The role of the synchrotron component in the mid infrared spectrum of M 87

TL;DR

This study analyzes the mid-infrared spectrum of M 87, revealing that its emission is dominated by synchrotron radiation from the active nucleus, with minimal contribution from a dusty torus.

Contribution

It provides a detailed spectral decomposition of M 87's mid-infrared emission, demonstrating the dominance of synchrotron emission over thermal dust components.

Findings

The residual spectrum is a power-law consistent with synchrotron emission.

The 10 micron silicate feature is explained by stellar populations, not a dusty torus.

No significant thermal dust emission detected in the mid-infrared spectrum.

Abstract

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