Structural axes of sources in the SUMSS survey and their alignments
Coherent orientations of extragalactic radio sources have been reported on large angular scales, but their statistical significance and susceptibility to observational systematics remain debated. Searches based on very long baseline interferometry (VLBI) also leave the far southern sky comparatively sparsely sampled. We test for local alignments of kiloparsec-scale radio source structural axes in the Sydney University Molonglo Sky Survey (SUMSS), providing a complementary probe in the undersampled southern sky. We constructed a catalog of structural position angles for 4087 sources from 629 SUMSS mosaics. A convolutional neural network supplied initial angle estimates, which were corrected for image-to-sky coordinate distortion and visually vetted. We searched for local alignments using a coordinate-invariant parallel transport statistic on HEALPix tessellations, then refined candidate regions with a genetic algorithm. We calibrated the significance of the full search against mock skies that preserve the declination-dependent position angle distribution and undergo the same search procedure. Several regions show apparently strong local alignment, but none remains statistically significant after accounting for the look-elsewhere effect. The candidate signals are consistent with chance fluctuations in the full search. We find no statistically significant evidence for alignment of structural axes of radio sources in SUMSS images. The resulting southern sky position angle catalog provides a useful resource for future studies of radio source morphology and population properties.