AGN unification

For decades astronomers kept naming new kinds of active galaxy — quasars, Seyfert 1s and 2s, radio galaxies, blazars, BL Lacs — as if they were a whole zoo of different beasts. The unification idea is a beautifully simple rescue: maybe most of these are not different objects at all, but the same basic engine seen under different circumstances. Change a few things — chiefly the angle you view it from — and one creature wears all the costumes.

The shared engine is always a supermassive black hole, an accretion disk, fast broad-line gas, slower narrow-line gas, a dusty torus, and sometimes jets. Unification says the apparent zoo comes mostly from three knobs. First, viewing angle: look down the torus's hole and you see the broad-line region (a 'type 1'); look through the torus edge-on and the broad lines are hidden, leaving only narrow lines (a 'type 2'); look straight down a jet and you get a blazar; look at that same jet from the side and you get a radio galaxy. Second, luminosity (how fast the hole is feeding) separates a faint Seyfert from a brilliant quasar. Third, whether the AGN launches strong jets ('radio-loud' versus 'radio-quiet').

Unification is one of astrophysics' great organizing ideas, and it has strong evidence behind it — most strikingly, finding hidden broad lines in the polarized light of type 2 AGN, exactly as a buried 'type 1' engine would predict. But it is a framework, not a finished law: some type 2 AGN genuinely seem to lack a broad-line region, and what makes an AGN radio-loud is still unsettled. The honest summary is that orientation explains a great deal of the AGN zoo, but not quite all of it.