Broad Absorption Lines Quasars: VLBI survey

Jets and outflows are two main powerful processes transporting material out of the active galactic nuclei (AGN). Both are important feedback processes which means that they can efficiently interact with the surrounding medium or even self-regulate the growth of the supermassive black hole. Outflows are moving outward at lower speeds than jets but they can carry thousands of times more mass flux per unit of kinetic luminosity than collimated relativistic jets observed in 10% of all AGNs. We can observe them indirectly as blueshifted Broad Absorption Lines (BALs) in quasar spectrum. However, only about 15% of the whole population of quasars shows BALs in their spectra. This could be explained by the quasar unification scheme in which outflows are present in every quasar but appear as BALs in the spectrum only if they are seen under specific inclination. Another explanation, which emerged with the discovery of radio-loud BAL quasars, suggests that absorption lines are only present in the early evolution phase of quasars.

Discovery of the existence of radio-loud BAL quasars gave us another opportunity to study the BAL phenomenon, this time on the basis of radio emission. The radio emission is an additional tool to understand their orientation and age by the VLBI imaging (detection of radio jets and their direction, size determination), the radio-loudness parameter distribution and variability study. The studies of the radio properties carried out by us on the sample of compact radio-loud BAL quasars brought some clues on the BAL phenomenon.

We performed multi-frequency high resolution radio observations of a sample of BAL quasars with EVN at 1.7 GHz and with VLBA at 5 and 8.4 GHz. Most of the sources were resolved showing, typical for radio-loud quasars, core-jet morphology. Their high radio luminosities and small linear sizes (less than 6 kpc) indicate they are strong young AGNs (Fig.1).

Fig.1: Quasar 1223+5037 observed with EVN at 1.7 GHz and VLBA at 5 and 8.5 GHz. Contours increase by a factor 2, and the first contour level corresponds to 3σ. J1 and J2 indicate parts of a radio jet. C indicates a radio core resolved at higher frequencies into two components.

We then used the largest available sample of BAL quasars to study the relationships between the radio and optical properties in these objects. We found that the stronger absorption is connected with the lower values of the radio- loudness parameter, log R , and thus probably with larger viewing angles. Therefore, the orientation is an important parameter that affects the amount of the measured absorption. Nevertheless the lack of correlation between the value of absorption and parameter R suggest that some additional factors may be significant here, e.g. weak radio emission.

Fig.2: (Left) The distribution of radio-loudness parameter log R for the studied sample of radio-loud BALQSOs selected from Gibson et al.(2009). (Right) Radio-loudness vs. the value of modified balnicity index BI0 for the same sample of radio-loud BALQSOs.