NLS1

Narrow-Line Seyfert 1 galaxies (NLS1) are a subclass of active galactic nuclei characterized by distinctive optical emission-line properties. The class was defined by Osterbrock and Pogge in 1985. The defining criteria are that the broad component of the Hβ emission line has a full width at half maximum (FWHM) of 2000 km/s or less, the [O III] λ5007 line is relatively weak with [O III]/Hβ less than about 3, and there is strong Fe II emission in the optical spectrum, often quantified by a high Fe II to Hβ ratio (R4570).

NLS1s exhibit strong soft X-ray excesses and typically possess steep soft X-ray spectra along with rapid X-ray variability. They tend to have relatively narrow broad-line regions, which, combined with other features, is interpreted as indicating high accretion rates relative to the Eddington limit. Their black hole masses are generally considered to be lower than those of typical broad-line Seyfert 1 galaxies, often in the range of 10^6 to 10^7 solar masses, which complements the picture of high Eddington ratios.

Host galaxies of NLS1s are commonly late-type spirals with ongoing star formation, though the host properties can vary. Some NLS1s appear to be in an early stage of black hole growth or early AGN evolution, contributing to discussions about the connection between accretion physics and host galaxy development.

Most NLS1s are radio-quiet, but a subset is radio-loud, and a few have been detected in gamma rays by facilities such as Fermi-LAT, indicating the presence of relativistic jets in at least some objects. The study of NLS1s informs models of black hole growth, accretion physics, and the diversity of AGN phenomena.