The LOFAR view of massive early-type galaxies: Transition from radio AGN to host emission

¹ INAF – Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: alessandro.capetti@inaf.it
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
³ Dipartimento di Fisica e Astronomia, Università di Bologna, Via P. Gobetti 93/2, Bologna, Italy

Received: 29 March 2023
Accepted: 16 June 2023

Abstract

We extend the study of the radio emission in early-type galaxies (ETGs) in the nearby Universe (recession velocity < 7500 km s⁻¹) as seen by the 150 MHz Low-Frequency ARray (LOFAR) observations and extend the sample from giant ETGs to massive (∼6 × 10¹⁰ − 3 × 10¹¹ M_⊙) ETGs (mETGS) with −25 < M_K < −23.5. Images from the second data release of the LOFAR Two-metre Sky Survey were available for 432 mETGs, 48% of which are detected above a typical luminosity of ∼3 × 10²⁰ W Hz⁻¹. Most (85%) of the detected sources are compact, with sizes ≲4 kpc. The radio emission of 31 mETGs is extended on scales ranging from 2 to 180 kpc (median 12 kpc). In several cases, it is aligned with the host galaxy. We set a limit of ≲1% to the fraction of remnant or restarted objects, which is ≲16% of the extended sources. We found that the properties of the radio sources are connected with the stellar mass of the ETGs (the median radio power, the fraction of extended radio sources, and the link with the large-scale environment). However, these results only describe statistical trends because the radio properties of sources of similar stellar mass and environment show a large spread of radio properties. These trends break at the lowest host luminosities (M_K > −24.5). This effect is strengthened by the analysis of even less massive ETGs, with −23.5 < M_K < −21.5. This suggests that at a mass of ∼2 × 10¹¹ M_⊙, a general transition occurs from radio emission produced from radio-loud active galactic nuclei (AGN) to processes related to the host galaxy and (or) radio quiet AGN. At this luminosity, a transition in the stellar surface brightness profile also occurs from Sérsic galaxies to those with a depleted stellar core, the so-called core galaxies. This finding is in line with previous results that indicated that only core galaxies host radio-loud AGN.