An Infrared Excess Identified in Radio-Loud Broad Absorption Line Quasars

TL;DR

This study reveals that radio-loud BAL quasars exhibit a significant infrared excess compared to normal quasars, indicating potential evolutionary factors influencing the presence of broad absorption lines.

Contribution

First analysis of IR properties of radio-loud BAL quasars showing an infrared excess, suggesting an evolutionary component in BAL quasar characteristics.

Findings

BAL quasars have higher mid-infrared luminosities at 1.5 and 4 microns.

Infrared excess supports an evolutionary scenario for BAL quasars.

Results imply a need to revise current BAL quasar models.

Abstract

If broad absorption line (BAL) quasars represent a high covering fraction evolutionary state (even if this is not the sole factor governing the presence of BALs), it is expected that they should show an excess of mid-infrared radiation compared to normal quasars. Some previous studies have suggested that this is not the case. We perform the first analysis of the IR properties of radio-loud BAL quasars, using IR data from WISE and optical (rest-frame ultraviolet) data from SDSS, and compare the BAL quasar sample with a well-matched sample of unabsorbed quasars. We find a statistically significant excess in the mid- to near-infrared luminosities of BAL quasars, particularly at rest-frame wavelengths of 1.5 and 4 microns. Our sample was previously used to show that BALs are observed along many lines of sight towards quasars, but with an overabundance of more edge-on sources, suggesting that orientation factors into the appearance of BALs. The evidence here---of a difference in IR luminosities between BAL quasars and unabsorbed quasars---may be ascribed to evolution. This suggests that a merging of the current BAL paradigms is needed to fully describe the class.