Dependence of the BALQSO fraction on Radio Luminosity

TL;DR

This study investigates how the fraction of BALQSOs varies with radio luminosity, revealing a significant decrease at higher powers and suggesting different physical origins for subclasses of BALQSOs.

Contribution

It provides the first comprehensive analysis of BALQSO fraction dependence on radio luminosity across a large sample, confirming previous trends and identifying potential subclasses.

Findings

BALQSO fraction drops from ~20% at low radio power to <8% at high power

The trend is consistent across redshift, optical, and radio selection criteria

AI-BALQSOs without classical features do not show the same decline

Abstract

We find that the fraction of classical Broad Absorption Line quasars (BALQSOs) among the FIRST radio sources in the Sloan Data Release 3, is 20.5^{+7.3}_{-5.9}% at the faintest radio powers detected (L_{\rm 1.4 GHz}~10^{32} erg/s), and rapidly drops to <8% at L_{\rm 1.4 GHz}~3*10^{33} erg/s. Similarly, adopting the broader Absorption Index (AI) definition of Trump et al. (2006) we find the fraction of radio BALQSOs to be 44^{+8.1}_{-7.8}% reducing to 23.1^{+7.3}_{-6.1}% at high luminosities. While the high fraction at low radio power is consistent with the recent near-IR estimates by Dai et al. (2008), the lower fraction at high radio powers is intriguing and confirms previous claims based on smaller samples. The trend is independent of the redshift range, the optical and radio flux selection limits, or the exact definition of a radio match. We also find that at fixed optical magnitude, the highest bins of radio luminosity are preferentially populated by non-BALQSOs, consistent with the overall trend. We do find, however, that those quasars identified as AI-BALQSOs but \emph{not} under the classical definition, do not show a significant drop in their fraction as a function of radio power, further supporting independent claims for which these sources, characterized by lower equivalent width, may represent an independent class with respect to the classical BALQSOs. We find the balnicity index, a measure of the absorption trough in BALQSOs, and the mean maximum wind velocity to be roughly constant at all radio powers. We discuss several plausible physical models which may explain the observed fast drop in the fraction of the classical BALQSOs with increasing radio power, \emph{although no one is entirely satisfactory}. (abridged).