Abstract
Broad absorption line quasars (BAL QSOs) are objects showing absorption from relativistic outflows, with velocities up to 0.2c. These manifest, in about 15% of quasars, as absorption troughs on the blue side of UV emission lines, such as C iv and Mg ii. In this work, we complement the information collected in the cm band for our previously presented sample of radio loud BAL QSOs with new observations at m and mm bands. Our aim is to verify the presence of old, extended radio components in the MHz range, and probe the emission of dust (linked to star formation) in the mm domain. We observed 5 sources from our sample, already presenting hints of low-frequency emission, with the GMRT at 235 and 610 MHz. Other 17 sources (more than half the sample) were observed with bolometer cameras at IRAM-30m and APEX. All sources observed with the GMRT present extended emission at a scale of tens of kpc. In some cases these measurements allow us to identify a second component in the SED, at frequencies below 1.4 GHz, beyond the one already studied in the GHz domain. In the mm-band, only one source shows emission clearly ascribable to dust. Upper limits were obtained for the remaining targets. These findings confirm that BAL QSOs can also be present in old radio sources, or even in restarting ones, where favourable conditions for the outflow launching/acceleration are present. A suggestion that these outflows could be precursors of the jet comes from the fact that ~70% of our sample can be considered in a GigaHertz Peaked Spectrum (GPS) or Compact Steep Spectrum (CSS)+GPS phase. This would confirm the idea proposed by other authors that these outflows could be recollimated to form the jet. Comparing with previous works in the literature, dust emission seems to be weaker than the what expected in 'normal' QSOs, suggesting that a feedback mechanism could inhibit star formation in radio-loud BAL QSOs.
Highlights
In the context of active galactic nuclei (AGNs), outflows from the central region are commonly detected as absorption lines in more bands
A suggestion that these outflows could be precursors of the jet comes from the possibility that ∼70% of our sample is in a GigaHertz Peaked Spectrum (GPS) or Compact Steep Spectrum (CSS)+GPS phase
We present the results from our observing campaign: the morphology could be studied with the GMRT interferometer, while images from the IRAM and Atacama Pathfinder Experiment (APEX) bolometers
Summary
In the context of active galactic nuclei (AGNs), outflows from the central region are commonly detected as absorption lines in more bands. The two main scenarios discussed in the literature tend to ascribe the BAL phenomenon to: 1) young objects, in which the strong nuclear starburst activity is still expelling a dust cocoon (Briggs et al 1984; Sanders 2002; Farrah et al 2007); or 2) normal QSOs, whose outflows intercept the line of sight of the observer (Elvis 2000). In this case, relativistic outflows are supposed to be commonly present in QSOs, but detected only when orientation is favourable. The variability of the BAL troughs has been explored by many authors in past years, thanks to the increase in available spectroscopic surveys data (Gibson et al 2008, 2010; Capellupo et al 2011, 2012; Vivek et al 2012) and a typical duty cycle of about a thousand years for the BAL-producing outflow has been found (Filiz Ak et al 2012, 2013)
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