I report the discovery of blueshifted broad absorption line ( BAL ) troughs in at least six transitions of the Balmer series of hydrogen ( H \beta to H9 ) and in Ca ii , Mg ii and excited Fe ii in the quasar SDSS J125942.80+121312.6 .
This is only the fourth active galactic nucleus known to exhibit Balmer absorption , all four in conjunction with low-ionization BAL systems containing excited Fe ii .
The substantial population in the n = 2 shell of H i in this quasar ’ s absorber likely arises from Ly \alpha trapping .
In an absorber sufficiently optically thick to show Balmer absorption , soft X-rays from the quasar penetrate to large \tau _ { Ly \alpha } and ionize H i. Recombination then creates Ly \alpha photons that increase the n = 2 population by a factor \tau _ { Ly \alpha } since they require \simeq \tau _ { Ly \alpha } scatterings to diffuse out of the absorber .
Observing Ly \alpha trapping in a quasar absorber requires a large but Compton-thin column of gas along our line of sight which includes substantial H i but not too much dust .
Presumably the rarity of Balmer-line BAL troughs reflects the rarity of such conditions in quasar absorbers .