The mid-infrared emission of M87

We discuss Subaru and Spitzer Space Telescope imaging and spectroscopy of M87 in the mid- infrared ( mid- IR) from 5 to 35 mu m. These observations allow us to investigate mid- IR emission mechanisms in the core of M87 and to establish that the flaring, variable jet component HST- 1 is not a major contributor to the mid- IR flux. The Spitzer data include a high signal- to- noise ratio 15Y35 mu m spectrum of the knot A/ B complex in the jet, which is consistent with synchrotron emission. However, a synchrotron model cannot account for the observed nuclear spectrum, even when contributions from the jet, necessary due to the degrading of resolution with wavelength, are included. The Spitzer data show a clear excess in the spectrum of the nucleus at wavelengths longer than 25 mu m, which we model as thermal emission from cool dust at a characteristic temperature of 55 +/- 10 K, with an IR luminosity similar to 10(39) ergs s (-1). Given Spitzer's few arcsecond angular resolution, the dust seen in the nuclear spectrum could be located anywhere within similar to 5 '' ( 390 pc) of the nucleus. In any case, the ratio of active galactic nucleus ( AGN) thermal to bolometric luminosity indicates that M87 does not contain the IR- bright torus that classical unified AGN schemes invoke. However, this result is consistent with theoretical predictions for low- luminosity AGNs.