Editing Active galactic nucleus (section)

=== Radio-quiet AGN ===
* [[Low-ionization nuclear emission-line region]]s (LINERs). As the name suggests, these systems show only weak nuclear emission-line regions, and no other signatures of AGN emission. It is debatable<ref>{{cite journal |last1=Belfiore |first1=Francesco |title=SDSS IV MaNGA – spatially resolved diagnostic diagrams: a proof that many galaxies are LIERs |journal=Monthly Notices of the Royal Astronomical Society |date=September 2016 |volume=461 |issue=3 |page=3111  |doi=10.1093/mnras/stw1234 |doi-access=free |arxiv=1605.07189 |bibcode=2016MNRAS.461.3111B |s2cid=3353122 |url=https://academic.oup.com/mnras/article/461/3/3111/2608476}}</ref> whether all such systems are true AGN (powered by accretion on to a supermassive black hole). If they are, they constitute the lowest-luminosity class of radio-quiet AGN. Some may be radio-quiet analogues of the low-excitation radio galaxies (see below).
* [[Seyfert galaxies]]. Seyferts were the earliest distinct class of AGN to be identified. They show optical-range nuclear continuum emission, narrow and occasionally broad emission lines, occasionally strong nuclear X-ray emission, and sometimes a weak small-scale radio jet. Originally they were divided into two types, known as Seyfert 1 and Seyfert 2: Seyfert 1s show strong broad emission lines, while Seyfert 2s do not, and Seyfert 1s are more likely to show strong low-energy X-ray emission. Various forms of elaboration upon this scheme exist: For example, Seyfert 1s with relatively narrow broad lines are sometimes referred to as narrow-line Seyfert 1s. The host galaxies of Seyferts are usually spiral or irregular galaxies.
* Radio-quiet [[quasar]]s/QSOs. These are essentially more-luminous versions of Seyfert 1s. The distinction is arbitrary, and is usually expressed in terms of a limiting optical magnitude. 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.