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

TL;DR

This paper reports the discovery of rare Balmer broad absorption lines in the quasar LBQS 1206+1052, providing detailed spectral analysis and insights into the physical conditions of the absorbing gas.

Contribution

It is the first detailed analysis of Balmer BALs in a quasar, revealing their origin in a partially ionized, high-density region and explaining their rarity.

Findings

Balmer BALs detected in LBQS 1206+1052

Two absorption components with different velocities identified

Physical conditions favor Lyα resonant scattering pumping as the origin

Abstract

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 $2^3S$ 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 $\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 $-v_{\rm c}\approx 726$ km\ s$^{-1}$. The other component is detected in \hei* and \mgii with $-v_{\rm c}\approx 1412$ 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 $N_{\rm H}\sim 10^{21-22}$ cm$^{-2}$ for an electron density of $n_e\sim 10^{6-8} $cm$^{-3}$ via Ly$\alpha$ 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.