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| AGN | |
|---|---|
| Name | Active Galactic Nucleus |
| Type | Astronomical object |
| Mass | Supermassive black hole |
AGN
An active galactic nucleus (AGN) is a compact, luminous region at the center of a galaxy powered by accretion onto a supermassive black hole. AGN produce radiation across the electromagnetic spectrum and drive phenomena observed in systems ranging from nearby Messier 87 to distant quasars discovered in surveys like the Sloan Digital Sky Survey and the Hubble Deep Field. Studies of AGN tie together work by teams at institutions such as the European Southern Observatory, National Radio Astronomy Observatory, Max Planck Institute for Astronomy, and missions including Chandra X-ray Observatory and Hubble Space Telescope.
AGN are identified by excess emission relative to stellar populations in galaxies such as Andromeda Galaxy and NGC 1068 that cannot be explained by star formation alone. Historically, sources now classified as AGN were cataloged as Seyfert galaxies, quasars, and radio galaxies from observational programs at facilities like Palomar Observatory and the Arecibo Observatory. Landmark discoveries linking compact cores to supermassive black holes involved measurements by teams using instruments at Very Large Array and very long baseline interferometry networks including the European VLBI Network.
Classification schemes separate AGN into populations such as Seyfert 1 and Seyfert 2 based on optical emission-line diagnostics developed in part by researchers at Mount Wilson Observatory and groups using the Keck Observatory. Quasars (quasi-stellar objects) include luminous examples like 3C 273 identified in radio catalogs such as the Third Cambridge Catalogue of Radio Sources. Radio-loud AGN include Centaurus A and Cygnus A, while radio-quiet AGN include many Seyferts and optically selected quasars found by the Two-degree Field Galaxy Redshift Survey. Blazars — exemplified by BL Lacertae and Markarian 421 — show relativistic jet signatures studied by collaborations including the Fermi Gamma-ray Space Telescope team and the VERITAS consortium.
The central engine consists of a supermassive black hole often measured via stellar or gas dynamics in systems like NGC 4258 and Milky Way's center (studied by groups at Max Planck Institute for Extraterrestrial Physics and teams led by Andrea Ghez and Sheperd S. Doeleman). Accretion disks modeled by frameworks from researchers at Princeton University and Harvard-Smithsonian Center for Astrophysics produce thermal emission, while magnetohydrodynamic simulations by groups at University of California, Berkeley and Columbia University explain jet launching observed in M87 by the Event Horizon Telescope collaboration. Broad-line regions and narrow-line regions produce Doppler-broadened and narrow emission lines seen in objects such as NGC 5548 and Mrk 231. Obscuring structures resembling a dusty torus are inferred in systems studied with the Spitzer Space Telescope and James Webb Space Telescope.
AGN spectra display multiwavelength signatures from radio through gamma rays; classic examples include the radio lobes of 3C 273 and X-ray coronae observed by Chandra X-ray Observatory in NGC 1275. Optical spectroscopic diagnostics trace emission lines like Hα and [O III] used in classification schemes developed at Palomar Observatory and employed in surveys by Anglo-Australian Observatory. Variability studies on timescales from hours to years involve monitoring programs at observatories including Las Cumbres Observatory and long-term projects such as International Ultraviolet Explorer. Polarization measurements by teams at University of Arizona and reverberation mapping campaigns led by groups at University of California, Irvine probe geometry and scale of emitting regions.
AGN reside in diverse hosts from massive ellipticals like M87 to spirals like NGC 1068; host studies involve imaging with Hubble Space Telescope and surveys by the Sloan Digital Sky Survey. AGN feedback — including mechanical jets in Perseus Cluster sources and radiative winds in systems such as PDS 456 — affects star formation and intracluster medium properties investigated by collaborations at Space Telescope Science Institute and the European Space Agency. Environmental correlations with galaxy mergers are studied in samples from the COSMOS survey and by observers at Keck Observatory and Subaru Telescope linking interactions like those in Antennae Galaxies to AGN fueling.
Quasar luminosity functions measured by teams behind the Sloan Digital Sky Survey and the Two Micron All Sky Survey show a peak in activity near redshift z ~ 2 found in deep fields such as the Hubble Ultra Deep Field. Co-evolution of black holes and galaxies is constrained by the M–sigma relation observed in local ellipticals and studies from institutions like University of Cambridge and California Institute of Technology. High-redshift AGN discovered by surveys with the Atacama Large Millimeter/submillimeter Array and the Submillimeter Array inform reionization-era models produced by theorists at Institute for Advanced Study and Kavli Institute for Cosmology.
AGN detection employs radio arrays such as the Very Large Array and LOFAR, optical/IR facilities including Keck Observatory, Very Large Telescope, and space telescopes like Chandra X-ray Observatory and Fermi Gamma-ray Space Telescope. Spectroscopic surveys by the Sloan Digital Sky Survey and time-domain programs from the Zwicky Transient Facility identify candidates for follow-up with interferometers like the Event Horizon Telescope and instruments on James Webb Space Telescope. Multiwavelength campaigns coordinated among institutions such as the National Radio Astronomy Observatory, European Southern Observatory, and NASA enable comprehensive characterization from parsec to kiloparsec scales.