Jet collimation in NGC 315 and other nearby AGN

Abstract

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.

Identifying changing jets through their radio variability

“…Different VLBI studies can also produce discrepant results depending on the time-span used in the analysis (e.g., Boccardi et al 2021;Park et al 2021). Interestingly, Boccardi et al (2021) find a parabolic geometry (k-index ≤ 0.6) for four sources common with our Sample A, namely 0316+413, 0430+052, 0415+379, and 1807+698 (3 radio galaxies and 1 BL Lac object), where we do not detect a significant trend. This discrepancy could be related to either the caveats discussed below (Sect.…”

“…However, it is likely that the distance to the transition region (∼10 5 R g ) is not universal. The transition from a parabolic to a conical geometry can be different for different sources and occur closer to the black hole (Boccardi et al 2016(Boccardi et al , 2021. Different VLBI studies can also produce discrepant results depending on the time-span used in the analysis (e.g., Boccardi et al 2021;Park et al 2021).…”

“…The transition from a parabolic to a conical geometry can be different for different sources and occur closer to the black hole (Boccardi et al 2016(Boccardi et al , 2021. Different VLBI studies can also produce discrepant results depending on the time-span used in the analysis (e.g., Boccardi et al 2021;Park et al 2021). Interestingly, Boccardi et al (2021) find a parabolic geometry (k-index ≤ 0.6) for four sources common with our Sample A, namely 0316+413, 0430+052, 0415+379, and 1807+698 (3 radio galaxies and 1 BL Lac object), where we do not detect a significant trend.…”

“…Very long baseline interferometry (VLBI) observations show that the jet in M87 has a parabolic shape up to the HST-1 region (e.g., Biretta et al 1999) and then transitions to conical (Asada & Nakamura 2012). Recent observations of other nearby radio galaxies have found similar geometries suggesting this is a common feature of AGN jets (Kovalev et al 2020;Boccardi et al 2021), while theoretical works predict differences in the location of the transition region between blazar subclasses (Potter & Cotter 2015). Based on our current understanding of blazar jets (e.g., Blandford & Königl 1979;Marscher 1995;Marscher et al 2008), gigahertz radio emission should arise at parsec scales where the jets are likely conical and nonaccelerating.…”

The jet collimation profile at high resolution in BL Lacertae

“…The wind (which may be magnetized) coming from the outer parts of the accretion disk may play a crucial role in the confinement of jets (McKinney 2006;Tchekhovskoy et al 2008). Boccardi et al (2020) show that in high-excitation radio galaxies (HERGs), the ACZ, expressed in Schwarzschild radii, extends over larger distances than it does in low-excitation radio galaxies (LERGs) and that this may be associated with the different accretion mechanisms in these two classes of objects. A HERG, hosting radiatively efficient standard disks, would have a stronger disk wind component that keeps the jet collimated for larger distances.…”