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

Low spectral resolution (delta lambda/lambda ~ 50) mid-infrared observationswith high angular resolution (0.3--0.5'') using the Long WavelengthSpectrometer on the Keck I Telescope are reported of the nuclei of five highlyluminous infrared bright galaxies. Spectra of eight distinct nuclei, ranging inluminosity from 10^11 L_sun to >10^12 L_sun have been obtained. Four of thenuclei show the characteristic PAH emission features, i.e. 11.3 micron emissionas well as the 8.6 micron shoulder of the 7.7 micron band. The other nucleishow either weak PAH emission bands or no evidence for these bands. The highspatial resolution of the observations reveals extended emission in the 11.3micron PAH band associated with several of the compact nuclear sources. Whenproper account is taken of the diffuse PAH emission, most of the compactsources show little or no directly associated PAH emission. The diffuse PAHemission is extended over spatial scales of 100--500 pc; its presence showsthat there is significant circumnuclear UV/optical emission exciting thearomatic bands, most likely associated with circumnuclear starbursts. After thespectra of the nuclear sources are corrected for the spectrum of the diffusePAH emission, the peak apparent silicate optical depth at 9.7 microns can be asgreat as 15, corresponding to > 150 magnitudes of visible light extinction.Because of the large silicate optical depths, mid-infrared spectra are notprobing the nature of the true nuclei in the most opaque compact sources.Comment: 21 pages, 4 tables, 4 figures, Accepted for publication in A