The environments of radio-loud AGN from the LOFAR Two-Metre Sky Survey (LoTSS)

¹ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
e-mail: Judith.Croston@open.ac.uk
² Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, AL10 9AB, UK
³ SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
⁴ ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁵ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁶ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
⁷ CSIRO Astronomy and Space Science, PO Box 1130 Bentley, WA, 6102, Australia
⁸ School of Physics and Astronomy, University of Southampton, Highfield, Southampton, SO17 1BJ, UK
⁹ Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK
¹⁰ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany

Received: 3 August 2018
Accepted: 10 October 2018

Abstract

An understanding of the relationship between radio-loud active galaxies and their large-scale environments is essential for realistic modelling of radio-galaxy evolution and environmental impact, for understanding AGN triggering and life cycles, and for calibrating galaxy feedback in cosmological models. We use the LOFAR Two-Metre Sky Survey (LoTSS) Data Release 1 catalogues to investigate this relationship. We cross-matched a sample of 8745 radio-loud AGN with 0.08 <  z <  0.4, selected from LoTSS, with two Sloan Digital Sky Survey (SDSS) cluster catalogues, and find that only 10 percent of LoTSS AGN in this redshift range have a high-probability association, so that the majority of low-redshift AGN (including a substantial fraction of the most radio-luminous objects) must inhabit haloes with M <  10¹⁴ M_⊙. We find that the probability of a cluster association, and the richness of the associated cluster, is correlated with AGN radio luminosity, and we also find that, for the cluster population, the number of associated AGN and the radio luminosity of the brightest associated AGN is richness-dependent. We demonstrate that these relations are not driven solely by host-galaxy stellar mass, supporting models in which large-scale environment is influential in driving AGN jet activity in the local Universe. At the lowest radio luminosities we find that the minority of objects with a cluster association are located at larger mean cluster-centre distances than more luminous AGN, an effect that appears to be driven primarily by host-galaxy mass. Finally, we also find that FRI radio galaxies inhabit systematically richer environments than FRIIs, consistent with previous work. The work presented here demonstrates the potential of LoTSS for AGN environmental studies. In future, the full northern-sky LoTSS catalogue, together with the use of deeper optical/IR imaging data and spectroscopic follow-up with WEAVE-LOFAR, will provide opportunities to extend this type of work to much larger samples and higher redshifts.