Cosmic evolution of FRI and FRII sources out to z = 2.5

Abstract

Context. Radio-loud active galactic nuclei (RLAGN) play an important role in the evolution of galaxies through the effects on their environment. The two major morphological classes are core-bright (FRI) and edge-bright (FRII) sources. With the LOw-Frequency ARray (LOFAR), we can now compare the FRI and FRII evolution down to lower flux densities and with larger samples than before. Aims. Our aim is to examine the cosmic space density evolution for FRIs and FRIIs by analyzing their space density evolution between L150 ∼ 1024.5 W Hz−1 and L150 ∼ 1028.5 W Hz−1 and up to z = 2.5. In particular, we look at the space density enhancements and compare the FRI and FRII evolution with the total RLAGN evolution. Methods. We constructed radio luminosity functions (RLFs) from FRI and FRII catalogues based on recent data from LOFAR at 150 MHz to study the space densities as a function of radio luminosity and redshift. These catalogues contain over 100 times the number of FRIs with associated redshifts greater than z = 0.3, compared to the most recent FRI/FRII RLF study. To derive the maximum distance according to which a source can be classified and to correct for detection limits, we conducted simulations of how sources appear across a range of redshifts. Results. Our RLFs do not show any sharp transitions between the space density evolution of FRI and FRII sources as a function of radio luminosity and redshift. We report a space density enhancement from low to high redshift for FRI and FRII sources brighter than L150 ∼ 1027 W Hz−1. Furthermore, while we observe a tentative decrease in the space densities of FRIs with luminosities below L150 ∼ 1026 W Hz−1 and at redshifts beyond z = 0.8, this may be due to residual selection biases. The FRI/FRII space density ratio does not appear to evolve strongly as a function of radio luminosity and redshift. Conclusions. We argue that the measured space density enhancements above L150 ∼ 1027 W Hz−1 are related to the higher gas availability in the earlier, denser universe. The constant FRI/FRII space density ratio evolution as a function of radio luminosity and redshift suggests that the jet-disruption of FRIs might be primarily caused by events occurring on scales within the host galaxy, rather than being driven by changes in the overall large-scale environment. The remaining selection biases in our results also highlight the need to resolve more sources at angular scales below 40″, thereby strengthening the motivation for further developing and automating the calibration and imaging pipeline of LOFAR data to produce images at a sub-arcsecond resolution.