The Physics of Active Galactic Nuclei

Gas accreting onto a black hole through an accretion disk emits an incredible amount of X-ray, ultraviolet and optical emission, and spectral/variability studies of this emission reveal the physical conditions in the immediate vicinity of the black hole. Jets can also be fired out from the black hole region at nearly the speed of light.

Chandra, XMM-Newton, BeppoSAX and RXTE are the main satellite missions currently providing new X-ray results on active galaxies. The vastly improved spectral resolution and collecting area provided by these satellites is revolutionizing our understanding of the X-ray emitting regions around the black hole.

The Hobby-Eberly Telescope (HET), with its large collecting area and advanced focal plane instrumentation, is now allowing excellent optical/near-infrared spectroscopy of AGN. Queue scheduling of the HET facilitates near-simultaneous observations with space-based observatories, rapid response observations of variable AGN, and long-term AGN monitoring programs.

Current AGN research efforts at Penn State include studies of Narrow-Line Seyfert 1 Galaxies (Prof. Niel Brandt); AGN with double-peaked optical emission lines (Prof. Mike Eracleous); Broad Absorption Line Quasars (Prof. Niel Brandt); Narrow Absorption Line Quasars (Prof. Mike Eracleous and Prof. Jane Charlton); and Seyfert 2 galaxies (Dr. Rita Sambruna and Prof. Niel Brandt).

• Narrow-Line Seyfert 1 Galaxies show extremely strong soft X-ray emission which is thought to originate in the innermost accretion disk. They also have remarkable optical emission line properties, hard X-ray spectra, and X-ray variability properties. It is likely that Narrow-Line Seyfert 1s have extreme values of a primary physical parameter such as mass accretion rate onto the black hole.
• Double-peaked optical emission lines are found in about 10 per cent of radio-loud AGN. These AGN may be a small but distinct subclass which show broad optical line emission from an accretion disk. Tests are underway to determine if the double-peaked emission lines indeed arise in a disk.
• Major outflows of ionized gas from quasars can be studied via X-ray and ultraviolet observations of Broad and Narrow Absorption Line Quasars. A surprising recent insight into the quasar phenomenon is that these objects may well be ejecting as much material as the black hole ultimately swallows!
• Seyfert 2 galaxies have their black hole regions hidden from direct sight by obscuring gas thought to be in the form of a torus. They can show strong emission lines at X-ray wavelengths, and the Chandra and XMM-Newton grating spectrometers are allowing excellent X-ray spectroscopy of some of the brightest Seyfert 2s.

---

Web page by Niel Brandt (niel@astro.psu.edu)
Last update: 2000, August 27