Jet Collimation Profile of the Low-luminosity Active Galactic Nucleus M84: Insight into the Jet Formation in the Low-accretion Regime

Recent advancements in high-resolution very long baseline interferometry have significantly improved our understanding of jet collimation near supermassive black holes in active galactic nuclei (AGNs), particularly in high-power systems. However, the collimation properties of jets in low-luminosity AGNs (LLAGNs) remain poorly explored. In this study, we investigate the jet structure of M84, a nearby radio galaxy and a representative LLAGN, to probe jet collimation properties in the low-accretion regime. Utilizing astrometric phase-referencing observations from the Very Long Baseline Array, supplemented by archival Very Large Array data, we trace the jet geometry of M84 over a broad range of scales, from ∼10 2 to ∼10 7 Schwarzschild radii ( r s ). Our analysis reveals a well-defined transition from a semiparabolic profile, W ( r ) ∝ r 0.71 , to a conical shape, W ( r ) ∝ r 1.16 , occurring at approximately 1.67 × 10 4 r s . This indicates that the M84 jet is notably less collimated than those in other known LLAGN sources. Our findings provide new insights into the relationship between jet collimation and accretion rate, offering crucial constraints for jet formation models of LLAGNs.