The LOFAR view of giant, early-type galaxies: Radio emission from active nuclei and star formation

We studied the properties and the origin of the radio emission in the most luminous, early-type galaxies (ETGs) in the nearby Universe (M_K ≤ −25, recession velocity ≤7500 km s⁻¹), as seen by the 150 MHz Low-Frequency ARray (LOFAR) observations. LOFAR images are available for 188 of these giant ETGs (gETGs), and 146 (78%) of them are detected above a typical luminosity of ∼10²¹ W Hz⁻¹. They show a large spread in power, reaching up to ∼10²⁶ W Hz⁻¹. We confirm a positive link between the stellar luminosity of gETGs and their median radio power, the detection rate, and the fraction of extended sources. About two-thirds (91) of the detected gETGs are unresolved, with sizes ≲4 kpc, confirming the prevalence of compact radio sources in local sources. Forty-six gETGs show extended emission on scales ranging from 4 to 340 kpc, at least 80% of which have a FR I class morphology. Based on the morphology and spectral index of the extended sources, ∼30% of them might be remnant or restarted sources, but further studies are needed to confirm this. Optical spectroscopy (available for 44 gETGs) indicates that for seven gETGs the nuclear gas is ionized by young stars suggesting a contribution to their radio emission from star forming regions. Their radio luminosities correspond to a star formation rate (SFR) in the range 0.1−8 M_⊙ yr⁻¹ and a median specific SFR of 0.8 × 10⁻¹² yr⁻¹. The gas flowing toward the center of gETGs can accrete onto the supermassive black hole but also stall at larger radii and form new stars, an indication that feedback does not completely quench star formation. The most luminous gETGs (25 galaxies with M_K < −25.8) are all detected at 150 MHz; however, they are not all currently turned on: at least four of them are remnant sources and at least one is likely powered by star formation.