Active Galactic Nuclei

Active galactic nuclei (AGNs) are centers of galaxies that emit large amounts of radiation with a spectrum that differs from that of normal stars. Quasi-stellar Objects (QSOs), or quasars, are luminous examples of the AGN phenomenon, in which the galaxy nucleus outshines the remainder of the galaxy. The tremendous power and compact nature of these sources is believed to originate from matter falling into an enormous black hole, with a mass as large as a billion times that of the Sun. Material moving under the influence of the black hole's gravity is heated and radiates the light that we observe. AGN research at OU employs observational as well as theoretical methods to probe the structure of these exotic objects.

Image of a quasar and host galaxy acquired with the Hubble Space Telescope:

Quasars represent the most distant galaxies we can see in the universe. Shields and former student Anca Constantin have recently completed a study of quasars at redshifts z>4, which trace some of the most distant known galaxies in the universe. A collaboration with Dr. Fred Hamann (Univ. Florida), Dr. Craig Foltz (MMT Observatory), and Dr. Fred Chaffee (Keck Observatory), this effort made use of spectra obtained with the 4.5-meter Multiple Mirror Telescope in Arizona, and the 10-meter Keck Telescopes in Hawaii. The results reinforce evidence that the quasar phenomenon is associated with, or preceded by, a phase of vigorous star formation that gives rise to the heavy elements that reveal their presence in the emitted spectra.

Because of the time required for light to reach us, quasars observed at high redshift are seen at a time when the universe was much younger than it is today. No quasars are seen in the environs of the Milky Way, despite the fact that they were common at high redshift, in the past. Evidently quasars fade over time, but the massive black holes that drove these systems should persist in the centers of normal galaxies. In some cases these remnants may be seen today as low luminosity AGNs (LLAGNs) , in which the rate of infall into the black hole is reduced to a trickle of matter. Shields is part of a team of astronomers using the Hubble Space Telescope to study nearby LLAGNs, and to determine the masses of the underlying black holes in these systems.

AGNs emit strongly across the electromagnetic spectrum, so that multi-wavelength diagnostics are available for studying these sources. Shields currently has active programs for the study of low-luminosity AGNs using the Hubble Space Telescope for optical and ultraviolet observations, and the Chandra Observatory for X-ray measurements. These programs are supported with funding from NASA. Additional observations for the study of absorption by gas residing near the black hole are being carried out at the MDM Observatory.

A subset of AGNs display powerful jets of relativistic matter that can extend to large distances from the central accretion structure. These sources can emit strongly across the electromagnetic spectrum. The radiation is often strongly beamed in the direction of the jet motion, due to relativistic effects, and the beamed radiation dominates our view in objects where the jet is pointed toward us. Böttcher is deeply involved in multi-wavelength observational studies and theoretical analysis of such jets.


Markus Böttcher
Joe Shields

Updated: 2007 November 18