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

TL;DR

This study investigates the incidence and properties of broad absorption line (BAL) winds in hyper-luminous quasars, revealing higher BAL fractions and velocities compared to lower luminosity AGN, with implications for quasar feedback mechanisms.

Contribution

It provides the first detailed analysis of BAL quasar properties in hyper-luminous quasars, highlighting their higher BAL fraction and wind velocities, and discusses their potential role in galaxy feedback.

Findings

Higher BAL fraction (24%) in hyper-luminous quasars compared to lower luminosity AGN.

BAL winds in these quasars have larger velocities, up to 17,000 km/s.

BAL winds likely driven by radiation pressure, with sufficient kinetic power for galaxy feedback.

Abstract

The WISSH quasars project aims at studying the nuclear and host galaxy properties of the most luminous quasars ($L_{bol}>10^{47}$ erg/s, $1.8<z<4.6$). Nuclear winds are manifested as UV broad ($\geq$ 2,000 km/s) absorption lines (BAL) in $\sim$ 15\% of quasars. We aim at studying the incidence and properties of such winds in the WISSH sample, to investigate possible differences with respect to lower luminosity AGN regimes. We collected optical spectra from the SDSS data release 12, and identified those showing absorption troughs in the region between \siiv and \civ emission lines. We find a higher observed fraction of \civ BAL quasars in the WISSH sample (24\%) with respect to previous catalogues (10-15\%). These WISSH BAL quasars are also characterized by a larger average BI ($\sim$4,000 km/s) and maximum velocity ($\sim$17,000 km/s). Moreover, for two objects we discovered BAL features bluewards of the \siiv peak, which can be associated to \civ absorption with velocity of 0.15c. Finally, we updated previous studies on the dependence of maximum outflow velocity upon bolometric luminosity, showing that BAL winds have intermediate properties compared to molecular/ionized winds and ultra fast outflows (UFOs). 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. Our results suggest that the higher $L_{\rm bol}$ 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 to these winds in hyperluminous quasars is sufficient, for the highest column density and fastest winds, to provide efficient feedback onto the host galaxy.