Gaia-assisted discovery of a detached low-ionisation BAL quasar with very large ejection velocities

TL;DR

This paper reports the discovery of a unique detached low-ionisation BAL quasar with extremely high ejection velocities, identified through Gaia data and novel systemic redshift determination methods involving Lyman-alpha emission.

Contribution

It introduces a new systemic redshift measurement technique using extended Lyman-alpha emission for BAL quasars, revealing an extreme case with very large ejection velocities.

Findings

Discovery of a detached low-ionisation BAL quasar with velocities 22000-40000 km/s.

First systemic redshift measurement based on extended Lyman-alpha emission in a BAL quasar.

Identification of a quasar with very large ejection velocities and peculiar spectral features.

Abstract

We report on the discovery of a peculiar Broad Absorption Line (BAL) quasar identified in our Gaia-assisted survey of red quasars. The systemic redshift of this quasar was difficult to establish due to the absence of conspicuous emission lines. Based on deep and broad BAL troughs (at least SiIV, CIV, and AlIII), a redshift of z=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 AA most likely due to Lyman-alpha emission, which implies a systemic redshift of z=2.66 if correctly identified. There is also evidence for the onset of Lyman-alpha forest absorption bluewards of 4450 AA and evidence for H-alpha emission in the K-band consistent with a systemic redshift of z=2.66. If this redshift is correct, the quasar is an extreme example of a detached low-ionisation BAL quasar. The BAL lines must originate from material moving with very large velocities ranging from 22000 to 40000 km/s. To our knowledge, this is the first case of a systemic-redshift measurement based on extended Lyman-$\alpha$ emission for a BAL quasar, a method that should also be useful in cases of sufficiently distant BL Lac quasars without systemic-redshift information.