The environments of radio galaxies and quasars in LoTSS data release 2

¹ Leiden Observatory, Leiden University, PO Box 9513 NL-2300 RA Leiden, The Netherlands
² Kapteyn Astronomical Institute, University of Groningen, PO Box 800 NL-9700 AV Groningen, The Netherlands
³ Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB, UK
⁴ Institute for Astronomy, University of Edinburgh, Edinburgh EH9 3HJ, UK

^⋆ Corresponding author; tpan@strw.leidenuniv.nl

Received: 25 November 2024
Accepted: 30 January 2025

Abstract

Aims. The orientation-based unification scheme of radio-loud active galactic nuclei (AGNs) asserts that radio galaxies and quasars are essentially the same type of object, but viewed from different angles. To test this unification model, we compared the environments of radio galaxies and quasars, which would reveal similar properties when an accurate model is utilized.

Methods. Using the second data release of the LOFAR Two-metre Sky Survey (LoTSS DR2), we constructed a sample of 26 577 radio galaxies and 2028 quasars at 0.08 < z < 0.4. For radio galaxies with optical spectra, we further classified them as 3631 low-excitation radio galaxies (LERGs) and 1143 high-excitation radio galaxies (HERGs). We crossmatched these samples with two galaxy cluster catalogs from the Sloan Digital Sky Survey (SDSS).

Results. We find that 17.1 ± 0.2% of the radio galaxies and 4.1 ± 0.4% of the quasars are associated with galaxy clusters. Luminous quasars are very rare in clusters, while 18.7 ± 0.7% LERGs and 15.2 ± 1.1% HERGs reside in clusters. We also note that in radio galaxies, both HERGs and LERGs tend to reside in the centers of clusters, while quasars do not show a strong preference for their positions in clusters.

Conclusions. This study shows that local quasars and radio galaxies exist in different environments, challenging the orientation-based unification model. This means that factors other than orientation may play an important role in distinguishing radio galaxies from quasars. The future WEAVE-LOFAR survey will offer high-quality spectroscopic data for a large number of radio sources and allow for a more comprehensive exploration of the environments of radio galaxies and quasars.