The origin of Broad Absorption Line Quasars (BAL QSOs) is still an open issue. Accounting for ∼ 20% of the QSO population, these objects present broad absorption lines in their optical spectra generated from outflows with velocities up to 0.2c. There is still no consensus about the origin of the absorbing gas in BAL QSOs, the mechanism which accelerates it, or the relationship between BAL QSOs and the quasar population as a whole.

Nowadays, the hypotheses about their nature are principally related to orientation or evolutionary scenarios. In the first one, absorption lines are produced by outflows originated by the accretion disk, basically present in all QSOs, but seen only when they intercept the line of sight. In the second hypothesis, BAL QSOs would be young or recently re-fueled QSOs, still ejecting their dust cocoon. In this case orientation would not play a role, since the absorption features would be produced by spherically ejected matter.

In this work we present the results of a multi-frequency study of a Radio-Loud BAL QSO sample, and a comparison sample of Radio-Loud non-BAL QSOs, both selected from the Sloan Digital Sky Survey (SDSS). We performed observations in the radio band, to sample the SED in the GHz range and study the characteristics of the synchrotron emission, aiming at collecting indications about the age and the orientation of the central radio source, as well as the polarisation properties. The comparison with the non-BAL QSO sample allows us to conclude that no particular orientation is present in BAL QSOs. Nevertheless, similar fractions of GHz-peaked sources in the two sample (36±12% vs 23±8%) does not seem to suggest a young age for BAL QSOs, and this conclusion is favoured by the presence, in some cases, of low-frequency, presumably old components in the radio spectrum.

Through the VLBI technique, it was possible to study the pc-scale radio-morphology of half the sample. About 80% of sources present a resolved structure, with projected linear sizes comprised between tens and hundreds of pc. This fraction is comparable with previous results from literature. The missing flux density with respect to observations at kpc-scale resolution suggests a low frequency, diffuse component in some cases. The variety of morphologies does not support a particular orientation.

We also investigated the dust grey-body emission at mm-wavelengths, to verify whether BAL QSOs are dust-rich, physical condition requested by the evolutionary model. Only 7% of the sources present a clear dust contribution at 250 GHz. Not being dust rich, BAL QSOs should not present an high star-formation rate. Thus, again, they should not be a particularly young class of objects.

Finally, using a sample of optically-bright Radio-Loud and Radio-Quiet QSOs, we collected spectra in the Near-Infrared band, to estimate the central black hole mass, the Eddington ratio, and the Broad Line Region radius, in order to underline differences induced by the Radio-Loud phase. From an analysis of the SDSS DR7 QSO catalogue, the Eddington ratio is the only physical quantity found to be significantly different in Radio-Loud BAL QSOs, 26% of these objects being super-Eddington, while only 13% of Radio-Quiet BAL QSOs show the same property. Even a bigger difference has been found between BAL and non-BAL QSOs as a whole (13% vs 2%).

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