Speaker
Description
The morphologies of extended, double-lobed radio quasars beyond z>2.5 remain largely uncharted, and no systematic census of their structural properties across cosmic time has yet been undertaken. Filling this gap is essential for understanding how powerful jets couple to their evolving environments from the early Universe to the present day. Understanding how powerful FR II radio quasars evolve from the local to the very high–redshift Universe requires large, deep radio-optical data sets that can both detect faint bridge emission and resolve the compact core–jet–hotspot structure of each source. No single large-sky radio survey delivers the necessary combination of sensitivity and resolution. Hence, we employ multiple complementary radio surveys to carry out this study. LoTSS provides the necessary sensitivity to trace low-surface brightness bridges, VLASS resolves compact cores and hotspots, and FIRST/NVSS deliver robust total fluxes. By combining these with spectroscopic quasar catalogues (e.g. SDSS), we have built the largest, wide-area sample of extended radio quasars spanning 0.13 < z < 4.2.
Starting from ∼7000 candidates, we inspected each source across the three surveys and used a Python package we developed to measure total angular size (and estimated total projected size), arm-length ratio (ALR) and misalignment angle (MA). After excluding core-dominated objects via VLASS/FIRST core flux densities and NVSS for total flux densities, 1569 FR II quasars remain. NVSS recovers the total flux in all but seven cases, giving reliable core-dominance factors; LoTSS-DR2 data were essential for identifying every object above z = 2.5, owing to its superior sensitivity to low-surface-brightness emission. The final catalogue covers projected sizes from 33 to 989 kpc and includes 35 extended quasars at z > 3, with the most distant at z = 4.2 (190 kpc). This is the largest RQ above z>4. Our newly identified 35 extended FR II quasars at z>3 are the largest high-redshift set yet assembled, which span projected sizes of 45–792 kpc (median ≈ 254 kpc) and include three “giants’’ (> 700 kpc), the most distant giant radio sources known. Their ≥100 kpc lobes imply sustained jet activity in substantial gaseous haloes, making each source a compelling signpost of a forming massive structure; together they provide a uniquely powerful tracer ensemble for proto-cluster searches at the dawn of cosmic time.
Our analysis shows that when the sample is split into contiguous redshift bins, we find a modest but significant increase in both ALR and MA, and a decrease in projected size with redshift, implying that high-z quasars are systematically more distorted. Intriguingly, those trends disappear when sources are instead binned by projected size. MA and ALR rise steeply as doubles grow from ∼30 kpc to ∼300 kpc, then flatten in >300 kpc sources, implying that larger lobes are intrinsically straighter and more balanced.
Nearly 80 % of the catalogue lies in the forthcoming LoTSS-DR3 footprint; combining DR3 fluxes with NVSS will allow us to examine the spectral properties of these sources and help us to understand ICCMB-induced fading of bridge emission out to z ≈ 4.
The 1569-object sample - by far the largest set of extended radio quasars with measured morphology in this redshift range, opens a path to calibrate the evolution of core-to-lobe ratios, test jet–jet-environment interactions across cosmic time. Our new measurement tool already streamlines the geometrical analysis and will be vital for inspecting large new samples. Beyond the immediate science, the catalogue supplies a unique set of labelled images for training convolutional networks to identify FR II morphology automatically. This will be crucial for harvesting the millions of RLQs as well as radio galaxies expected from LoTSS-DR3, LoTSS-Deep, SKA pathfinders, and ultimately with the SKA.
