An extended and extremely thin gravitational arc from a lensed compact symmetric object at redshift 2.059

TL;DR

This work addresses resolving the morphology of a high-redshift compact symmetric object (CSO) that is gravitationally lensed. It combines global VLBI imaging at 1.7 GHz with a Bayesian forward-modelling framework to reconstruct both image- and source-plane brightness, correcting for lensing distortions. The analysis reveals an extremely thin gravitational arc and two radio mini-lobes separated by 642 pc, with brightness temperatures up to $10^{9.15}$ K and a rest-frame luminosity density of about $10^{26.34}$ W Hz$^{-1}$ at 1.7 GHz, supporting a type-2 CSO classification and consistency with a bow-shock interaction model in a dense ISM. The results demonstrate the power of lensing plus advanced imaging to study the high-redshift radio AGN population and underscore the potential of future SKA+VLBI efforts to extend such investigations.

Abstract

Compact symmetric objects (CSOs) are thought to be short-lived radio sources with two lobes of emission that are separated by less than a kpc in projection. However, studies of such systems at high redshift is challenging due to the limited resolution of present-day telescopes, and can be biased to the most luminous objects. Here we report imaging of a gravitationally lensed CSO at a redshift of 2.059 using very long baseline interferometry at 1.7 GHz. The data are imaged using Bayesian forward modelling deconvolution, which reveals a spectacularly extended and thin gravitational arc, and several resolved features within the lensed images. The surface brightness of the lensing-corrected source shows two mini-lobes separated by 642 pc in projection, with evidence of multiple hotspots that have brightness temperatures of 10^8.6 to 10^9.2 K, and a total luminosity density of 10^26.3 W / Hz. By combining the well-resolved radio source morphology with previous multi-wavelength studies, we conclude that this object is likely a CSO of type 2, and that the properties are consistent with the bow-shock model for compact radio sources. Our analysis highlights the importance of combining high quality data sets with sophisticated imaging and modelling algorithms for studying the high redshift Universe.

Figures (3)

• Figure 1: The Bayesian forward modelling deconvolution of the global VLBI imaging of JVAS B1938+666 at 1.7 GHz. The image has an rms of 34.3 $\mu$Jy beam$^{-1}$ and the deconvolved image has been made with a restoring beam of $7.4\times4.7$ mas$^{2}$ at a position angle of 32.1 deg east of north. The centre of the image is ${\rm RA}=19^{\rm h}38^{\rm m}25\fs3407$, ${\rm Dec}= +66\degr48\arcmin52\farcs809\,40$ (J2000).
• Figure 2: The source-plane brightness temperature distribution of JVAS B1938+666. The surface brightness of the rest-frame 0.7 $\mu$m emission from the host galaxy is shown with the black contours at $(0.25, 0.5, 0.75) \times I_{\rm 0.7~\mu m}$, the peak optical brightness. The two insets are $50\times50$ pc$^2$ in area and provide a zoom-in on the two radio components. The inset contours are $(0.25, 0.5, 0.75) \times T_{b,{\rm peak}}$, the peak brightness temperature of each radio component. We apply a mask to noise features with $T_b < 10^8$ K. The centre of the image is ${\rm RA}=19^{\rm h}38^{\rm m}25\fs3229$, ${\rm Dec}= +66\degr48\arcmin52\farcs871\,36$ (J2000).
• Figure 3: The radio spectral energy distribution of JVAS B1938+666 between 74 MHz and 97 GHz, which shows a clear turnover towards low frequencies. The data are taken from stacey2018, and references therein. In addition, we include $S_{\rm 74~MHz} = 281\pm72$ mJy (VLA Low-Frequency Sky Survey; cohen2007), $S_{\rm 150~MHz} = 531\pm54$ mJy (Tata Institute of Fundamental Physics Giant Metrewave Radio Telescope Sky Survey Alternative Data Release; intema2017) and $S_{\rm 325~MHz} = 750\pm31$ mJy (Westerbork Northern Sky Survey; rengelink1997).