sy2

Sy2, short for Seyfert type 2 galaxy, is a class of active galactic nuclei (AGN) defined by its optical spectrum, which shows strong narrow emission lines but lacks the broad emission lines produced by fast-moving gas near the central black hole. The emission lines are primarily from the narrow-line region and include lines such as [O III] 5007, [N II], and [S II], with velocities of a few hundred kilometers per second.

The central engine of a Sy2 is a accreting supermassive black hole, similar to other Seyfert types, but the broad-line region is obscured by a surrounding dusty torus. In the widely cited unified model of AGN, the observed differences between Seyfert 1 and Seyfert 2 galaxies arise mainly from orientation: when the torus blocks the line of sight to the broad-line region, only the narrow lines from the outer regions are visible, yielding a Sy2 spectrum.

In some Sy2 galaxies, a broad-line region can be detected indirectly through polarized light or infrared spectroscopy, revealing a hidden broad-line region (HBLR). X-ray observations often show strong absorption, consistent with a line of sight through the torus; in some cases the nucleus is Compton-thick, making the core difficult to detect at soft X-ray energies. The host galaxies are frequently spiral or lenticular, and Sy2 activity can coexist with circumnuclear star formation.

NGC 1068 is a canonical example. Sy2 galaxies are important for studies of AGN unification, obscuration, and the interaction between supermassive black holes and their host galaxies.