Radio Loud Broad Absorption Line Quasars

This thesis is concerned with Broad Absorption Line Quasars (BAL QSOs). At the moment it is not clear how many quasars host outflows and why these can only be seen in a fraction of the quasar population. The most popular hypotheses proposed to explain this mainly differ in the role given to orientation. In this work, we concentrate on a sample of BAL QSOs showing radio emission at 1.4 GHz and we try to find evidence about the nature of these objects through radio observations with various telescopes.

We present multi-frequency observations of a sample of 15 radio-emitting BAL QSOs, covering a spectral range between 74 MHz and 43 GHz. They display mostly convex radio spectra which typically peak at about 1-5 GHz (in the observer’s rest-frame), flatten at MHz frequencies, probably due to synchrotron self-absorption, and become steeper at high frequencies, i.e., ν >~ 20 GHz. VLA 22-GHz maps (HPBW ∼ 80 mas) show unresolved or very compact sources, with linear projected sizes of ≤ 1 kpc. About 2/3 of this sample look unpolarised or weakly polarised at 8.4 GHz, a frequency in which reasonable upper limits could be obtained for polarised intensity. Statistical comparisons have been made between the spectral index distributions of samples of BAL and non-BAL QSOs, both in the observed and the rest-frame, finding steeper spectra among non-BAL QSOs. However constraining this comparison to compact sources results in no significant differences between both distributions. This comparison is consistent with BAL QSOs not being oriented along a particular line of sight. In addition, our analysis of the spectral shape, the variability and polarisation properties shows that radio BAL QSOs share several properties common to young radio sources like Compact Steep Spectrum (CSS) or Gigahertz-Peaked Spectrum (GPS) sources.

A synchrotron ageing analysis has been done and we discuss the different parameters contributing to the uncertainty in the radiative age determination. With the current observational constraints, and the assumption of equipartition conditions, radiative ages have been determined in the range 1-50 kyr, suggesting that the radio brightest components in BAL QSOs would actually be young.

Since BAL QSOs have been found to be associated with extremely compact radio sources, we present VLBA milliarcsecond-scale observations of 5 BAL QSOs at 5.0 and 8.4 GHz. We discuss the pc-scale morphology of these objects and compare them with observations of other radio BAL QSOs in the literature. A relatively high fraction of unresolved, unpolarised sources and indications of restarting radio activity in some of the targets favours the evolutionary hypothesis to explain the origin of radio BAL QSOs.

Finally, we make a detection experiment trying to detect H i in absorption in one of the most interesting radio BAL QSOs in our sample. A non-detection allows us to estimate some upper limits of the contents of H i for this source.