A striking relationship between dust extinction and radio detection in DESI QSOs: evidence for a dusty blow-out phase in red QSOs

TL;DR

This study reveals a strong correlation between dust extinction and radio emission in red QSOs, supporting the idea that they are in a transitional phase where outflows clear dust, revealing the central engine.

Contribution

It provides the first evidence linking dust reddening and radio detection in QSOs, suggesting a dusty blow-out phase in QSO evolution.

Findings

Positive correlation between dust extinction and radio detection in QSOs.

Red QSOs likely represent a transitional phase with winds and outflows.

Radio emission is associated with low-powered jets or shocks in dusty environments.

Abstract

We present the first eight months of data from our secondary target program within the ongoing Dark Energy Spectroscopic Instrument (DESI) survey. Our program uses a mid-infrared and optical colour selection to preferentially target dust-reddened QSOs that would have otherwise been missed by the nominal DESI QSO selection. So far we have obtained optical spectra for 3038 candidates, of which ~70% of the high-quality objects (those with robust redshifts) are visually confirmed to be Type 1 QSOs, consistent with the expected fraction from the main DESI QSO survey. By fitting a dust-reddened blue QSO composite to the QSO spectra, we find they are well-fitted by a normal QSO with up to Av~4 mag of line-of-sight dust extinction. Utilizing radio data from the LOFAR Two-metre Sky Survey (LoTSS) DR2, we identify a striking positive relationship between the amount of line-of-sight dust extinction towards a QSO and the radio detection fraction, that is not driven by radio-loud systems, redshift and/or luminosity effects. This demonstrates an intrinsic connection between dust reddening and the production of radio emission in QSOs, whereby the radio emission is most likely due to low-powered jets or winds/outflows causing shocks in a dusty environment. On the basis of this evidence we suggest that red QSOs may represent a transitional "blow-out" phase in the evolution of QSOs, where winds and outflows evacuate the dust and gas to reveal an unobscured blue QSO.