The radio loudness of SDSS quasars from the LOFAR Two-metre Sky Survey: ubiquitous jet activity and constraints on star formation

TL;DR

This study models radio emission in ~42,000 SDSS quasars using LOFAR data, revealing ubiquitous jet activity, a continuous radio-loudness distribution, and a strong link between star formation and quasar luminosity across redshifts.

Contribution

It introduces a two-component model of radio emission in quasars, showing jets operate in all quasars with varying efficiency and no clear bimodality in radio-loudness.

Findings

Jet activity is present in all quasars, with a wide power distribution.

Star formation rate correlates with quasar luminosity and redshift.

Quasars contribute a small but increasing fraction to cosmic star formation at higher redshifts.

Abstract

We examine the distribution of radio emission from ~42,000 quasars from the Sloan Digital Sky Survey, as measured in the LOFAR Two-Metre Sky Survey (LoTSS). We present a model of the radio luminosity distribution of the quasars that assumes that every quasar displays a superposition of two sources of radio emission: active galactic nuclei (jets) and star-formation. Our two-component model provides an excellent match to the observed radio flux density distributions across a wide range of redshifts and quasar optical luminosities; this suggests that the jet-launching mechanism operates in all quasars but with different powering efficiency. The wide distribution of jet powers allows for a smooth transition between the 'radio-quiet' and 'radio-loud' quasar regimes, without need for any explicit bimodality. The best-fit model parameters indicate that the star-formation rate of quasar host galaxies correlates strongly with quasar luminosity and also increases with redshift at least out to z~2. For a model where star-formation rate scales as $SFR \propto L_{bol}^\alpha (1+z)^\beta$, we find $\alpha = 0.47 \pm 0.01$ and $\beta = 1.61 \pm 0.05$, in agreement with far-infrared studies. Quasars contribute ~0.15 per cent of the cosmic star-formation rate density at z=0.5, rising to 0.4 per cent by z=2. The typical radio jet power is seen to increase with both increasing optical luminosity and black hole mass independently, but does not vary with redshift, suggesting intrinsic properties govern the production of the radio jets. We discuss the implications of these results for the triggering of quasar activity and the launching of jets.