Identifying active galactic nuclei via brightness temperature with sub-arcsecond international LOFAR telescope observations

Monthly Notices of the Royal Astronomical Society, vol. 515, pp. 5758–5774

Abstract

ABSTRACT Identifying active galactic nuclei (AGNs) and isolating their contribution to a galaxy’s energy budget is crucial for studying the co-evolution of AGNs and their host galaxies. Brightness temperature (Tb) measurements from high-resolution radio observations at GHz frequencies are widely used to identify AGNs. Here, we investigate using new sub-arcsecond imaging at 144 MHz with the International LOFAR Telescope to identify AGNs using Tb in the Lockman Hole field. We use ancillary data to validate the 940 AGN identifications, finding 83 percent of sources have AGN classifications from SED fitting and/or photometric identifications, yielding 160 new AGN identifications. Considering the multiwavelength classifications, brightness temperature criteria select over half of radio-excess sources, 32 percent of sources classified as radio-quiet AGNs, and 20 percent of sources classified as star-forming galaxies. Infrared colour–colour plots and comparison with what we would expect to detect based on peak brightness in 6 arcsec LOFAR maps imply that the star-forming galaxies and sources at low flux densities have a mixture of star-formation and AGN activity. We separate the radio emission from star-formation and AGN in unresolved, Tb-identified AGNs with no significant radio excess and find the AGN comprises 0.49 ± 0.16 of the radio luminosity. Overall, the non-radio excess AGNs show evidence for having a variety of different radio emission mechanisms, which can provide different pathways for AGNs and galaxy co-evolution. This validation of AGN identification using brightness temperature at low frequencies opens the possibility for securely selecting AGN samples where ancillary data are inadequate.

Authors 14

1. Leah K. Morabito corresponding

   Durham University

   Affiliation as printed

   Centre for Extragalactic Astronomy, Department of Physics, Durham University , Durham DH1 3LE, UK

   Institute for Computational Cosmology, Department of Physics, University of Durham , South Road, Durham DH1 3LE, UK

2. Frits Sweijen Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University , P.O. Box 9513, NL-2300 RA Leiden, the Netherlands

3. J. F. Radcliffe

   University of Manchester · University of Pretoria

   Affiliation as printed

   Department of Physics, University of Pretoria , Lynnwood Road, Hatfield, Pretoria 0083, South Africa

   Jodrell Bank Centre for Astrophysics, University of Manchester , Oxford Road, Manchester M13 9PL, UK

4. P. N. Best

   Royal Observatory

   Affiliation as printed

   Institute for Astronomy, Royal Observatory , Blackford Hill, Edinburgh EH9 3HJ, UK

5. R. Kondapally

   Royal Observatory

   Affiliation as printed

   Institute for Astronomy, Royal Observatory , Blackford Hill, Edinburgh EH9 3HJ, UK

6. M. Bondi

   Istituto di Radioastronomia di Bologna · National Institute for Astrophysics

   Affiliation as printed

   INAF - Istituto di Radioastronomia , Via P. Gobetti 101, I-40129 Bologna, Italy

7. Matteo Bonato

   Istituto di Radioastronomia di Bologna · National Institute for Astrophysics

   Affiliation as printed

   INAF - Istituto di Radioastronomia , Via P. Gobetti 101, I-40129 Bologna, Italy

   Italian ALMA Regional Centre , Via Gobetti 101, I-40129 Bologna, Italy

8. K. J. Duncan

   Royal Observatory

   Affiliation as printed

   Institute for Astronomy, Royal Observatory , Blackford Hill, Edinburgh EH9 3HJ, UK

9. Isabella Prandoni

   Istituto di Radioastronomia di Bologna · National Institute for Astrophysics

   Affiliation as printed

   INAF - Istituto di Radioastronomia , Via P. Gobetti 101, I-40129 Bologna, Italy

10. T. W. Shimwell Aachen Leiden Observatory

    Leiden University · Leiden Observatory · Netherlands Institute for Radio Astronomy

    Affiliation as printed

    ASTRON, Netherlands Institute for Radio Astronomy , Oude Hoogeveensedijk 4, NL-7991 PD Dwingeloo, the Netherlands

    Leiden Observatory, Leiden University , P.O. Box 9513, NL-2300 RA Leiden, the Netherlands

11. Wendy L. Williams Aachen Leiden Observatory

    Leiden University · Leiden Observatory

    Affiliation as printed

    Leiden Observatory, Leiden University , P.O. Box 9513, NL-2300 RA Leiden, the Netherlands

12. Reinout J. van Weeren Aachen Leiden Observatory

    Leiden University · Leiden Observatory

    Affiliation as printed

    Leiden Observatory, Leiden University , P.O. Box 9513, NL-2300 RA Leiden, the Netherlands

13. John Edward Conway

    Chalmers University of Technology

    Affiliation as printed

    Department of Space, Earth and Environment, Chalmers University of Technology , Onsala Space Observatory, SE-439 92 Onsala, Sweden

14. G. Calistro Rivera

    European Southern Observatory

    Affiliation as printed

    European Southern Observatory , Karl-Schwarzschild-Straße 2, D-85748 Garching bei München, Germany