into such category. They seem to represent the galaxies in another phase of evolution. These special objects include Seyfert galaxies, radio galaxies, quasars and extremely red objects (ERO) in a rough order of ascending redshift (distance). Thanks to systematic surveys, the latest (2007) catalogs contain more than 13,000 quasars - a number that could eventually reach 100,000.

Black Hole in Galaxy By 2007, it is recognized that most galaxies other than dwarfs have central black holes. The idea is that black hole "seeds" either attracted matter into forming galaxies or formed within young galaxies in the early universe. This action produced quasars and explains why most quasars are extremely distant. As the black hole acquired more and more matter from galaxies' centers, the fuel became exhausted, so they slowly quieted down. Most galaxies in the recent universe have slumbering giants in their centers. They can be re-activated when interact with other galaxies, starbursts, or gas clouds falling into the central region. This scenario explains active galactic nucleus (AGN) in the nearby universe. Study of such galaxies reveals that the more massive a galaxy's central bulge, the more massive its black hole. Figure 05-01e shows two deep field galaxies in both optical and infrared. The black holes are displayed prominently in the infrared images.

Figure 05-01e Black Hole in Galaxy [view large image]

Views of AGN An Unified View of AGNs Figure 05-01f presents a unified view of the AGN. The basic ingredient consists of a central black hole surrounded by a dusty disk. The kind sporting a jet or two usually associates with strong radio emission; it includes quasar, and radio galaxy. While QSO and Seyfert galaxy belong to the radio quiet kind. Energy consumed by the jet and various forms of radiations is extracted from the in-falling gas cloud and the dusty torus - ultimately it is converted from the gravitational potential and angular momentum. The same kind of object presents a different aspect depending on the viewing angle of the observer. Figure 05-01g is an updated version of the AGN. It adds an accretion disk in between

Figure 05-01f An Unified View of AGN

Figure 05-01g Another Unified View of AGN
[view large image]

the dust torus and the black hole and classifies the Seyfert galaxies into two types (see Figure 05-01g). While the EROs (Extremely Red Objects) are these kinds of objects further away in the early universe.

See an "Introduction to Active Galaxies".

A 2010 report reveals that the Fermi gamma-ray observatory has detected in 3C279 (a blazar as shown in Figure 05-01h) a giant
Jet from 3C279 gamma-ray flare lasting for about 20 days. What is unique about this observation is that the degree and direction of visible-light polarization also changed drastically during the same period. It indicates that the gamma-ray and visible-light regions are connected probably at the same location. The changing polarization could be related to a blob of gas flowing around a bend in the jet. Combining the speed of the jet with the duration of the flare provides an estimate of the size of the emitting region in the jet and thus an estimate of the distance of the emission region from the black hole. It is

Figure 05-01h Blob in Jet from 3C279

found to be probably more than 105 the radius of the black hole. This is much more than the tens of radius predicted by some models, and suggests that the region is not intimately associated with the innermost regions of the accretion disk.

X-ray AGN An international team of astronomers using NASA's Swift satellite and the Japanese/United States Suzaku X-ray observatory has discovered a new class of active galactic nuclei (AGN) in 2007. These objects are so heavily shrouded in gas and dust that virtually no light gets out. Only the high-energy X-rays can punch through such thick layer. These objects comprise about 20 percent of point sources in the X-ray background, a glow of X-ray radiation that pervades our Universe. It implies that there must be a large number of yet unrecognized obscured AGNs in the local universe. By missing this new class, previous AGN surveys were heavily biased, and thus gave an incomplete picture of how supermassive black holes and their host galaxies have evolved over cosmic history. Figure 05-01i is an artist's illustration of the X-ray AGN.

Figure 05-01i X-ray AGN [view large image]

See an animation on traveling to an AGN by NASA's GSFC.

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