Connection between steep radio spectral slopes and dust extinction in QSOs: evidence for outflow-driven shocks in dusty QSO

Abstract

Recent studies have found a striking positive correlation between the amount of dust obscuration and enhanced radio emission in quasi-stellar objects (QSOs). However, what causes this connection remains unclear. In this paper we analyse uGMRT Band-3 (400 MHz) and Band-4 (650 MHz) data of a sample of 38 $1.0 < z < 1.5$ QSOs with existing high-resolution $0.2''$ e-MERLIN 1.4 GHz imaging. In combination with archival radio data, we have constructed sensitive 4-5 band radio SEDs across 0.144-3 GHz to further characterize the radio emission in dusty QSOs. We find that the dusty QSOs (those with E(B-V) $> 0.1$ mag) are more likely to exhibit steep spectral slopes ($\alpha < -0.5$; $S_{\nu} \propto \nu^{\alpha}$) than the non-dusty QSOs (E(B-V) $< 0.1$ mag), with fractions of 46$\pm$12 and 12$\pm$4 per cent, respectively. A higher fraction of the non-dusty QSOs have peaked radio SEDs (48$\pm$9 per cent) compared to the dusty QSOs (23$\pm$8 per cent). We discuss the origin of the radio emission, finding that the majority of the peaked, predominantly non-dusty, QSOs have consistent sizes and luminosities with compact jetted radio galaxies. However, the connection between steepness and dust obscuration implies an outflow-driven shock origin for the enhanced radio more commonly found in dusty QSOs. These results add to the emerging picture whereby dusty QSOs are in an earlier blow-out phase, with shocks that heat and destroy the surrounding dust, eventually revealing a typical non-dusty QSO.