narrow-line region
/ NLR /
An AGN's spectrum often shows two kinds of emission lines at once: some smeared broad, and others staying sharp and narrow. The sharp ones come from gas that is moving much more sedately. That gas, sitting far out from the black hole, is the narrow-line region. If the broad-line region is the fast inner whirl, the narrow-line region is the slow, sprawling outskirts.
Narrow-line gas moves at only hundreds of kilometres per second — fast by everyday standards but slow for an AGN — so its emission lines are barely Doppler-smeared and appear narrow. Being slow means it orbits far from the black hole, typically tens to thousands of light-years out, well beyond the dusty torus. It is also more thinly spread (lower density), which lets it produce certain 'forbidden' emission lines that dense inner gas cannot. Because it sits so far out, the narrow-line region is the one part of an AGN that we sometimes can resolve as an extended glow rather than a single point.
The narrow-line region is the great equalizer of AGN classification: it lies outside the obscuring torus, so we see its narrow lines from essentially any viewing angle. That is why both type 1 and type 2 AGN show narrow lines, while only type 1s show broad ones. The narrow lines also map where the AGN's radiation and outflows are striking the surrounding galaxy — making the narrow-line region a window onto AGN feedback, the way the central engine pushes back on its host.
In the nearby galaxy NGC 1068, telescopes resolve a cone-shaped glow of narrow-line gas stretching hundreds of light-years from the core — lit up where the hidden central engine's radiation escapes through gaps and ionizes the surrounding interstellar gas.
Slow, far-out gas making sharp lines — visible from almost any angle.
Narrow does not mean weak or unimportant — these lines are 'narrow' only relative to the broad-line region's frantic motion. Because the narrow-line region sits outside the torus, its survival in type 2 AGN is exactly the clue that the broad lines were merely hidden, not absent.