The WISSH quasars project

VI. Fraction and properties of BAL quasars in the hyper-luminosity regime

¹ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
e-mail: gabriele.bruni@inaf.it
² INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Roma, Italy
³ School of General Education, Shinshu University 3-1-1 Asahi, Matsumoto 390-8621, Japan
⁴ ASI – Space Science Data Center, Via del Politecnico snc, 00133 Roma, Italy
⁵ Excellence Cluster Universe, Technische Universität München, Boltzmannstr. 2, 85748 Garching, Germany
⁶ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁷ DiFA – Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁸ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy
⁹ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
¹⁰ INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
¹¹ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹² Department of Astronomy, University of Maryland, College Park, MD 20742, USA
¹³ NASA/Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA

Received: 20 December 2018
Accepted: 20 August 2019

Abstract

Context. The WISSH quasars project aims at studying the nuclear and host galaxy properties of the most luminous quasars (L_bol >  10⁴⁷ erg s⁻¹, 1.8 <  z <  4.6), with special emphasis on the occurrence and physical parameters of winds at different scales.

Aims. Nuclear winds are manifested as UV-broad (≥2000 km s⁻¹) absorption lines (BAL) in about 15% of quasars. We aim at studying the incidence and properties of such winds in the WISSH sample to investigate possible differences to active galactic nucleus regimes with lower luminosity.

Methods. We collected optical spectra from the Sloan Digital Sky Survey (SDSS) data release 12, and identified those showing absorption troughs in the region between the Si IV and C IV emission lines. We used three different indices for BAL absorption: the classic balnicity index (BI), the absorption index (AI), and the intermediate AI₁₀₀₀.

Results. We find a higher observed fraction of C IV BAL quasars in the WISSH sample (24%) than in previous catalogues (10−15%). These WISSH BAL quasars are also characterised by a higher average BI (∼4000 km s⁻¹) and maximum velocity (∼17 000 km s⁻¹). Moreover, for two objects we discovered BAL features bluewards of the Si IV peak, which can be associated with C IV absorption with a velocity of 0.15c. We also updated previous studies on the dependence of maximum outflow velocity upon bolometric luminosity, showing that BAL winds have intermediate properties compared to molecular or ionised winds and ultra-fast outflows. Finally, the radio properties of the WISSH BAL quasars as a whole are in line with those of samples at lower luminosities from previous studies.

Conclusions. Our results suggest that the higher L_bol of the WISSH quasars likely favours the acceleration of BAL outflows and that their most likely driving mechanism is radiation pressure. Furthermore, we estimate that the kinetic power associated with these winds in hyperluminous quasars for the highest column density and fastest winds is sufficient to provide efficient feedback onto the host galaxy.