The structure of an active galactic nucleus

The nuclei of most galaxies host supermassive black holes containing millions to billions of solar-masses of material. The immediate environments of these black holes typically include a tori of dust and gas and, as material falls toward the black hole, the gas radiates copiously at all wavelengths. Although the models for these active galactic nuclei (AGN) work reasonably well, it is difficult to obtain direct evidence of the inner structures of AGN because they are so far away and their dimensions are thought to be only tens to hundreds of light-years.

CfA astronomer David Wilner and his colleagues used the ALMA millimeter telescope facility to study the nearest AGN, Arp 220, which is thought to be particularly active after having recently undergone a merger with another galaxy. The two merging nuclei are about 1200 light-years apart, and each has a rotating disk of molecular gas a few hundred light-years in scale. Vigorous star formation is evident in the region as well as at least one molecular outflow inferred from the large velocities seen. But there are numerous unresolved structural issues about these inner regions, including how gas flows to, from, and between the two mergering nuclei and precisely which subregions are responsible for the dominant luminosity sources. The astronomers used these high-resolution millimeter observations to tackle these questions because thick dust, which blocks much of the view at shorter wavelengths, is relatively transparent in these bands.

Source

 

Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University

Hubble image of the superluminous merging galaxy Arp220. Astronomers have measured structures only a hundreds of light-years in size around the two supermassive black holes in the nuclear region, as well as evidece for an outflow.

Source / Image Courtesy

At the hearts of most big galaxies, including our own Milky Way, there lurks a supermassive black hole weighing millions to billions of times the sun's mass. As gas falls toward a supermassive black hole, it gathers into a so-called accretion disk and becomes compressed and heated, ultimately emitting X-rays. The centers of some galaxies produce unusually powerful emission that exceeds the sun's energy output by billions of times. These are active galactic nuclei, or AGN.

Using data from NASA's Rossi X-ray Timing Explorer (RXTE) satellite, an international team has uncovered a dozen instances where X-ray signals from active galaxies dimmed as a result of a cloud of gas moving across our line of sight. The new study triples the number of cloud events previously identified in the 16-year archive.

 

This animation provides a close-up look at the behavior of a central black hole in the nucleus of an active galaxy. An accretion disk of infalling material surrounds a central black hole that may be millions or billions of times the mass of our Sun. Some of the superheated material escapes along the black hole's rotation axis. Powerful magnetic fields keep the material confined to an extragalactic jet. Hubble Space Telescope spectroscopic observations of active galactic nuclei reveal how stellar material is blown back into intergalactic space.