Balmer decrement

• HII region: case B Balmer recombination decrement Ha/Hb = 2.85 for T = 10^4 K and Ne=10^4 cm-3

(Brocklehurst 1971).

• In AGNs, however, the harder photoionizing spectrum results in a large transition zone, or partly ionized region, in which H° coexists with H

+ and free electrons. In this zone collisional excitation is also important in addition to recombination collisional excitation (Ferland and Netzer 1983; Halpem and Steiner 1983). The main effect of collisional excitation is to enhance Ha. The higher Balmer lines are less affected because of their larger excitation energies and smaller excitation cross sections.(Veilleux & Osterbrock (1987).

• Ha/Hb = 3.1 (Ferland and Netzer 1983, Péquignot 1984, Gaskell 1984; Gaskell and Ferland 1984).