  I report the discovery of blueshifted broad absorption line (BAL) troughs in
at least six transitions of the Balmer series of hydrogen (Hbeta to H9) and in
CaII, MgII and excited FeII 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 about
\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.
