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

الأصل · EN

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 λλ2796, 2803 doublet, BALs are also detected in He I* multiplet at λλ2946, 3189, 3889 Åarising from metastable helium 2³S level, and in Hα and Hβ from excited hydrogen H I* n=2 level, which are rarely seen in quasar spectra. We identify two components in the BAL troughs of Δv 2000 km s⁻¹ width: One component shows an identical profile in H I*, and with its centroid blueshifted by -v c≈ 726 kms⁻¹. The other component is detected in and with -v c≈ 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 H 10²¹⁻²² cm⁻² for an electron density of nₑ 10⁶⁻⁸ cm⁻³ via Lyα 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.