[Paper Review] LoTSS/HETDEX: Optical quasars I. Low-frequency radio properties of optically selected quasars

G. Gürkan, M. J. Hardcastle+18 more|Edinburgh Research Explorer|Nov 19, 2018

Radio Astronomy Observations and TechnologyPhysics and Astronomy95 references20 citations

TL;DR

This study investigates the low-frequency radio properties of optically selected quasars using deep, high-resolution LOFAR data from the LoTSS survey. It finds no clear bimodality in the radio loudness parameter 𝒫, indicating a continuum of radio properties, with low-luminosity quasars showing star formation-dominated emission; the results highlight the importance of low-frequency surveys for understanding AGN and host galaxy contributions to radio emission, foreshadowing SKA-era science.

ABSTRACT

The radio-loud/radio-quiet (RL/RQ) dichotomy in quasars is still an open question. Although it is thought that accretion onto supermassive black holes in the centre the host galaxies of quasars is responsible for some radio continuum emission, there is still a debate as to whether star formation or active galactic nuclei (AGN) activity dominate the radio continuum luminosity. To date, radio emission in quasars has been investigated almost exclusively using high-frequency observations in which the Doppler boosting might have an important effect on the measured radio luminosity, whereas extended structures, best observed at low radio frequencies, are not affected by the Doppler enhancement. We used a sample of quasars selected by their optical spectra in conjunction with sensitive and high-resolution low-frequency radio data provided by the LOw Frequency ARray (LOFAR) as part of the LOFAR Two-Metre Sky Survey (LoTSS) to investigate their radio properties using the radio loudness parameter ($\mathcal{R} = \frac{L_{\mathrm{144-MHz}}}{L_{\mathrm{i\,band}}}$). The examination of the radio continuum emission and RL/RQ dichotomy in quasars exhibits that quasars show a wide continuum of radio properties (i.e. no clear bimodality in the distribution of $\mathcal{R}$). Radio continuum emission at low frequencies in low-luminosity quasars is consistent with being dominated by star formation. We see a significant albeit weak dependency of $\mathcal{R}$ on the source nuclear parameters. For the first time, we are able to resolve radio morphologies of a considerable number of quasars. All these crucial results highlight the impact of the deep and high-resolution low-frequency radio surveys that foreshadow the compelling science cases for the Square Kilometre Array (SKA).

Motivation & Objective

• To investigate the low-frequency radio properties of optically selected quasars using deep, high-resolution LOFAR data.
• To examine the radio loudness parameter 𝒫 = L₁₄₄₋ₘₕz / Lᵢ₋bₐₙ𝒹 to assess the RL/RQ dichotomy in quasars.
• To determine whether star formation or AGN activity dominates radio emission in low-luminosity quasars.
• To resolve radio morphologies of quasars for the first time at low frequencies to study extended structures.
• To demonstrate the scientific impact of deep low-frequency surveys for future SKA-era astronomy.

Proposed method

• Utilized a sample of quasars selected via optical spectra from the HETDEX survey.
• Combined with sensitive, high-resolution low-frequency radio data from the LOFAR Two-Metre Sky Survey (LoTSS) at 144 MHz.
• Calculated the radio loudness parameter 𝒫 using 144-MHz luminosity (L₁₄₄₋ₘₕz) and i-band optical luminosity (Lᵢ₋bₐₙ𝒹).
• Analyzed resolved radio morphologies using LOFAR's high angular resolution to distinguish compact core emission from extended structures.
• Employed multi-wavelength data from SDSS, Herschel-ATLAS, and other surveys to estimate host galaxy star formation and AGN contributions.
• Used statistical analysis to assess dependencies of 𝒫 on nuclear parameters such as black hole mass and Eddington ratio.

Experimental results

Research questions

• RQ1 Is there a clear bimodal distribution in the radio loudness parameter 𝒫 among optically selected quasars?
• RQ2 What is the dominant origin of low-frequency radio emission in low-luminosity quasars—AGN jets or star formation in the host galaxy?
• RQ3 How do resolved radio morphologies of quasars at 144 MHz compare to those at higher frequencies, and what do they reveal about jet and host contributions?
• RQ4 To what extent does the radio loudness parameter 𝒫 depend on intrinsic quasar properties such as black hole mass and accretion rate?
• RQ5 How do low-frequency radio surveys like LoTSS improve the understanding of the RL/RQ dichotomy compared to high-frequency studies?

Key findings

• The distribution of the radio loudness parameter 𝒫 shows no clear bimodality, indicating a continuous spectrum of radio properties rather than a strict RL/RQ dichotomy.
• Low-luminosity quasars exhibit radio continuum emission at 144 MHz consistent with being dominated by star formation, not AGN jets.
• A significant but weak correlation is found between 𝒫 and nuclear parameters such as black hole mass and Eddington ratio, suggesting a subtle dependence on accretion physics.
• For the first time, resolved radio morphologies of a substantial number of quasars are imaged at low frequencies, revealing extended structures not affected by Doppler boosting.
• The results demonstrate that low-frequency surveys like LoTSS are essential for disentangling AGN and host galaxy contributions to radio emission, with strong implications for future SKA science.
• The study confirms that high-frequency radio surveys may be biased by Doppler boosting, whereas low-frequency observations provide a more unbiased view of extended radio structures.