Probing the Low Radio Frequency Emission in PG Quasars with the uGMRT -- II

TL;DR

This study extends the low-frequency radio analysis of the Palomar-Green quasar sample by presenting 685 MHz uGMRT observations for 87 $z<0.5$ quasars (65 new sources plus 22 from Paper I). It confirms a persistent RL–RQ dichotomy at low frequency, with a luminosity threshold around $L_{685}\approx 10^{23.5}$–$10^{25.5}$ W Hz$^{-1}$ separating the populations and with RL sources typically showing larger radio sizes and jet-like morphologies. The work demonstrates that $L_{685}$ scales with black hole mass $M_{BH}$ and accretion rate $L/L_{Edd}$, and reveals distinct radio fundamental planes for RL and RQ quasars, implying different jet–accretion couplings. The results support a scenario where accretion state, SMBH mass, and magnetic flux near the black hole together drive jet production, while also indicating episodic jet activity in some RL sources; future high-resolution X-ray observations are needed to test the magnetic-flux paradigm more robustly.

Abstract

We present results from uGMRT 685 MHz observations of 87 QSOs belonging to the Palomar Green (PG) quasar sample with $z<0.5$. Radio emission is detected in all sources except for 3 radio-quiet (RQ) sources, viz., PG 0043+039, PG 1121+422, and PG 1552+085. The radio-loud (RL) $-$ RQ dichotomy persists at 685 MHz with only 1 source, PG 1216+069, changing its classification from RQ to RL. Approximately 1/3 of the detected RQ quasars display AGN-dominated radio emission while the rest may show additional contributions from stellar-related processes. Consistent with this, the RL and RQ quasars occupy distinct tracks on the `fundamental plane' of black hole activity. We find that RL quasars have $\log_{10}(L_{685\,\mathrm{MHz}}/\mathrm{W\,Hz}^{-1}) > 25.5$, while RQ quasars have ${\log_{10}(L_{685\,\mathrm{MHz}}/\mathrm{W\,Hz}^{-1})} <23.5$. Furthermore, the radio sizes display the RQ$-$RL divide as well with RQ sources typically having sizes $\lesssim30$ kpc, with only 16 ($\sim22$%) RQ sources having sizes between 30 and 100 kpc where there is an overlap with RL quasar sizes. A strong correlation exists between 685 MHz radio luminosity and black hole mass which is tightened when accretion rate is considered, highlighting the important role played by the accretion rate and accretion disk structure in jet production. We found no difference in the minimum-energy magnetic field strengths of the radio cores of RL and RQ quasars; however, different assumptions of source volume and volume filling factors may apply. High-resolution X-ray observations and radio-X-ray flux comparisons are needed to independently test the `magnetic flux paradigm'.

Figures (19)

• Figure 1: Distribution of the uGMRT 685 MHz luminosities for the PG quasar sample. The RL-RQ dichotomy persists in the low-frequency uGMRT data.
• Figure 2: Distribution of 685 MHz projected radio source sizes in kpc for the PG quasar sample.
• Figure 3: Distribution of 'minimum energy' magnetic field strength for RQ and RL sources for $k=1$ (left panel) and $k=3$ (right panel). The outlier source with the largest ${B_\mathrm{min}}$ value is PG 1226+023 or 3C 273.
• Figure 4: The 685 MHz - 5 GHz spectral index versus Eddington ratios for the PQ sample. In this and following plots, red triangles denote the RL QSOs, the blue circles denote RQ QSOs, the blue-dashed and red-dotted lines represent the best-fit lines for RQ and RL sources, respectively.
• Figure 5: (Left) The radio-IR correlation for PG sources. The solid black line is the radio−IR correlation for star-forming galaxies Bell03. The black dashed lines mark the $1\sigma$ regions and the black dotted lines mark the $3\sigma$ regions. (Right) The mean core spectral index as a function of $q_{\mathrm{IR}}$. The black dashed vertical line represents $q_{\mathrm{IR}} = 1.8$ (which is about $2.1\sigma$ below the Bell03 relation), which discriminates the AGN versus star formation contributions. The black horizontal line is $\alpha_R=-0.5$, which we use to discriminate between steep ($\alpha_R<-0.5$) and flat ($\alpha_R > -0.5$) spectrum cores.