The Spitzer view of FR-I radio galaxies: on the origin of the nuclear mid-infrared continuum

We present Spitzer MIR spectra of 25 FR-I radio galaxies and investigate the nature of their MIR continuum emission. MIR spectra of star-forming galaxies and quiescent elliptical galaxies are used to identify host galaxy contributions while radio/optical core data are used to isolate the nuclear non-thermal emission. Out of the 15 sources with detected optical compact cores, four sources are dominated by emission related to the host galaxy. Another four sources show signs of warm, nuclear dust emission: 3C15, 3C84, 3C270, and NGC 6251. It is likley that these warm dust sources result from hidden AGN of optical spectral type 1. The MIR spectra of seven sources are dominated by synchrotron emission, with no significant component of nuclear dust emission. In parabolic SED fits of the non-thermal cores FR-Is tend to have lower peak frequencies and stronger curvature than blazars. This is roughly consistent with the common picture in which the core emission in FR-Is is less strongly beamed than in blazars.