How to Turn Jets into Cylinders near Supermassive Black Holes in 3D GRMHD Simulations

Accreting supermassive black holes (SMBHs) produce highly-magnetized relativistic jets that tend to collimate gradually as they fly away from their SMBHs. However, recent radio interferometric observations of the relativistic jets in the 3C84 galaxy have revealed a stunning, cylindrical jet very close – within hundreds of Schwarzschild radii – to the black hole! To explore how such extreme collimation comes to be, we use 3D general-relativistic magnetohydrodynamic (GRMHD). For this, I consider a SMBH surrounded by a magnetized torus immersed in a constant-density ambient medium. I demonstrate that the jet-ambient medium interactions produce backflows, which – alongside the disk winds – provide the necessary lateral pressure support to collimate the jets into cylinders. However, this collimation happens at much larger distances than observed in 3C84. I will then demonstrate that radiative disk cooling, which is inferred to take place in 3C84, reduces the distance at which the jets turn into cylinders and brings it into agreement with the observations. I will discuss how cooling affects the mass-accretion rate, jet power, and lateral jet power and velocity distributions.