The Compact Structure of Radio-Loud Broad Absorption Line Quasars

TL;DR

This study uses VLBI polarization observations to analyze the radio structures of BAL quasars, revealing their compactness, polarization properties, and possible evolutionary stage as young radio sources.

Contribution

It provides high-resolution polarization data on BAL quasars, suggesting they are young, compact radio sources with similar properties across different ionization and spectral types.

Findings

Most BAL quasars have unresolved or core-jet structures at sub-kiloparsec scales.

Polarization levels indicate synchrotron emission from the cores.

BAL quasars are likely young radio sources at an early evolutionary stage.

Abstract

We present the results of EVN+MERLIN VLBI polarization observations of 8 Broad Absorption Line (BAL) quasars at 1.6 GHz, including 4 LoBALs and 4 HiBALs with either steep or flat spectra on VLA scales. Only one steep-spectrum source, J1122+3124, shows two-sided structure on the scale of 2 kpc. The other four steep-spectrum sources and three flat-spectrum sources display either an unresolved image or a core-jet structure on scales of less than three hundred parsecs. In all cases the marginally resolved core is the dominant radio component. Linear polarization in the cores has been detected in the range of a few to 10 percent. Polarization, together with high brightness temperatures (from 2*10^9-5*10^10 K), suggest a synchrotron origin for the radio emission. There is no apparent difference in the radio orphologies or polarization between low-ionization and high-ionization BAL QSOs nor between flat- and steep-spectrum sources. We discuss the orientation of BAL QSOs with both flat and steep spectra, and consider a possible evolutionary scenario for BAL QSOs. In this scenario, BAL QSOs are probably the young population of radio sources, which are Compact Steep Spectrum or GHz peaked radio source analog at the low end of radio power.