A SCUBA-2 survey of FeLoBAL QSOs: Are FeLoBALs in a `transition phase' between ULIRGs and QSOs?

TL;DR

This study used deep submillimetre observations to investigate whether FeLoBAL QSOs are a transitional phase between ULIRGs and QSOs, finding no evidence of enhanced starburst activity and suggesting they are not a distinct transition population.

Contribution

The paper provides the first systematic submillimetre survey of FeLoBAL QSOs, showing they are not uniquely associated with intense starburst activity, challenging the transition-phase hypothesis.

Findings

FeLoBALs are not luminous in the submillimetre range.

None of the FeLoBALs are individually detected at 850 μm.

FeLoBALs have properties similar to other QSOs, with no evidence of a cold starburst component.

Abstract

It is thought that a class of broad absorption line (BAL) QSOs, characterised by Fe absorption features in their UV spectra (called `FeLoBALs'), could mark a transition stage between the end of an obscured starburst event and a youthful QSO beginning to shed its dust cocoon, where Fe has been injected into the interstellar medium by the starburst. To test this hypothesis we have undertaken deep SCUBA-2 850 $\mu$m observations of a sample of 17 FeLoBAL QSOs with 0.89 $\leq$ z $\leq$ 2.78 and -23.31 $\leq$ M$_{B}$ $\leq$-28.50 to directly detect an excess in the thermal emission of the dust which would probe enhanced star-formation activity. We find that FeLoBALs are not luminous sources in the submillimetre, none of them are individually detected at 850 $\mu$m, nor as a population through stacking ($F_{s}=1.14\pm0.58$ mJy). Statistical and survival analyses reveal that FeLoBALs have sub-mm properties consistent with BAL and non-BAL QSOs with matched redshifts and magnitudes. An SED fitting analysis shows that the FIR emission is dominated by AGN activity, and a starburst component is required only in 6/17 sources of our sample; moreover the integrated total luminosity of 16/17 sources is L$\geq$10$^{12}$L$_{\odot}$, high enough to classify FeLoBALs as infrared luminous. In conclusion, we do not find any evidence in support of FeLoBAL QSOs being a transition population between a ULIRG and an unobscured QSO; in particular, FeLoBALs are not characterized by a cold starburst which would support this hypothesis.