We study in detail the mid-infrared Spitzer -IRS spectrum of M 87 in the range 5 to 20 \mu m. 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 \mu m silicate emission shown in 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 \alpha \sim 0.77-0.82 ( S _ { \nu } \propto \nu ^ { - \alpha } ) , consistent with optically thin synchrotron emission .