Narrow Line Active Galaxies

S2.0

S2.0 (Sy2.0, Seyfert 2.0, also may be given as S2, as there are no other subtypes) – AGN with spectra displaying relatively narrow (compared to Sy1) resolved emission Balmer and forbidden lines with almost equal FWHM (≥300km/s), usually in the range of 300-1000 km/s (Khachikian & Weedman, 1974, Weedman & Khachikyan, 1968) that originate in a low-density medium (n_e ≈ 10³-10⁶ cm^-3). The broad component is not visible. The condition [OIII]5007/H_β ≥ 3, serves as a secondary classification criterion, so they can be distinguished against NLS1s (Lawrence, 1987, Veilleux & Osterbrock, 1987). NGC 1068 is the prototype.

LINER

LINER (Low-Ionization Nuclear Emission-line Region, also given as S3, Seyfert 3). Galactic nuclei with emission lines formed in low ionization regions. Introduced by Heckman (1980), they are low activity AGN, the weakest form of AGN activity. They have S2-like spectra with relatively strong low-ionization lines ([OI], [OII]). The ratios [OII]3727/[OIII]5007 ≥ 1, [OI]6300/[OIII]5007 ≥ 1/3. [NII]6584/H_α > 0.6 according to Kauffmann et al. (2003). According to Ho et al. (1997), there are 2 classes of LINERs: type 1 shows broad Balmer emission analogous to S1s (weak broad H_α visible), and type 2, without broad H_α analogous to S2s. May be either radio quiet or radio loud. Most of the nuclei of nearby galaxies are LINERs. However, their emission line spectra are not necessarily caused by active nuclei.

HII

HII (HII regions, H2) – Isolated extragalactic HII regions as defined by Sargent & Searle (1970) or HII galaxies as defined by Terlevich et al. (1991). Have spectra similar to SB, i. e., spectra with strong narrow (FWHM ≤ 300 km/s) emission lines but with ratios [OIII]/H_β ≥ 3 and [NII]6583/H_α < 0.6, combined with a blue continuum (Veilleux & Osterbrock, 1987, Winkler, 1992). Essentially the same as SB, but for classification of SB the value of the star formation rate (SFR) is needed.