George J. Bendo et al. 2006 ApJ 645 134 doi:10.1086/504033
Spitzer and JCMT Observations of the Active Galactic Nucleus in the Sombrero Galaxy (NGC 4594)
FREEGeorge J. Bendo1,2, Brent A. Buckalew3, Daniel A. Dale4, Bruce T. Draine5, Robert D. Joseph6, Robert C. Kennicutt, Jr.2,7, Kartik Sheth3, John-David T. Smith2, Fabian Walter8, Daniela Calzetti9, John M. Cannon8, Charles W. Engelbracht2, Karl D. Gordon2, George Helou3, David Hollenbach10, Eric J. Murphy11 and Hélène Roussel8
Show affiliationsWe present Spitzer 3.6-160 μm images, Spitzer mid-infrared spectra, and JCMT SCUBA 850 μm images of the Sombrero Galaxy (NGC 4594), an Sa galaxy with a 109 M
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 μm 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 μm emission. We examine the possible emission mechanisms that could give rise to the 850 μm 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 μm flux density by themselves. The remaining possibilities for the source of the 850 μm emission include a combination of known emission mechanisms, synchrotron emission that is self-absorbed at wavelengths longer than 850 μm, or unidentified spectral lines in the 850 μm band.
- Subject headings
galaxies: active; galaxies: individual (NGC 4594); galaxies: ISM; galaxies: nuclei; infrared: galaxies
- Dates
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Issue 1 (2006 July 1)
Received 2006 January 5, accepted for publication 2006 March 7
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Spitzer and JCMT Observations of the Active Galactic Nucleus in the Sombrero Galaxy (NGC 4594)
George J. Bendo et al. 2006 ApJ 645 134
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