Spitzer and JCMT observations of the active galactic nucleus in the Sombrero Galaxy (NGC 4594)

We present Spitzer 3.6-160 mu m images, Spitzer mid-infrared spectra, and JCMT SCUBA 850 mu m images of the Sombrero Galaxy ( NGC 4594), an Sa galaxy with a 10(9) M-circle dot low-luminosity active galactic nucleus ( AGN). The brightest infrared sources in the galaxy are the nucleus and the dust ring. The spectral energy distribution of the AGN demonstrates that, while the environment around the AGN is a prominent source of mid- infrared emission, it is a relatively weak source of far-infrared emission, as had been inferred for AGNs in previous research. The weak nuclear 160 mu m emission and the negligible polycyclic aromatic hydrocarbon emission from the nucleus also implies that the nucleus is a site of only weak star formation activity and the nucleus contains relatively little cool interstellar gas needed to fuel such activity. We propose that this galaxy may be representative of a subset of low-ionization nuclear emission region galaxies that are in a quiescent AGN phase because of the lack of gas needed to fuel circumnuclear star formation and Seyfert-like AGN activity. Surprisingly, the AGN is the predominant source of 850 mu m emission. We examine the possible emission mechanisms that could give rise to the 850 mu m emission and find that neither thermal dust emission, CO line emission, bremsstrahlung emission, nor the synchrotron emission observed at radio wavelengths can adequately explain the measured 850 mu m flux density by themselves. The remaining possibilities for the source of the 850 mu m emission include a combination of known emission mechanisms, synchrotron emission that is self-absorbed at wavelengths longer than 850 mu m, or unidentified spectral lines in the 850 mu m band.