Spitzerand JCMT Observations of the Active Galactic Nucleus in the Sombrero Galaxy (NGC 4594)

We present Spitzer 3.6-160 micron images, Spitzer mid-infrared spectra, andJCMT SCUBA 850 micron images of the Sombrero Galaxy (NGC 4594), an Sa galaxywith a 10^9 M_solar low luminosity active galactic nucleus (AGN). The brightestinfrared sources in the galaxy are the nucleus and the dust ring. The spectralenergy distribution of the AGN demonstrates that, while the environment aroundthe AGN is a prominent source of mid-infrared emission, it is a relatively weaksource of far-infrared emission, as had been inferred for AGN in previousresearch. The weak nuclear 160 micron emission and the negligible polycyclicaromatic hydrocarbon emission from the nucleus also implies that the nucleus isa site of only weak star formation activity and the nucleus contains relativelylittle cool interstellar gas needed to fuel such activity. We propose that thisgalaxy may be representative of a subset of low ionization nuclear emissionregion galaxies that are in a quiescent AGN phase because of the lack of gasneeded to fuel circumnuclear star formation and Seyfert-like AGN activity.Surprisingly, the AGN is the predominant source of 850 micron emission. Weexamine the possible emission mechanisms that could give rise to the 850 micronemission and find that neither thermal dust emission, CO line emission,bremsstrahlung emission, nor the synchrotron emission observed at radiowavelengths can adequately explain the measured 850 micron flux density bythemselves. The remaining possibilities for the source of the 850 micronemission include a combination of known emission mechanisms, synchrotronemission that is self-absorbed at wavelengths longer than 850 microns, orunidentified spectral lines in the 850 micron band.Comment: Accepted to ApJ, 200