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

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Bendo, George J., Buckalew, Brent A., Dale, Daniel A., Draine, Bruce T., Joseph, Robert D., Kennicutt, Robert C., Jr., Sheth, Kartik, Smith, John-David T., Walter, Fabian, Calzetti, Daniela, Cannon, John M., Engelbracht, Charles W., Gordon, Karl D., Helou, George, Hollenbach, David, Murphy, Eric J., & Roussel, Hÿl��ne
2006, The Astrophysical Journal, 645, 134

We present Spitzer 3.6-160 �?m images, Spitzer mid-infrared spectra, and JCMT SCUBA 850 �?m images of the Sombrero Galaxy (NGC 4594), an Sa galaxy with a 109 Msolar 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 �?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 �?m emission. We examine the possible emission mechanisms that could give rise to the 850 �?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 �?m flux density by themselves. The remaining possibilities for the source of the 850 �?m emission include a combination of known emission mechanisms, synchrotron emission that is self-absorbed at wavelengths longer than 850 �?m, or unidentified spectral lines in the 850 �?m band.

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