Alignment in the orientation of LOFAR radio sources
  Various studies have laid claim to finding an alignment of the polarization
vectors or radio jets of active galactic nuclei (AGN) over large distances, but
these results have proven controversial and so far, there is no clear
explanation for this observed alignment. To investigate this case further, we
tested the hypothesis that the position angles of radio galaxies are randomly
oriented in the sky by using data from the Low-Frequency Array (LOFAR)
Two-metre Sky Survey (LoTSS). A sample of 7,555 double-lobed radio galaxies was
extracted from the list of 318,520 radio sources in the first data release of
LoTSS at 150 MHz.
  We performed statistical tests for uniformity of the two-dimensional (2D)
orientations for the complete 7,555 source sample. We also tested the
orientation uniformity in three dimensions (3D) for the 4,212 source sub-sample
with photometric or spectroscopic redshifts. Our sample shows a significant
deviation from uniformity (p-value < $10^{-5}$) in the 2D analysis at angular
scales of about four degrees, mainly caused by sources with the largest flux
densities. No significant alignment was found in the 3D analysis. Although the
3D analysis has access to fewer sources and suffers from uncertainties in the
photometric redshift, the lack of alignment in 3D points towards the cause of
the observed effect being unknown systematics or biases that predominantly
affect the brightest sources, although this has yet to be demonstrated
irrefutably and should be the subject of subsequent studies.
