# Identifying active galactic nuclei via brightness temperature with sub-arcsecond International LOFAR Telescope observations

Leah K. Morabito*, F. Sweijen, J. F. Radcliffe, P. N. Best, Rohit Kondapally, Marco Bondi, Matteo Bonato, K. J. Duncan, Isabella Prandoni, T. W. Shimwell, W. L. Williams, R. J. van Weeren, J. E. Conway, G. Calistro Rivera

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

## Abstract

Identifying active galactic nuclei (AGN) and isolating their contribution to a galaxy's energy budget is crucial for studying the co-evolution of AGN and their host galaxies. Brightness temperature ($T_b$) measurements from high-resolution radio observations at GHz frequencies are widely used to identify AGN. Here we investigate using new sub-arcsecond imaging at 144 MHz with the International LOFAR Telescope to identify AGN using $T_b$ 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 multi-wavelength classifications, brightness temperature criteria select over half of radio-excess sources, 32 percent of sources classified as radio-quiet AGN, 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, $T_b$-identified AGN with no significant radio excess and find the AGN comprises $0.49\pm 0.16$ of the radio luminosity. Overall the non-radio excess AGN show evidence for having a variety of different radio emission mechanisms, which can provide different pathways for AGN 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 is inadequate.
Original language English 5758-5774 17 Monthly Notices of the Royal Astronomical Society 515 4 29 Jul 2022 https://doi.org/10.1093/mnras/stac2129 Published - 1 Oct 2022

## Keywords

• acceleration of particles
• galaxies: active
• galaxies: jets