Discovery of Balmer Broad Absorption Lines in the Quasar LBQS 1206+1052

We report the discovery of Balmer broad absorption lines (BALs) in the quasar LBQS 1206+1052 and present a detailed analysis of the peculiar absorption line spectrum. Besides Mg II $\lambda \lambda 2796,2803$ doublet, BALs are also detected in He I* multiplet at $\lambda \lambda 2946,3189,3889$ \AA arising from metastable helium $2^{3}S$ level, and in H$\alpha$ and H$\beta$ from excited hydrogen H I* $n=2$ level, which are rarely seen in quasar spectra. We identify two components in the BAL troughs of $\mathrm{\Delta }v\sim$2000 km s${}^{-1}$ width: One component shows an identical profile in H I*, \hei* and \mgii with its centroid blueshifted by $-v_{\mathrm{c}}\approx 726$ km\ s${}^{-1}$. The other component is detected in \hei* and \mgii with $-v_{\mathrm{c}}\approx 1412$ km s${}^{-1}$. We estimate the column densities of H I*, He I*, and Mg II, and compare them with possible level population mechanisms. Our results favor the scenario that the Balmer BALs originate in a partially ionized region with a column density of $N_{\mathrm{H}}\sim {10}^{21-22}$ cm${}^{-2}$ for an electron density of $n_e\sim 10^{6-8} $cm$^{-3}$viaLy$\alpha$ resonant scattering pumping. The harsh conditions needed may help to explain the rarity of Balmer absorption line systems in quasar spectra. With an $i$-band PSF magnitude of 16.50, LBQS 1206+1052 is the brightest Balmer-BAL quasar ever reported. Its high brightness and unique spectral properties make LBQS 1206+1052 a promising candidate for follow-up high-resolution spectroscopy, multi-band observations, and long-term monitoring.