Aims. The collimation of relativistic jets in galaxies is a poorly understood process. Detailed radio studies of the jet collimation region have been performed so far in a few individual objects, providing important constraints for jet formation models. However, the extent of the collimation zone as well as the nature of the external medium possibly confining the jet are still debated. Methods. In this article, we present a multifrequency and multiscale analysis of the radio galaxy NGC 315, including the use of mm-VLBI data up to 86 GHz, aimed at revealing the evolution of the jet collimation profile. We then consider results from the literature to compare the jet expansion profile in a sample of 27 low-redshift sources, mainly comprising radio galaxies and BL Lacs, which were classified based on the accretion properties as low-excitation (LEG) and high-excitation (HEG) galaxies. Results. We propose that the jet collimation in NGC 315 is completed on sub-parsec scales. A transition from a parabolic to conical jet shape is detected at zt = 0.58 ± 0.28 parsecs or ∼5 × 103 Schwarzschild radii (RS) from the central engine, a distance which is much smaller than the Bondi radius, rB ∼ 92 pc, estimated based on X-ray data. The jet in this and in a few other LEG in our sample may be initially confined by a thick disk extending out to ∼103 − 104RS. A comparison between the mass-scaled jet expansion profiles of all sources indicates that jets in HEG are surrounded by thicker disk-launched sheaths and collimate on larger scales with respect to jets in LEG. These results suggest that disk winds play an important role in the jet collimation mechanism, particularly in high-luminosity sources. The impact of winds on the origin of the FRI and FRII dichotomy in radio galaxies is also discussed.

Aims. Hot accretion flows are thought to be able to power the relativistic jets observed in active galactic nuclei. They can be presented as standard and normal evolution (SANE) disks or magnetically arrested disks (MAD) -two states that imply profound differences in the physical properties of the disks themselves and the outflows they produce. Methods. We used a multifrequency and multiepoch data set to study the giant radio galaxy NGC 315, with the goal of exploring the properties of its accretion disk and sub-parsec jet. We analyzed the source maps with a pixel-based analysis and we used theoretical models to link the observational properties of the jet to the physical state of the accretion disk. Results. We propose that the bulk flow in NGC 315 accelerates on sub-pc scales, concurrently with the parabolic expansion. We show that this fast acceleration can be theoretically reconciled with a magnetically driven acceleration. Along the acceleration and collimation zone, we observe an unexpected spectral behavior, with very steep spectral index values 𝛼 ∼ −1.5 (𝑆 𝜈 ∝ 𝜈 𝛼 ) between 22 GHz and 43 GHz. Based on the properties of this region, we predict the black hole of NGC 315 to be rotating fast and the magnetic flux threading the accretion disk to be in excellent agreement with that expected in the case of a MAD. Using a new formalism based on the core shift effect, we modeled the magnetic field downstream a quasi-parabolic accelerating jet and we reconstructed it up to the event horizon radius. In the MAD scenario, we compared it with the expected magnetic saturation strengths in the disk, finding a good agreement.

In this article, we summarize our recent progress in observational studies of jet collimation in active galactic nuclei. We focus on the investigation of a relation between the properties of the collimation zone and the AGN accretion mode, recently proposed by Boccardi, B., Perucho, M., Casadio, C. et al., 2021, A&A, 647, A67. We also discuss the efforts made by our team to increase the still limited number of sources that can be imaged on sufficiently small spatial scales to probe such zone, and we present first results for new candidates.