摘要：We report on the discovery of a peculiar broad absorption line (BAL) quasar identified in ourGaia-assisted survey of red quasars. The systemic redshift of this quasar was difficult to establish because of the absence of conspicuous emission lines. Based on deep and broad BAL troughs of at least Si IV, C IV, and Al III, a redshift ofz = 2.41 was established under the assumption that the systemic redshift can be inferred from the red edge of the BAL troughs. However, we observe a weak and spatially extended emission line at 4450 Å that is most likely due to Lyman-αemission, which implies a systemic redshift ofz = 2.66 if correctly identified. There is also evidence for the onset of Lyman-αforest absorption bluewards of 4450 Å and evidence for Hαemission in theKband consistent with a systemic redshift ofz = 2.66. If this redshift is correct, the quasar is an extreme example of a detached low-ionisation BAL quasar. The BALs must originate from material moving with very large velocities ranging from 22 000 km s−1to 40 000 km s−1. To our knowledge, this is the first case of a systemic-redshift measurement based on extended Lyman-αemission for a BAL quasar. This method could also be useful in cases of sufficiently distant BL Lac quasars without systemic-redshift information.
