Evidence that the AGN dominates the radio emission in z ∼ 1 radio-quiet quasars

Abstract

In order to understand the role of radio-quiet quasars (RQQs) in galaxy evolution, we must determine the relative levels of accretion and star-formation activity within these objects. Previous work at low radio flux-densities has shown that accretion makes a significant contribution to the total radio emission, in contrast with other quasar studies that suggest star formation dominates. To investigate, we use 70 RQQs from the Spitzer-Herschel Active Galaxy Survey. These quasars are all at z ∼ 1, thereby minimising evolutionary effects, and have been selected to span a factor of ∼ 100 in optical luminosity, so that the luminosity dependence of their properties can be studied. We have imaged the sample using the Karl G. Jansky Very Large Array (JVLA), whose high sensitivity results in 35 RQQs being detected above 2 σ. This radio dataset is combined with far-infrared luminosities derived from grey-body fitting to Herschel photometry. By exploiting the far-infrared-radio correlation observed for starforming galaxies, and comparing two independent estimates of the star-formation rate, we show that star formation alone is not sufficient to explain the total radio emission. Considering RQQs above a 2-σ detection level in both the radio and the far-infrared, 92 per cent are accretion-dominated, and the accretion process accounts for 80 per cent of the radio luminosity when summed across the objects. The radio emission connected with accretion appears to be correlated with the optical luminosity of the RQQ, whilst a weaker luminosity-dependence is evident for the radio emission connected with star formation.

Galaxy Evolution in the Radio Band: The Role of Star-forming Galaxies and Active Galactic Nuclei

“…Our result agrees very well with the observational determination by Mauch & Sadler (2007), Best & Heckman (2012) and Padovani et al (2015). The most relevant point here is that the luminosity function of non-RL sources is dominated by emission from star formation below, and nuclear emission from radio AGNs above, the luminosity threshold of L 1.4 GHz ∼ 10 30 erg s −1 Hz −1 (see also Kimball et al 2011;Kellermann et al 2016;White et al 2017). For bright, but manifestly non-RL sources (discerned on the basis of, e.g., the 24 µm to 1.4 GHz flux ratio), it will be important to test the presence of substantial radio-emission from the nucleus via future high-resolution observations.…”

“…Moreover, in the figure we report the data by White et al (2015; see also White et al 2017). Basing on a sample of mostly unobscured RQ quasars at z 3 from the VIDEO survey, these authors claimed that the shape of their counts is suggestive of a nuclear origin for the radio emission of these objects.…”

“…Note, however, that the conclusions by White et al (2015White et al ( , 2017 regarding the nuclear origin in the radio emission of RQ systems are somewhat driven by their selection criteria, that tend to pick up radio powers larger than several 10 30 erg s −1 Hz −1 , a portion of the radio luminosity function populated by RQ AGNs (cf. Fig.…”

“…16 we illustrate the locus occupied by RQ AGNs in a diagram where the radio luminosity 1.4 GHz from the nucleus is plotted against that from star formation. First of all, we plot the data for RQ quasars at z ∼ 1 by White et al (2017); these authors followed-up an optically selected quasar sample with radio (FIRST) and far-IR (Herschel) observations to probe the relative contribution from the nucleus and from star formation to the radio emission.…”

“…On the one hand, Padovani et al (2015) and Bonzini et al (2015) have shown that RQ AGNs feature infrared-to-radio flux ratios, evolving radio luminosity functions, host galaxy colors, optical morphologies and stellar masses similar to those of star-forming systems, suggesting that in RQ AGNs the radio emission is on the average dominated by star formation (see also Kimball et al 2011;Condon et al 2013;Kellermann et al 2016). On the other hand, White et al (2015White et al ( , 2017 have argued that RQ AGNs show a radio luminosity exceeding that of SFGs of similar stellar masses. Note, however, that these distinct findings may be partly attributed to the different luminosity and redshift ranges probed by the above studies.…”