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 ≤10¹¹ 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 μn 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.