The Impact of a Clumpy Ambient Medium on the Dynamics and Synchrotron Emission of AGN Jets

Abstract

Relativistic jets from AGN are expected to propagate through an inhomogeneous ISM, but the role of small-scale gas inhomogeneities in shaping their early evolution remains uncertain. We investigate this problem using 3D special-relativistic magnetohydrodynamic simulations with PLUTO, following a jet with a Lorentz factor of 10 propagating through a pc-scale medium. We compare a homogeneous ambient medium with two clumpy ambient-medium models, motivated by dense gas in the nuclear ISM, in which clouds occupy volume filling factors of $0.1$ and $1$ per cent. We compute synthetic synchrotron maps and spectra from Lagrangian macro-particles that have been accelerated by diffusive shock acceleration. Jet-cloud interactions deflect the flow, enhance shock formation, and produce a more asymmetric cocoon than in the homogeneous ISM case. Altogether, this results in a substantially stronger mass loading: the entrained mass increases from $0.43$ M$_{\odot}$ in the homogeneous run to $5.3$ and $8.8$ M$_{\odot}$ in the clumpy runs. The clumpy ISM also modifies the turbulence and the material mixing within the jet cocoon. The dynamical differences we find translate into brighter and more irregular synchrotron emission. For the same injected jet, the models with a clumpy medium produce stronger frequency-integrated synchrotron emission than the homogeneous run, with the largest differences in the SED occurring from the sub-mm to the infrared/optical bands. Our results demonstrate that including a small-scale ISM structure provides a more complete description of the early dynamical evolution of young AGN jets and their radiative properties.