Restarting radio activity and dust emission in radio-loud broad absorption line quasars

TL;DR

This study investigates radio-loud broad absorption line quasars, revealing their potential to be old or restarting radio sources with suppressed dust emission, and explores the connection between outflows and jet formation.

Contribution

It provides new low-frequency and mm-band observations of BAL QSOs, identifying extended radio emission and dust emission properties, and suggests outflows may be precursors to jets.

Findings

Extended radio emission detected at tens of kpc scales.

Most sources are in a GPS or CSS+GPS phase, indicating restarting activity.

Dust emission is weaker than in typical QSOs, implying suppressed star formation.

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.