[Paper Review] Probing the Wind Component of Radio Emission in Luminous High-Redshift Quasars

Gordon T. Richards, Trevor V. McCaffrey+5 more|arXiv (Cornell University)|Jun 14, 2021

Galaxies: Formation, Evolution, Phenomena141 references19 citations

TL;DR

This study probes the contribution of accretion disk wind shocks to radio emission in luminous, high-redshift radio-quiet quasars using deep 3-GHz VLA and LOFAR survey observations. It finds that radio detection fraction non-linearly correlates with C IV emission line properties, suggesting multiple radio emission origins—star formation, weak jets, coronae, and wind-driven shocks—depending on Eddington ratio, with wind-related shocks becoming significant in high-luminosity quasars.

ABSTRACT

We discuss a probe of the contribution of wind-related shocks to the radio emission in otherwise radio-quiet quasars. Given 1) the non-linear correlation between UV and X-ray luminosity in quasars, 2) that such correlation leads to higher likelihood of radiation-line-driven winds in more luminous quasars, and 3) that luminous quasars are more abundant at high redshift, deep radio observations of high-redshift quasars are needed to probe potential contributions from accretion disk winds. We target a sample of 50 $z\simeq 1.65$ color-selected quasars that span the range of expected accretion disk wind properties as traced by broad CIV emission. 3-GHz observations with the Very Large Array to an rms of $\approx10\mu$Jy beam$^{-1}$ probe to star formation rates of $\approx400\,M_{ m Sun}\,{ m yr}^{-1}$, leading to 22 detections. Supplementing these pointed observations are survey data of 388 sources from the LOFAR Two-metre Sky Survey Data Release 1 that reach comparable depth (for a typical radio spectral index), where 123 sources are detected. These combined observations reveal a radio detection fraction that is a non-linear function of \civ\ emission-line properties and suggest that the data may require multiple origins of radio emission in radio-quiet quasars. We find evidence for radio emission from weak jets or coronae in radio-quiet quasars with low Eddingtion ratios, with either (or both) star formation and accretion disk winds playing an important role in optically luminous quasars and correlated with increasing Eddington ratio. Additional pointed radio observations are needed to fully establish the nature of radio emission in radio-quiet quasars.

Motivation & Objective

• To test whether wind-related shocks contribute to radio emission in radio-quiet quasars, particularly at high redshift.
• To determine how radio emission origins (e.g., star formation, jets, coronae, shocks) vary with accretion disk wind properties.
• To assess whether the radio detection fraction in high-luminosity, high-redshift quasars correlates non-linearly with C IV emission line diagnostics such as equivalent width and blueshift.
• To challenge the assumption that weak radio emission in radio-quiet quasars is dominated solely by star formation, by probing alternative origins like wind shocks.

Proposed method

• Targeted 50 z ≈ 1.65 color-selected quasars with diverse C IV emission line properties, using 3-GHz Very Large Array (VLA) pointed observations to an rms depth of ≈10 µJy beam⁻¹.
• Supplemented with 388 sources from the LOFAR Two-metre Sky Survey Data Release 1, achieving comparable depth for typical radio spectral indices.
• Used C IV emission line parameters—equivalent width and blueshift relative to rest wavelength—as proxies for accretion disk wind strength.
• Applied Independent Component Analysis (ICA) to reconstruct quasar spectra and derive robust C IV emission parameters from noisy data.
• Correlated radio detection fractions and luminosities with C IV diagnostics to identify non-linear trends indicative of multiple emission mechanisms.
• Evaluated the role of star formation via far-IR SFR estimates and considered the potential bias from AGN-heated dust on SFR indicators.

Experimental results

Research questions

• RQ1 Does the radio detection fraction in high-redshift, radio-quiet quasars correlate non-linearly with C IV emission line properties, suggesting multiple emission mechanisms?
• RQ2 Is there evidence for wind-driven shocks contributing to radio emission in luminous, high-redshift quasars, particularly in those with high Eddington ratios?
• RQ3 How do the relative contributions of star formation, weak jets, coronae, and wind shocks to radio emission vary with quasar accretion properties?
• RQ4 To what extent are SFR estimates based on far-IR emission biased by AGN-heated dust in the host galaxies of these quasars?

Key findings

• The radio detection fraction in the sample is a non-linear function of C IV emission line properties, indicating that multiple radio emission mechanisms coexist in radio-quiet quasars.
• Quasars with low Eddington ratios show evidence for weak jets or coronal emission, while those with high Eddington ratios show strong correlations with both star formation and wind-driven shocks.
• The data suggest that wind-related shocks become a significant contributor to radio emission in luminous quasars, particularly at high Eddington ratios.
• Non-detections are consistent with star formation-only models, but only if AGN-heated dust does not bias far-IR SFR estimates, implying a potential hidden contribution from winds.
• The observed trends are inconsistent with a single dominant origin of radio emission and necessitate multi-component modeling of radio emission in radio-quiet quasars.
• Future work should include deeper, high-resolution observations to detect compact jets and spectral index analysis (e.g., using VLASS data) to distinguish optically thick vs. thin radio emission components.