5.9 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Active galactic nucleus | 3/4 | https://en.wikipedia.org/wiki/Active_galactic_nucleus | reference | science, encyclopedia | 2026-05-05T13:31:44.530837+00:00 | kb-cron |
Radio-quiet quasars or QSOs. These are essentially more-luminous versions of Seyfert 1s. The distinction between radio-loud and radio-quiet is arbitrary, and is usually expressed in terms of a limiting optical magnitude or a optical-to-radio luminosity ratio. Quasars were originally 'quasi-stellar' in optical images, because they had optical luminosities that were greater than that of their host galaxy. They always show strong optical continuum emission, X-ray continuum emission, and broad and narrow optical emission lines. Some astronomers use the term QSO (Quasi-Stellar Object) for this class of AGN, reserving 'quasar' for radio-loud objects, while other astronomers talk about radio-quiet and radio-loud quasars. The host galaxies of quasars can be spirals, irregulars, or ellipticals. There is a correlation between the quasar's luminosity and the mass of its host galaxy, in that the most luminous quasars inhabit the most massive galaxies (ellipticals). 'Quasar 2s'. By analogy with Seyfert 2s, these are objects with quasar-like luminosities, but without strong optical nuclear continuum emission or broad line emission. They are scarce in surveys, though a number of possible candidate quasar 2s have been identified.
=== Radio-loud AGN === There are several subtypes of radio-loud active galactic nuclei.
Radio-loud quasars behave exactly like radio-quiet quasars with the addition of emission from a jet. Thus they show strong optical continuum emission, broad and narrow emission lines, and strong X-ray emission, together with nuclear and often extended radio emission. "Blazars" (BL Lacertae (BL Lac) objects and optically violent variable (OVV) quasars) are distinguished by rapidly variable, polarized optical, radio, and X-ray emissions. BL Lac objects show no optical emission lines, broad or narrow, so that their redshifts can only be determined from features in the spectra of their host galaxies. The emission-line features may be intrinsically absent, or simply swamped by the additional variable component. In the latter case, emission lines may become visible when the variable component is at a low level. OVV quasars behave more like standard radio-loud quasars with the addition of a rapidly variable component. In both classes of source, the variable emission is believed to originate in a relativistic jet that is oriented close to the line of sight. Relativistic effects amplify both the luminosity of the jet and the amplitude of variability. Radio galaxies. These objects show nuclear and extended radio emission. Their other AGN properties are heterogeneous. They can broadly be divided into low-excitation and high-excitation classes. Low-excitation objects show no strong narrow or broad emission lines, and the emission lines they do have may be excited by a different mechanism. Their optical and X-ray nuclear emission is consistent with originating purely in a jet. They may be the best current candidates for AGN with radiatively inefficient accretion. By contrast, high-excitation objects (narrow-line radio galaxies) have emission-line spectra similar to those of Seyfert 2s. The small class of broad-line radio galaxies, which show relatively strong nuclear optical continuum emission probably includes some objects that are simply low-luminosity radio-loud quasars. The host galaxies of radio galaxies, whatever their emission-line type, are essentially always ellipticals.
== Unified models ==
Unified models propose that different observational classes of AGN are a single type of physical object observed under different conditions. A "strict" unification model proposes that the apparent differences between different types of objects arise simply because of their different orientations of the jet and obscuring torus as viewed on Earth. The obscuring torus, also called a "dusty torus" is a cool outer layer surrounding an accretion disk. This model has had partial success, showing for example that Seyfert galaxies of type 1 and 2 are the same kinds of AGN viewed differently. Other effects that might lead the same kind of astrophysical object to have distinctive observational characteristics include the accretion rate, the strength of the relativistic jet, obscuring effects of the galaxy surrounding the AGN, or time of observation relative to the formation of the AGN. AGNs are the subject of numerous on-going studies seeking to clarify the nature of AGNs.
== Effects on planets == It is expected that all supermassive black holes at the center of galaxies have gone through high AGN activity to reach the mass we see today. These periods of high AGN activity can potentially affect the atmospheres of planets and their habitability. Planets located in compact galaxies such as “Red Nuggets” are likely to be more impacted than planets located in typical elliptical galaxies such as M87 or spiral galaxies such as the Milky Way galaxy. For a planet with a high amount of initial oxygen in its atmosphere, AGN radiation may allow a thicker ozone layer, possibly by shielding it from other UV radiation. This would potentially increase the habitability of a planet. The supermassive black hole at the center of our galaxy (Sagittarius A*) experienced a phase of AGN activity 8 Gyr ago. This would have caused loss of atmospheres to planets within 1 kpc comparable to present-day Earth. The X-ray and extreme UV radiation also would have caused biological damage to surface life on planets without proper shielding. This would potentially hinder the development of complex life within a few kiloparsecs. The inherent energy from AGNs can also heat up the atmosphere of planets leading to atmospheric escape. The combined effect of AGN outflows would likely make all planets within 1kpc (~3,262 light-years) of the center of a galaxy uninhabitable.
== Examples ==