BROAD BALMER ABSORPTION LINE VARIABILITY: EVIDENCE OF GAS TRANSVERSE MOTION IN THE QSO SDSS J125942.80+121312.6

We report on the discovery of broad Balmer absorption lines variability in the QSO SDSS J125942.80+121312.6, based on the optical and near-infrared spectra taken from the SDSS-I, SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS), and TripleSpec observations over a timescale of 5.8 years in the QSO's rest -frame. The blueshifted absorption profile of Hi(3) shows a variation of more than 50-at a high velocity portion (>3000 km s-1) of the trough. We perform a detailed analysis for the physical conditions of the absorber using Balmer lines as well as metastable Her and optical Fen absorptions (lambda 4233 from b(4)P(5/2) level and lambda 5169 from a(6)55/2) at the same velocity. These Fe n lines are identified in the QSO spectra for the first time. According to the photoionization simulations, we estimate a gas density of n(H) approximate to 1091 cm(-3) and a column density of N-col (H) approximate to 10(23) cm(-2) for the BOSS data, but the model fails to predict the variations of ionic column densities between the SDSS and BOSS observations if changes in ionizing flux are assumed. We thus propose transverse motion of the absorbing gas being the cause of the observed broad Balmer absorption line variability. In fact, we find that the changes in covering factors of the absorber can well -reproduce all of the observed variations. The absorber is estimated -0.94 pc away from the central engine, which is where the outflow likely experiences deceleration due to the collision with the surrounding medium. This scheme is consistent with the argument that LoBAL QSOs may represent the transition from obscured star -forming galaxies to classic QSOs.