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

¹ Cosmic DAWN Center, Vibenshuset, Lyngbyvej 2, 2100 København Ø, Denmark
² Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, 2100 Copenhagen Ø, Denmark
e-mail: jfynbo@nbi.ku.dk
³ European Southern Observatory, Karl-Schwarzschildstrasse 2, 85748 Garching, Germany
⁴ Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107, Reykjavík, Iceland
⁵ University of California, 1156 High St., Santa Cruz, CA 95064, USA
⁶ DARK, Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, 2100 Copenhagen Ø, Denmark
⁷ Instituto de Astrofísica de Canarias, Vía Láctea, s/n, 38205, La Laguna, Tenerife, Spain
⁸ Gran Telescopio Canarias (GRANTECAN), 38205 San Cristóbal de La Laguna, Tenerife, Spain
⁹ Institut d’Astrophysique de Paris, CNRS-SU, UMR 7095, 98bis bd Arago, 75014 Paris, France
¹⁰ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001 Santiago, Chile
¹¹ Department of Astronomy, University of Massachusetts, 710 North Pleasant Street, Amherst, MA 01003, USA

Received: 19 October 2019
Accepted: 22 December 2019

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 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 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 Å that is most likely due to Lyman-α emission, which implies a systemic redshift of z = 2.66 if correctly identified. There is also evidence for the onset of Lyman-α forest absorption bluewards of 4450 Å and evidence for Hα 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 BALs must originate from material moving with very large velocities ranging from 22 000 km s⁻¹ to 40 000 km s⁻¹. To our knowledge, this is the first case of a systemic-redshift measurement based on extended Lyman-α emission for a BAL quasar. This method could also be useful in cases of sufficiently distant BL Lac quasars without systemic-redshift information.