Morphology and orientation of radio-loud Broad Absorption Line quasars

Abstract

ABSTRACT: BAL QSOs are still a not-well understood class of objects. In the UV spectra they show Broad Absorption Lines (BALs) in the blue wings of the UV resonance lines, due to ionized gas with outflow velocities up to 0.2 c. Two different models have been proposed to explain this phenomenon: in the orientation model BAL-producing outflows should be present in all QSOs, but seen only when they intercept the observer’s line of sight. In the evolutionary model BAL QSOs are young sources still expelling their dust cocoon. We performed VLBI observations with both the EVN (4.8 GHz) and VLBA (4.8 and 8.4 GHz) to map the pc-scale structure of the brightest radio-loud objects of our sample. A variety of morphologies and orientations have been found: 5 BAL QSOs in a total of 9 observed sources have a resolved structure, with a linear size < 1 kpc. In some cases the spectral index analysis of single components suggests a beamed emission toward the observer, in other cases a symmetric structure is evident from the map. From VLBI observations BAL QSOs do not seem to have a preferred orientation. Dimensions are typical of young GPS-CSS sources. This evidence could indicate an evolutionary scenario for the origin of this class of quasars.