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

Fawcett, V A; Harrison, C M; Alexander, D M; Morabito, L K; Kharb, P; Rosario, D J; Baghel, Janhavi; Ghosh, Salmoli; Sasikumar, Silpa; Petley, J; Sargent, C; Calistro Rivera, G

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 spectral energy distributions (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 (⁠α < −0.5⁠; S ∝ να⁠) than the non-dusty QSOs [E(B−V) < 0.1 mag], with fractions of 46±12 and 12±4 per cent, respectively. A higher fraction of the non-dusty QSOs have peaked radio SEDs (48±9 per cent) compared to the dusty QSOs (23±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.

Citation

Fawcett, V. A., Harrison, C. M., Alexander, D. M., Morabito, L. K., Kharb, P., Rosario, D. J., Baghel, J., Ghosh, S., Sasikumar, S., Petley, J., Sargent, C., & Calistro Rivera, G. (2025). Connection between steep radio spectral slopes and dust extinction in QSOs: evidence for outflow-driven shocks in dusty QSOs. Monthly Notices of the Royal Astronomical Society, 537(2), 2003-2023. https://doi.org/10.1093/mnras/staf105

• Journal Article Type: Article
• Acceptance Date: Jan 15, 2025
• Online Publication Date: Jan 20, 2025
• Publication Date: Jan 23, 2025
• Deposit Date: Feb 10, 2025
• Publicly Available Date: Feb 11, 2025
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 537
• Issue: 2
• Pages: 2003-2023
• DOI: https://doi.org/10.1093/mnras/staf105
• Keywords: galaxies: active, galaxies: evolution, quasars: general, quasars: supermassive black holes, radio continuum: galaxies
• Public URL: https://durham-repository.worktribe.com/output/3476019