The Structure of the M87 Jet: A Transition from Parabolic to Conical Streamlines (1110.1793v2)

Abstract: The structure of the M87 jet, from milli-arcsec to arcsecond scales, is extensively investigated, utilizing the images taken with the EVN, MERLIN and VLBA. We discover that the jet maintains a parabolic streamline over a range in sizescale equal to $10^{5}$ times the Schwarzschild radius. The jet then transitions into a conical shape further downstream. This suggests that the magnetohydrodynamic jet is initially subjected to the confinement by the external gas which is dominated by the gravitational influence of the supermassive black hole. Afterwards the jet is then freely expanding with a conical shape. This geometrical transition indicates that the origin of the HST-1 complex may be a consequence of the over-collimation of the jet. Our result suggests that when even higher angular resolution is provided by a future submm VLBI experiment, we will be able to explore the origin of active galactic nuclei jets.

The Structure of the M87 Jet: Transition from Parabolic to Conical Geometries

The paper presented in this paper investigates the structural characteristics of the relativistic jet emanating from the core of M87, one of the nearest active galaxies. The research focuses on the transition from a parabolic to a conical streamline, utilizing high-resolution imaging from the EVN, MERLIN, and VLBA radio telescopes.

Key Observations and Findings

A significant finding is the identification of a parabolic geometry in the jet over a substantial range—scaled up to $10^{5}$ times the Schwarzschild radius of the central supermassive black hole (SMBH). The radii of the jet exhibit a power-law function $z \propto r^{a}$, reinforcing a parabolic shape with $a = 1.73 \pm 0.05$. Transition to a conical shape ($a = 0.96 \pm 0.1$) occurs downstream, beyond the Bondi radius, estimated to be $3.8 \times 10^{5}\, r_{s}$.

The paper utilizes archival VLBA data and newly acquired EVN and MERLIN observations, marking a significant detection of continuous jet emission extending up to 500 mas from the core. Importantly, the core of synchrotron emission at 43 GHz has been constrained to be at 20 $r_{s}$ from the SMBH, presenting an opportunity to extrapolate a single power-law streamline to the innermost jet emission region.

Implications and Theoretical Speculations

The observed parabolic structure in the upstream segment is attributed to the magnetohydrodynamic (MHD) confinement exerted by surrounding gas influenced by the SMBH's gravity. Downstream, the transition to a conical structure suggests liberation from external pressure constraints, potentially indicating a highly magnetized regime consistent with theoretical models predicting a $p_{\text{ism}} \propto z^{-b}$ pressure gradient needed for such geometry.

Notably, the HST-1 complex is proposed as a site of over-collimation leading to recurrent flaring activities. Observational data supports the presence of superluminal motions within this region. The implications for MHD jet models are profound, offering insights into the mechanics of jet collimation and acceleration mechanisms occurring close to the SMBH—pertinent topics in current astrophysical research on AGN jets.

Future Directions and Conclusion

The paper concludes with anticipations for future submillimeter VLBI experiments which could illuminate regions closer to the core—potentially accessing the Alfvén surface. This would enable more precise studies of jet formation mechanisms, possibly tracing back the origin of relativistic jets to either the spinning SMBH (as posited by Blandford-Znajek processes) or the accretion disk.

By offering a detailed view into the structure of the M87 jet, this research provides critical insights into the dynamics of jet formation and evolution, driving forward our understanding of the complex processes that govern active galactic nuclei and their associated relativistic jets. Future observations at higher resolutions are expected to further refine these models and enhance the theoretical characterizations of jet instabilities and acceleration dynamics.