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Published November 2002 | Published + Accepted Version
Journal Article Open

Mid-Infrared Spectroscopy of Infrared-Luminous Galaxies with Subarcsecond Resolution


Low spectral resolution (Δλ/λ ~ 50) mid-infrared observations with high angular resolution (0."3–0."5) using the Long Wavelength Spectrometer on the Keck I Telescope are reported of the nuclei of five highly luminous infrared-bright galaxies. Spectra of eight distinct nuclei, ranging in luminosity from ~10^(11) to more than 10^(12) L_⊙ have been obtained. Four of the nuclei show the characteristic polycyclic aromatic hydrocarbon (PAH) emission features, i.e., 11.3 μm emission, as well as the 8.6 μm shoulder of the 7.7 μm band. The other nuclei show either weak PAH emission bands or no evidence for these bands. The high spatial resolution of the observations reveals extended emission in the 11.3 μm PAH band associated with several of the compact nuclear sources. When proper account is taken of the diffuse PAH emission, most of the compact sources show little or no directly associated PAH emission. The diffuse PAH emission is extended over spatial scales of 100–500 pc; its presence shows that there is significant circumnuclear UV–optical emission exciting the aromatic bands, most likely associated with circumnuclear starbursts. After the spectra of the nuclear sources are corrected for the spectrum of the diffuse PAH emission, the peak apparent silicate optical depth at 9.7 μm can be as large as 15, corresponding to more than 150 mag of visible light extinction. Because of the large silicate optical depths, mid-infrared spectra are not probing the nature of the true nuclei in the most opaque compact sources.

Additional Information

© 2002. The American Astronomical Society. Received 2002 January 22. Accepted 2002 July 18. We thank J. Aycock and R. Campbell for assistance with the observations and Barbara Jones, Rick Puetter, and the Keck team that brought the LWS into service, enabling these observations. S. Lord kindly provided the atmospheric model that was used to establish the wavelength calibration. G. Helou and L. Armus kindly read a draft of this manuscript. We thank the anonymous referee for providing helpful comments. The W. M. Keck Observatory is operated as a scientific partnership between the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. It was made possible by the generous financial support of the W. M. Keck Foundation. We extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented herein would have been possible. This research has made use of the NASA/IPAC Extragalactic Database, which is operated by the Jet Propulsion Laboratory, Caltech under contract with NASA. B. T. S., K. M., and E. E. are supported by grants from the NSF and NASA. SIRTF is carried out at the Jet Propulsion Laboratory. A. J. W. was supported by NASA grant NAG 5-3042.

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Published - Soifer_2002_AJ_124_2980.pdf

Accepted Version - 0207444.pdf


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