2026/06/10 by G. Fichet de Clairfontaine, M. Perucho, J. M. Martí +1
Physics and Astronomy · #Astrophysics and Cosmic Phenomena #Astrophysical Phenomena and Observations #Galaxies: Formation, Evolution, Phenomena
paper · pdf · doi:10.1051/0004-6361/202660436
Context. Recent Very Long Baseline Interferometry (VLBI) and Gaia astrometry reveal systematic milliarcsecond-scale offsets between the radio and optical centroids of active galactic nuclei (AGNs). These “radio-optical offsets” do not alter the standard opacity-driven interpretation of radio core shifts. Instead, they indicate that the optical emission centroid is frequently displaced downstream of the radio synchrotron optical depth τ = 1 surface, implying that additional dissipation and particle reacceleration occur beyond the opacity radio core within relativistic jets. Aims. We investigate whether energy dissipation via stellar wind mass-loading (a process by which stellar wind material is entrained into the jet flow, progressively increasing its inertia and triggering dissipation) can generate such offsets by refreshing the particle spectrum, and how this effect may depend on AGN jet power, host stellar distribution, and viewing angle. In addition, another goal of this work is to assess whether radio-optical offsets can serve as a diagnostic of jet composition and energetics across time. Methods. We performed steady-state, axisymmetric relativistic magnetohydrodynamic simulations of AGN jets, including baryonic mass-load from stellar winds, varying jet kinetic power, and stellar core radius. Synthetic synchrotron emission maps in radio and optical bands were generated via a radiative transfer code, and centroid offsets were extracted for comparison with observations. Results. Parsec-scale radio-optical offsets arise only for jet powers L j ∼ 10 42.5 − 10 44 erg s −1 . In this regime, stellar winds trigger jet deceleration at intrinsic distances of a few 10 2 − 10 3 pc, shifting the optical centroid downstream and producing offsets of ∼0.1 − 4 mas (a few tens of parsecs at z = 1). Offsets depend on stellar distribution, viewing angle, and optical jet dominance, and vanish outside this power range. We reproduce the observed redshift evolution of offset incidence, linking it to the cosmic evolution of thermally pulsing asymptotic giant branch mass loss. Conclusions. Radio-optical offsets provide a flux-weighted, geometry-dependent probe of jet dissipation and stellar entrainment. Although stellar mass loading is unlikely to be the sole dissipation mechanism, its unavoidable presence in galactic nuclei makes it a natural baseline for energy dissipation. Radio-optical offsets therefore offer a constraint on AGN jet power and jet-host coupling, independent of traditional lobe-based methods.