The role of the synchrotron component in the mid-infrared spectrum of m 87

Abstract

We study in detail the mid-infrared (MIR) Spitzer-IRS spectrum of M 87 in the range 5-20 μm. Thanks to the high sensitivity of our Spitzer-IRS spectra we can disentangle the stellar and nuclear components of this active galaxy. To this end, we have properly subtracted from the M 87 spectrum the contribution of the underlying stellar continuum derived from passive Virgo galaxies in our sample. The residual is a clear power law, without any additional thermal component, with a zero-point consistent with that obtained by high spatial resolution, ground-based observations. The residual is independent of the adopted passive template. This indicates that the 10 μm silicate emission shown in the spectra of M 87 can be entirely accounted for by the underlying old stellar population, leaving little room for a possible torus contribution. The MIR power law has a slope α ∼0.77-0.82 (Sν ∝ ν -α), consistent with optically thin synchrotron emission.