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We present the discovery of Balmer line absorption from H$\alpha$ toH$\gamma$ in an iron low-ionization broad absorption line (FeLoBAL) quasar SDSSJ152350.42+391405.2 (hereafter J1523), by the quasi-simultaneous optical andnear-infrared spectroscopy. The Balmer line absorption is at $z_{absor}$ =0.6039 +/-0.0021 and blueshifted by v=10,353 km/s with respect to the Balmeremission lines. All Balmer BALs have uniform absorption profile with the widthsof $\Delta$ v ~12,000 km/s. We also found the absorption trough in He 1*$\lambda$10830 with the same velocity and width in the H-band TripleSpecspectrum of J1523. This object is only the tenth active galactic nucleus knownto exhibit non-stellar Balmer absorption, and also the case with the highestvelocity and broadest Balmer absorption lines which have ever been found. ACLOUDY analysis shows that the absorbers require an gas density of $log_{10} n_e (cm^{-3})=9$ and an ionization parameter of $log_{10} U=-1.0$. They locate ata distance of ~0.2 pc from the central ionizing source which is slightlyfarther than that of BELRs. Furthermore, J1523 is one of the brightestBalmer-BAL quasar ever reported, with unique iron absorption variations, makingit as the most promising candidate for follow up high-resolution spectroscopy,multi-band observations, and long-term monitoring.

Discovery of Extremely Broad Balmer Absorption Lines in SDSS J152350.42+391405.2