The High-redshift Clusters Occupied by Bent Radio AGN (COBRA) Survey: Investigating the Role of Environment on Bent Radio AGNs Using LOFAR

Abstract

Bent radio active galactic nucleus (AGN) morphology depends on the density of the surrounding gas. However, bent sources are found inside and outside clusters, raising the question of how environment impacts bent AGN morphology. We analyze new LOw-Frequency Array the LOFAR Two-metre Sky Survey (LoTSS) Data Release II observations of 20 bent AGNs in clusters and 15 not in clusters from the high-z Clusters Occupied by Bent Radio AGN (COBRA) survey (0.35 < z < 2.35). We measure the impact of environment on size, lobe symmetry, and radio luminosity. We find that the most asymmetric radio lobes lie outside of clusters, and we uncover a tentative anticorrelation between the total projected physical area and cluster overdensity. Additionally, we, for the first time, present spectral index measurements of a large sample of high-z bent sources using LoTSS and Very Large Array Faint Images of the Radio Sky at Twenty-centimeters (VLA FIRST) observations. We find that the median spectral index for the cluster sample is −0.76 ± 0.01, while the median spectral index for the noncluster sample is −0.81 ± 0.02. Furthermore, 13 of 20 cluster bent AGNs have flat cores (α ≥ −0.6) compared to 4 of 15 of noncluster bent AGNs, indicating a key environmental signature. Beyond core spectral index, bent AGNs inside and outside clusters are remarkably similar. We conclude that the noncluster sample may be more representative of bent AGNs at large offsets from the cluster center (> 1.2 Mpc) or bent AGNs in weaker groups rather than the field.