Compact jets dominate the continuum emission in low-luminosity active galactic nuclei

Abstract: The disappearance of the accretion disc in low-luminosity active galactic nuclei (LLAGN) leaves behind a faint optical nuclear continuum whose nature has been largely debated, mainly due to serious observational limitations in the IR to UV range. We combine multi-wavelength sub-arcsecond resolution observations -- able to isolate the genuine nuclear continuum -- with nebular lines in the mid-IR, to indirectly probe the shape of the extreme UV continuum. We found that 8 of the nearest prototype LLAGN are compatible with pure compact jet emission (self-absorbed synchrotron plus the associated self-Compton component) over more than ten orders of magnitude in frequency. When compared with typical radio galaxies, the LLAGN continua show two peculiarities: $i)$ a very steep spectral slope in the IR-to-optical/UV range ($-3.7 < \alpha_0 < -1.3$; $F_\nu \propto \nu^{\alpha_0}$); and $ii)$ a very high turnover frequency ($0.2-30\, \rm{THz}$; $1.3\,\rm{mm}-10\,\rm{\mu m}$). These attributes can be explained if the synchrotron continuum is mainly dominated by thermalised particles at the jet base or corona with considerably high temperatures, whereas only a small fraction of the energy ($\sim 20\%$) would be distributed along the high-energy power-law tail of accelerated particles. On the other hand, the nebular gas excitation in LLAGN is in agreement with photo-ionisation from inverse Compton radiation ($\alpha_{\rm x} \sim -0.7$), which would dominate the nuclear continuum shortwards of $\sim 3000$ \r{A}. Our results suggest that the LLAGN continuum can be dominated at all wavelengths by undeveloped jets, powered by a thermalised particle distribution, similar to the behaviour observed in compact jets of quiescent black hole X-ray binaries. This has important implications in the context of galaxy evolution, since LLAGN may represent a major but underestimated source of kinetic feedback in galaxies.