Fundamental Plane relation differentiates between radio sources
15 August 2016
The era of very large all-sky radio surveys that probe down to very faint flux limits dawns upon us. Surveys such as the Evolutionary Map of the Universe (EMU), using the Australian Square Kilometre Array Pathfinder (ASKAP) telescope, will detect emission from many radio sources that can be star-forming galaxies or accreting supermassive black holes, or Active Galactic Nuclei (AGN), that sit at the centre of most galaxies. Observations of the 1.4 GHz radio emission can be a very useful dust-free tracer of a galaxy's star formation rate because the 1.4 GHz luminosity density correlates with the far-infrared (FIR) luminosity for star-forming systems. An increased observed 1.4 GHz luminosity density is usually attributed to AGN. Hence, many studies have used this radio – FIR correlation to differentiate between radio sources whose 1.4 GHz emission is predominantly arising from star formation and/or AGN.
In a new paper, CAASTRO Affiliate Dr Ivy Wong (ICRAR-UWA) and colleagues analysed the 1.4 GHz radio properties of nearby AGN with low X-ray energies ("hard X-ray" spectrum) from the Swift Burst Alert Telescope sample at 14-195 keV. Using observations from the Faint Images in the Radio Sky at Twenty-centimeters (FIRST) survey, they found that their AGN were radio-quiet and had 1.4 GHz luminosity densities which were comparable to those originating from star-forming galaxies. The sample was consistent with the standard radio – FIR correlation that is indicative of star-forming galaxies. This was somewhat surprising because the majority of the sample were compact radio sources located at the centre of galaxies. So the question arose: are the observed 1.4 GHz luminosity densities really due to star-formation or AGN?
The researchers examined the relationship between the 1.4 GHz emission with the X-ray luminosity and estimated black hole mass via the "Fundamental Plane" relation. This relationship is known to be scale-invariant and connects the accretion of matter with the radio outflow in Galactic stellar-mass black holes, as well as central supermassive black holes. They found that their hard X-ray selected sample was consistent with the black hole "Fundamental Plane", with the exception of one source where the extended 1.4 GHz emission seemed to include emission from the star-forming disk. Their results suggest that the observed 1.4 GHz emission originates from the low-luminosity radio AGN after all, not the star formation process. The team suggest that the observed consistency between the 1.4 GHz emission and the FIR emission might be due to a component of the 1.4 GHz emission that originates from the accretion disk corona. The radio emission from the disk corona is likely to correlate with the dust temperatures, and hence the FIR emission.