Active Galactic Nuclei and Quasar Phenomena
Summary
Active galactic nuclei (AGNs) are the luminous centres of galaxies powered by accretion onto supermassive black holes. When accretion rates and luminosities reach their highest levels, these objects appear as quasars, outshining entire host galaxies across the electromagnetic spectrum. Emission arises from a compact accretion disc, surrounded by fast-moving gas in the broad-line region, and often accompanied by powerful winds and relativistic jets. Warm, dusty tori obscure certain viewing angles, unifying different observational classes under a common physical framework. The diversity of AGN and quasar properties is organised through empirical schemes such as the four-dimensional eigenvector 1 main sequence, which links emission-line strength, broad-line widths and accretion rate. Outflows traced by blueshifted high-ionisation lines regulate star formation in host galaxies and contribute to cosmic feedback, while reverberation mapping of emission lines provides estimates of black hole masses. Quasars serve as probes of the early universe, cosmic reionisation, large-scale structure and the chemical enrichment of intergalactic gas. Advances in wide-field surveys and high-resolution instrumentation continue to deepen understanding of AGN physics, their role in galaxy evolution and their use as cosmological beacons.
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Active Galactic Nuclei and Quasar Phenomena publication trend
The graph below shows the total number of articles in active galactic nuclei and quasar phenomena across all publications each year (not limited to Nature Index journals).
Technical terms
Active Galactic Nucleus (AGN): The compact central region of a galaxy dominated by energy release from matter accreting onto a supermassive black hole.
Quasar: An extremely luminous AGN seen at high accretion rates and often large cosmological distances, outshining its host galaxy.
Eddington ratio: The ratio of an object’s luminosity to its Eddington luminosity, indicating the balance between radiative force and gravity in the accretion flow.
Broad-line Region (BLR): The inner gaseous region around the accretion disc where high-velocity clouds produce Doppler-broadened emission lines.
Reverberation mapping: A technique that measures time delays between continuum and line-emission variability to infer the size of the BLR and estimate black hole mass.
Eigenvector 1 main sequence: An empirical framework organising quasars by correlations among emission-line properties, line widths and accretion parameters.
References
- Fantastic Fits with fantasy of Active Galactic Nuclei Spectra: Exploring the Fe ii Emission near the Hα Line. The Astrophysical Journal Supplement Series (2023).
- High-redshift quasars along the main sequence⋆⋆⋆. Astronomy & Astrophysics (2023).
- Testing AGN outflow and accretion models with C iv and He ii emission line demographics in z ≈ 2 quasars. Monthly Notices of the Royal Astronomical Society (2023).
- Measuring the black hole masses of high-redshift quasars. Monthly Notices of the Royal Astronomical Society (2002).
- A Main Sequence for Quasars. Frontiers in Astronomy and Space Sciences (2018).
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