Multi-wavelength properties of three new radio-powerful $z\sim5.6$ quasi-stellar objects discovered from RACS

Abstract

We present a multi-wavelength study of three new $z\sim5.6$ quasi-stellar objects (QSOs) identified from dedicated spectroscopic observations. The three sources were selected as high-$z$ candidates based on their radio and optical/near-infrared properties as reported in the Rapid ASKAP Continuum Survey (RACS), the Dark Energy Survey (DES), and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) survey. These are among the most radio-bright QSOs currently known at $z>5.5$, relative to their optical luminosity, having $\rm R=S_{\rm 5GHz}/S_{\rm 4400A}>100$. In this work, we present their identification, and we also discuss their multi-wavelength properties (from the radio to the X-ray band) based on detections in public surveys as well as in dedicated radio and X-ray observations. The three sources present a wide range of properties in terms of relative intensity and spectral shape, highlighting the importance of multi-wavelength observations in accurately characterising these high-$z$ objects. In particular, from our analysis we found one source at $z=5.61$ that presents clear blazar properties (strong radio and X-ray emission), making it one of the most distant currently known in this class. Moreover, from the fit of the optical/near-infrared photometric measurements with an accretion disc model as well as the analysis of the CIV broad emission line in one case, we were able to estimate the mass and accretion rate of the central black holes in these systems, finding $\rm M_{\rm BH}\sim1-10\times10^9$~M$_\odot$ accreting at a rate $\lambda_{\rm Edd}\sim0.1-0.4$. The multi-wavelength characterisation of radio QSOs at $z>5.5$, such as the ones reported here, is essential to constraining the evolution of relativistic jets and supermassive black holes hosted in this class of objects.